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Quantum Information·Captain: Goku

On Cellular Algebras with Jucys Murphy ElementsResearch Paper

Motivation

We study analogues of Jucys-Murphy elements in cellular algebras arising from repeated Jones basic constructions. Examples include Brauer and BMW algebras and their cyclotomic analogues.

This mission is the statement-level formalization of On Cellular Algebras with Jucys Murphy Elements (arXiv:0907.3459v2). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

5 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Lemma 6.10 (see [4], Lemma 2.3) — degenerate affine skein power identity (Lemma 6.10 (see [4], Lemma 2.3), PDF p. 24). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

37 results of the paper are not stated yet, because the library lacks an interface they need:

  • Lemma 2.3 — globalizing arbitrary cell-module bases: CellModule, involution-twisted right module, cell-layer bimodule quotient, and compatible tensor-layer equivalence (hard)
  • Lemma 2.9 ([17], Lemma 2.20) — simple modules and filtration multiplicities: Coherent cellular tower, scalar-extended cell modules, cell filtrations and actual inclusion multiplicities (hard)
  • Corollary 2.10 — independence and Frobenius reciprocity of cell-filtration multiplicities: Filtrations of the actual induction and restriction modules and their cell-isomorphism counts (hard)
  • Corollary 2.12 — strong coherence implies multiplicity-free generic branching: Strong coherence and the actual generic inclusion matrices (hard)
  • Proposition 2.18 — restriction rule for the constructed path bases: Inductively constructed path bases, actual branching paths, restriction coefficients and higher-tail spans (hard)
  • Corollary 2.19 — restriction rule for globalized cellular path bases: Globalizations of constructed path bases and their cell-ideal quotient action (hard)
  • Proposition 3.6 — multiplicative JM triangularity and central product: Multiplicative JM family, scalar actions on constructed cell modules and path-basis triangularity (hard)
  • Proposition 3.7 — additive JM triangularity and central sum: Additive JM family and the constructed path-basis action (hard)
  • Lemma 3.10 — characterization of Gelfand–Zeitlin idempotents: Finite split semisimple tower, actual branching paths, primitive central idempotents and canonical GZ products (hard)
  • Proposition 3.11 — separating JM elements generate the GZ algebra: Scalar-extended separated JM family and the actual GZ subalgebra (hard)
  • Theorem 4.1 ([17], Theorem 3.2) — cellularity from basic constructions: All-eight-axiom basic-construction tower and its constructed cellular structure, cell modules and reflected branching diagram (hard)
  • Theorem 5.1 — lifting multiplicative JM families (paper headline): Strong-framework pair, quotient maps, multiplicative JM predicates and resulting A-cell structure (hard)
  • Corollary 5.2 — constant multiplicative lifting factor: Basic-construction labels and uniquely determined product-action scalars (hard)
  • Theorem 5.3 — lifting additive JM families: Strong-framework pair, additive JM predicates and the basic-construction cell action (hard)
  • Lemma 6.14 — generic injectivity of multiplication by e_n: Concrete universal admissible integer-polynomial omega sequence and canonical cyclotomic level map (hard)
  • Proposition 6.16 — degenerate cyclotomic BMW strong framework: Specific generic admissible degenerate tower, Hecke quotient and full strong framework predicate (hard)
  • Corollary 6.17 — strong coherence and path-labelled cells for admissible degenerate cyclotomic BMW algebras: Admissibility as freeness of W_2 e_1 and a strongly coherent multipartition-labelled cellular tower (hard)
  • Proposition 6.18 — explicit Temperley–Lieb cell ideals: Temperley--Lieb presentation, specialized basic-construction cellular datum and its named cell ideals (hard)
  • Lemma 6.20 — the Hecke quotient is the Temperley–Lieb algebra: Generic Hecke Murphy cell ideal and concrete Temperley--Lieb algebra over the enlarged ring (hard)
  • §2.7, unnumbered — existence of path bases and their globalizations: Coherent family of actual restriction-filtration lifts and cell-layer globalizations (hard)
  • §5, unnumbered — general multiplicative scalar formula: Lifted product action on basic-construction cell modules with alternating gamma indices (hard)
  • §5, unnumbered — additive scalar formula and constant-factor specialization: Lifted additive action on basic-construction cell modules and its scalar (hard)
  • §6.2, unnumbered — strongly coherent BMW cellular tower: Generic BMW presentation ring and constructed strongly coherent Young-diagram cellular datum (hard)
  • §6.2, unnumbered / (6.1) — BMW JM family and eigenvalues: Concrete BMW recursive L elements and their action on constructed up-down cell bases (hard)
  • §6.3, unnumbered — strongly coherent Brauer cellular tower: Brauer diagram algebra with loop-count multiplication and its constructed strongly coherent cell datum (hard)
  • §6.3, unnumbered / (6.2) — unshifted Brauer JM family and eigenvalues: Concrete unshifted Brauer recursion and its actual up-down cell-module action (hard)
  • §6.4.5, unnumbered — cyclotomic BMW cellular tower: Universal admissible nondegenerate coefficient ring and concrete cyclotomic BMW cellular tower (hard)
  • §6.4.6, unnumbered / (6.4) — cyclotomic BMW JM family: Concrete universal-admissible cyclotomic BMW tower, y_1 recursion and up-down cell action (hard)
  • §6.5.6, unnumbered / (6.7) — degenerate cyclotomic BMW additive JM family: Specific generic degenerate coefficient polynomials and constructed cell action of x_j (hard)
  • §6.6, unnumbered — Temperley–Lieb cellular tower and label order: Generic Temperley--Lieb presentation with specific reversed cell order and strong filtrations (hard)
  • §6.6, unnumbered — comparison of the two Temperley–Lieb cellular structures: Both actual Temperley--Lieb cellular structures, Hecke quotient map and cell-module comparison (hard)
  • §6.6, unnumbered — Temperley–Lieb JM triangularity, product scalar and local relation: Concrete Hecke-derived Temperley--Lieb L sequence and both identified cell structures (hard)
  • Example 3.1 / (3.1) — classical Hecke JM properties (background): Hecke presentation and literal Murphy cell modules and bases (hard)
  • §6.3, unnumbered — symmetric-group additive JM family (background): Actual symmetric group algebra, additive JM recursion and q=1 Murphy cell-module specialization (hard)
  • §6.4.6 / (6.3), unnumbered — cyclotomic Hecke multiplicative JM family (background): Cyclotomic Hecke presentation in the corrected quadratic convention and actual Murphy-type modules (hard)
  • §6.5.6 / (6.6), unnumbered — degenerate cyclotomic Hecke additive JM family (background): Degenerate cyclotomic Hecke quotient and its literal Murphy-type cellular tower and basis action (hard)
  • §3.1.1, unnumbered — maximal abelian GZ algebra and canonical idempotents: Actual multiplicity-free split semisimple tower, canonical central projectors and generated GZ algebra (hard)

Selected references

  • On Cellular Algebras with Jucys Murphy Elements, arXiv:0907.3459v2, https://arxiv.org/abs/0907.3459v2
6 thms2 active usersReviewed
🏆Completed
Quantum Information·Captain: Goku

Categorified central extensions, 'etale Lie 2-groups and Lie's Third Theorem for locally exponential Lie algebrasResearch Paper

Motivation

Lie's Third Theorem, asserting that each finite-dimensional Lie algebra is the Lie algebra of a Lie group, fails in infinite dimensions. The modern account on this phenomenon is the integration problem for central extensions of infinite-dimensional Lie algebras, which in turn is phrased in terms of an integration procedure for Lie algebra cocycles. This paper remedies the obstructions for integrating cocycles and central extensions from Lie algebras to Lie groups by generalising the integrating objects. Those objects obey the maximal coherence that one can expect. Moreover, we show that they are the universal ones for the integration problem. The main application of this result is that a Mackey-complete locally exponential Lie algebra (e.g., a Banach-Lie algebra) integrates to a Lie 2-group in the sense that there is a natural Lie functor from certain Lie 2-groups to Lie algebras, sending the integrating Lie 2-group to an isomorphic Lie algebra.

This mission is the statement-level formalization of Categorified central extensions, 'etale Lie 2-groups and Lie's Third Theorem for locally exponential Lie algebras (arXiv:0812.1673v3). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

2 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is A discrete generalized cocycle gives the explicit loop prolongation (Lemma II.3: discrete cocycle loop prolongation, PDF p. 12). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

42 results of the paper are not stated yet, because the library lacks an interface they need:

  • Abstract and Theorem VI.5: Lie's Third Theorem for Mackey-complete locally exponential Lie algebras: Mackey-complete locally exponential Lie algebra; actual etale Lie 2-group and its Lie differentiation (hard)
  • Lemma I.5: cycles and spherical 3-cocycle: Inconsistent printed diagonal filling normalization (5) (source-ambiguity)
  • Lemma I.6: comparison of filling systems: Comparison of filling systems satisfying the inconsistent printed (5) (source-ambiguity)
  • Lemma I.7: convex-chart fillers and comparison: Inconsistent filler extension claim and unspecified corner value of formula (9) (source-ambiguity)
  • Remark I.8: actual period-valued integrating cocycle: Actual simplex integral of a left-invariant locally convex 2-form and its spherical period homomorphism (hard)
  • Lemma I.9: differentiating a generalized cocycle: Intrinsic Bastiani differential and critical-point Hessian of a locally smooth quotient-valued cochain (hard)
  • Theorem I.11: constructed cocycle integrates omega: The computed integrating cocycle and its actual antisymmetric Hessian (hard)
  • Remark I.12: chart and cohomology representative independence: Both explicit integral comparison cochains for chart choice and cocycle representative (hard)
  • Lemma I.13: local triviality yields a subgroup-valued remainder: Lie differentiation of a local cocycle and subgroup-valued global remainder for an arbitrary coefficient subgroup (hard)
  • Remark I.14: obstruction for merely connected groups: Actual universal-cover obstruction determined by the constructed local splitting (hard)
  • Lemma I.15: period image is contained in the associativity subgroup: Actual period image and differentiated local cocycle, with arbitrary subgroup Gamma (hard)
  • Lemma I.16: three coefficient-compatibility equivalences: All three coefficient-compatibility assertions for the constructed spherical cocycle (hard)
  • Proposition I.17: unique period-compatible coefficient map: Universal spherical 3-cocycle and the unique induced coefficient homomorphism (hard)
  • Corollary I.18: universal integrating cocycle: Actual comparison morphisms and uniqueness of their coefficient map (hard)
  • Example II.4: no compatible globally smooth loop multiplication: Standard smooth SU(2) loop group, normalized Kac-Moody cocycle and compatible-global-smoothness obstruction (hard)
  • Lemma II.7: quotient of a locally smooth loop prolongation: Smooth local loop charts and quotient Lie structure for a generated loop prolongation (hard)
  • Remark II.8: Lie algebra of a locally associative loop: Canonical Lie algebra of a locally associative smooth local loop and independence of restriction (hard)
  • Proposition II.9: locally associative loop integration: Integration to an actual locally smooth and locally associative loop prolongation (hard)
  • Example III.4: loop prolongation to a 2-group: Missing strict common tensor inverse for a general loop prolongation (source-ambiguity)
  • Remark III.5: associated groups and source fibres: General nonstrict III.1 2-group, isomorphism-class group, identity component arrows and source fibres (hard)
  • Theorem IV.1: local-to-global smooth group structure: Locally convex maximal smooth structures, local charts and compatible local-to-global Lie structure (hard)
  • Example IV.2: cocycle central Lie group extension: Actual cocycle central extension with locally convex smooth atlas and a smooth local section (hard)
  • Example IV.2: path-section cocycle of universal cover: Compact-open based-path group, universal cover and its actual path-section cocycle (hard)
  • Proposition IV.5: strict 2-group globalization: Globalization of a strict smooth 2-space including strong source and target submersions (hard)
  • Lemma IV.6: trivial principal source-fibre bundle: Actual source-fibre submanifold, tensor action and trivial principal bundle for a nonstrict 2-group (hard)
  • Lemma IV.7: locally constant associator arrow part: Globally smooth nonstrict associator and its tensor-inverse arrow component (hard)
  • Example IV.10: Cartan 3-cocycle and string 2-group: Integral Cartan normalization and strict inverse-triple associator normalization (source-ambiguity)
  • Remark V.3: generalized cocycle extension 2-group: Failure of the stated strict two-sided tensor inversion for allowed generalized cocycles (source-ambiguity)
  • Lemma V.4: canonical central 2-group extension: Named extension object need not be a III.1 2-group (source-ambiguity)
  • Remark V.5: tangent 2-functor on pointed smooth 2-spaces: Actual tangent 2-functor from pointed locally convex Lie groupoids to internal topological vector categories (hard)
  • Lemma V.9: canonical smooth generalized central extension: Canonical smooth structure on the potentially invalid V.3 strict-inverse object (source-ambiguity)
  • Proposition V.10: ordinary band for discrete coefficient image: Band of a genuine smooth generalized central extension with discrete inherited coefficient image (hard)
  • Corollary V.12: band integrates central algebra extension: Band of the named potentially invalid V.3/V.9 2-group (source-ambiguity)
  • Remark V.13: canonical Lie differentiation 2-functor: Canonical Lie differentiation 2-functor of etale Lie 2-groups with a locally smooth associator (hard)
  • Proposition V.14: derived split central Lie algebra extension: Differentiated actual maps of a smooth generalized central extension and their continuous linear splitting (hard)
  • Theorem V.16: split central Lie algebra extensions integrate: Existence of a genuine smooth generalized central extension integrating a topologically split Lie algebra extension (hard)
  • Remark V.17: cover factorization exact diagram: Inverse-triple associator normalization for the zero-twist cover and the full factorization diagram (source-ambiguity)
  • Theorem VI.3 (cited): actual adjoint Lie group: Actual exponential-generated adjoint group and its locally exponential topology (hard)
  • Example VI.4: non-complemented center Banach Lie algebra: Uncontrolled Banach-space bracket series and invalid separating-vector assertion (source-ambiguity)
  • Remark VII.2: Cech transgression and principal 2-bundle: Printed Cech coefficient type and the actual principal 2-bundle construction (source-ambiguity)
  • Remark A.2: chain rules and intrinsic critical-point Hessian: Bastiani first/second differential, tangent bundle and intrinsic critical-point Hessian (hard)
  • Lemma A.4: exponential naturality: Actual locally convex tangent map and ODE-defined exponential function (hard)

Selected references

  • Categorified central extensions, 'etale Lie 2-groups and Lie's Third Theorem for locally exponential Lie algebras, arXiv:0812.1673v3, https://arxiv.org/abs/0812.1673v3
3 thms2 active usersReviewed
🏆Completed
Quantum Information·Captain: Goku

Exceptional Lie Algebras, SU(3) and Jordan PairsResearch Paper

Motivation

A simple unifying view of the exceptional Lie algebras is presented. The underlying Jordan pair content and role are exhibited. Each algebra contains three Jordan pairs sharing the same Lie algebra of automorphisms and the same external su(3) symmetry. Eventual physical applications and implications of the theory are outlined.

This mission is the statement-level formalization of Exceptional Lie Algebras, SU(3) and Jordan Pairs (arXiv:1112.1258v2). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

8 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is G2: the external A2 hexagon and two weight triangles (§4 opening, Fig. 2: the g2 prototype, PDF p. 5). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

10 results of the paper are not stated yet, because the library lacks an interface they need:

  • §§1,4, unnumbered main result: three Jordan pairs with a common reduced structure algebra, equation (4.1): Compatible concrete complex exceptional Lie, Hurwitz-Hermitian Jordan, and triple-preserving pair models (hard)
  • §4, unnumbered uniqueness assertion, specialized in §4.1.3: Equivalence relation and normalization of embedding uniqueness (source-ambiguity)
  • §4.1.3: parallel planes, Jordan pair and three shared-a2 copies of c3: Root-compatible complex F4/C3 models and the symmetric-matrix Jordan pair (hard)
  • §4.1.4: nested pair inside the central a2: Transport of the 1+2 matrix block Jordan pair to the specified central a2 (hard)
  • §4.2.3: A5 root correspondence, Jordan pair and three shared-a2+a2 copies: Root-compatible E6/A5 embeddings and full 3x3 matrix Jordan-pair equivalences (hard)
  • §4.2.4: ambiguous nested-pair multiplicity in a2+a2: Number and placement of nested (2,2bar) pairs in a2+a2 (source-ambiguity)
  • §4.3.3: three shared-a5 copies of d6 and quaternionic Hermitian Jordan pairs: Complexified quaternionic H3 Jordan pair and root-compatible E7/D6 embeddings (hard)
  • §4.3.4: the nested 3x3 matrix pair inside central a5: Root-compatible transport of the 3+3 sl6 grading to central a5 (hard)
  • §4.4.3: three shared-e6 copies of e7 and Albert Jordan pairs: Concrete complex Albert pair and compatible E8/E7 Lie embeddings (hard)
  • §4.4.4: four a2 factors and six Jordan pairs, equation (4.8): Compatible nested Lie/module/Jordan decomposition of actual complex E8 (hard)

Selected references

  • Exceptional Lie Algebras, SU(3) and Jordan Pairs, arXiv:1112.1258v2, https://arxiv.org/abs/1112.1258v2
9 thms2 active usersReviewed
🏆Completed
Quantum Information·Captain: Goku

Product formulas in functional calculi for sectorial operatorsResearch Paper

Motivation

We study the product formula (fg)(A)=f(A)g(A)(fg)(A) = f(A)g(A)(fg)(A)=f(A)g(A) in the framework of (unbounded) functional calculus of sectorial operators AAA. We give an abstract result, and, as corollaries, we obtain new product formulas for the holomorphic functional calculus, an extended Stieltjes functional calculus and an extended Hille-Phillips functional calculus. Our results generalise previous work of Hirsch, Martinez and Sanz, and Schilling.

This mission is the statement-level formalization of Product formulas in functional calculi for sectorial operators (arXiv:1309.0318v2). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

12 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Abstract regularised calculus product formula (Theorem 2.5, PDF p. 7). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

23 results of the paper are not stated yet, because the library lacks an interface they need:

  • Theorem 3.1: Sector-local H0-infinity contour calculus, B(S_phi), and H(A) (hard)
  • Corollary 3.2: Sector-local holomorphic values of the three rationally damped symbols (hard)
  • Proposition 4.3: Order-indexed bounded Stieltjes operator integral from a finite complex representing measure (hard)
  • Lemma 4.4: Both computed bounded Stieltjes and holomorphic evaluations (hard)
  • Proposition 4.5: Normed bounded Stieltjes evaluation for A and its scalar shifts (hard)
  • Proposition 4.6: The actual bounded Stieltjes integral map on the union algebra (hard)
  • Lemma 4.8: Extended Stieltjes regularisation from the bounded integral representation (hard)
  • Lemma 4.9: Extended Stieltjes graph value and actual powers of A (hard)
  • Proposition 4.10: Weighted complex-Radon Hirsch integral on dom(A^n), its graph closure, and resolvent conjugation (hard)
  • Proposition 4.11: Closed Hirsch evaluations for A and A+delta on the explicit common core (hard)
  • Theorem 4.12: Three independent computed Stieltjes, Hirsch, and holomorphic graph values (hard)
  • Corollary 4.13: Independent Stieltjes and Hirsch graph evaluations without injectivity (hard)
  • Lemma 4.14: Banach quotient by ker(A) and induced Stieltjes graphs (hard)
  • Remark 4.15: Closed induced quotient calculus graph with its exact common core (hard)
  • Theorem 4.16: Computed extended Stieltjes calculus for the scalar T_tilde algebra (hard)
  • Corollary 4.17: Ordered finite composition of computed extended Stieltjes values (hard)
  • Corollary 4.19: Computed extended Stieltjes sum on the intersection domain (hard)
  • Lemma 5.3: C0 semigroup quotient by ker(A) and its computed extended HP evaluations (hard)
  • Theorem 5.4: The same-semigroup HP strong integral and psi_k-regularised extension (hard)
  • Corollary 5.5: Ordered finite products in the computed psi_k-extended HP calculus (hard)
  • Example 5.6: Periodic differentiation generator on complex L2[0,1] and its HP counterexample graphs (hard)
  • Unnumbered weak product formula in Section 4.4: Strong regularisers for the actual extended Stieltjes graph evaluation (hard)
  • Unnumbered consistency of (5.7) with extended HP calculus: Computed HP evaluation of the geometric-mean square and the scalar product (hard)

Selected references

  • Product formulas in functional calculi for sectorial operators, arXiv:1309.0318v2, https://arxiv.org/abs/1309.0318v2
14 thms2 active usersReviewed
🏆Completed
Quantum Information·Captain: Goku

Entropic Fluctuations in Quantum Statistical Mechanics. An IntroductionResearch Paper

Motivation

These lecture notes provide an elementary introduction, within the framework of finite quantum systems, to recent developments in the theory of entropic fluctuations.

This mission is the statement-level formalization of Entropic Fluctuations in Quantum Statistical Mechanics. An Introduction (arXiv:1106.3786v1). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

49 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Paper seed: finite-time quantum Evans–Searles symmetry (Proposition 3.3(7)) (Paper seed: finite-time quantum Evans–Searles symmetry (Proposition 3.3(7)), PDF p. 74). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

73 results of the paper are not stated yet, because the library lacks an interface they need:

  • §1.4: classical entropy balance and nonnegative mean production: Finite pinned harmonic-chain Hamiltonian, its coupled Gaussian measure and relative-density entropy. (hard)
  • §1.6–1.7: scalar and vector finite-time Evans–Searles relations: Actual harmonic-chain time-averaged entropy law with measurable time-reversal and Radon–Nikodym cocycle. (hard)
  • Exercise 1.4 (explicit main result): Joint law of the two harmonic-chain time-averaged reservoir currents. (hard)
  • §1.7: generalized finite-time Evans–Searles symmetry: Extended logarithmic Laplace integral of the actual joint current law, including divergent moments. (hard)
  • §1.8: harmonic-chain thermodynamic limit: Finite-to-infinite Dirichlet-chain operators and Gaussian cylinder-measure convergence on configuration space. (hard)
  • §1.9: complete harmonic-chain wave operators and on-shell scattering matrix: Concrete coupled/decoupled infinite-chain generators, complete wave operators and their Fourier representation. (hard)
  • §1.10: Gaussian NESS and positive entropy production: Gaussian steady state built from the incoming complete wave operator on the actual harmonic chain. (hard)
  • Exercise 1.7 (explicit main result): Harmonic-chain NESS current observables, and arbitrary stationary states with finite local second moments. (hard)
  • §1.11: explicit asymptotic harmonic-chain pressures: Actual thermodynamic-then-time log-Laplace limits and their extended convergence domain. (hard)
  • Exercise 1.9 (explicit main result): Complex Fredholm log-determinant continuation of the actual harmonic finite-time pressure. (hard)
  • §1.12: harmonic-chain current central limit theorem: Harmonic current fluctuation random vector centered by its finite-time mean and degenerate Gaussian limit. (hard)
  • §1.13: Onsager, Einstein and Green–Kubo relations: Differentiable harmonic-chain Gaussian NESS family and equilibrium current correlation. (hard)
  • Exercise 1.12 (explicit main result): Actual equilibrium harmonic-chain correlations and the triangular Cesàro Green–Kubo average. (hard)
  • Exercise 1.13 (explicit main result): Discrete Klein–Gordon propagation matrix elements and symmetric improper current-correlation limits. (hard)
  • §1.14: harmonic-chain current LDP and rate symmetry: Borel laws of actual harmonic-chain mean currents, with extended rate and topological event bounds. (hard)
  • §1.14: local current fluctuation probabilities: Harmonic current laws and shrinking-interval logarithmic probabilities with ordered limits. (hard)
  • §1.15: harmonic-chain Gallavotti–Cohen theorem: Harmonic NESS Gaussian generating integrals and full LDP in that singular steady state. (hard)
  • Theorem 2.19: Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • §2.10: norm-convergent Dyson expansions and Gibbs perturbation series: Complex-time ordered-simplex integrals and recursively constructed Gibbs quotient coefficients. (hard)
  • §2.12.1–2: modular and Connes-cocycle identities: Corrected list of modular/cocycle identities specifying which evolution is implemented by conjugation, while retaining every valid claimed identity. (source-ambiguity)
  • Proposition 3.9: Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • Proposition 3.10: Canonical n-factor product-state embeddings and a joint commuting modular spectral law. (hard)
  • §3.8: joint FCS and modular-law identification: Canonical product factors and joint entropy-projector two-measurement law with vector modular identification. (hard)
  • §4.1: open-system entropy and current balance: Canonical sample/reservoir tensor model with commuting charges, local gauge-invariant couplings and product reference state. (hard)
  • §4.2: energy/charge joint FCS, cumulants and TRI symmetry: Energy/charge joint spectral FCS with all tensor embeddings and explicit deformed transfer generator. (hard)
  • Proposition 4.1: Coupled Gibbs reference on the actual sample/reservoir tensor algebra and four energy/charge transport blocks. (hard)
  • §4.3: transient response without time reversal: Actual coupled sample/reservoir Gibbs family and complex Kubo–Mori correlations for arbitrary observables. (hard)
  • Proposition 5.1: A1–A6 nested matrix embeddings into a norm-dense unital C*-algebra, with limiting dynamics and FCS measures. (hard)
  • Proposition 5.2: Modular GNS representation, A7 convergence and infinite signed relative entropy. (hard)
  • Proposition 5.3: A8 bounded modular perturbation of a closed infinite standard Liouvillean on its genuine domain. (hard)
  • Proposition 5.4: Infinite normal-state cone, generalized relative-entropy pressure and A5 flux family. (hard)
  • Proposition 5.5: Infinite A1–A7 flux family with C2 generalized pressure and normalized finite-time transport coefficients. (hard)
  • Proposition 5.6: Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • §5.5: resonance method (conditional outline): Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • Theorem 5.7: Real-time, finite-volume projection-test errors with iterated M liminf/limsup then t limit. (hard)
  • §5.6(iii)–(v): testing bounds in the enveloping von Neumann algebra: Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • §5.6(vi): quantum Hoeffding exponents: Normal states and all projection-test families in the enveloping von Neumann algebra, with three distinct exponents. (hard)
  • §5.6(vii): all Stein exponents and optimal error rates: Six separate epsilon/vanishing-error projection-testing exponents in the enveloping von Neumann algebra. (hard)
  • Proposition 5.8: A10 infinite analytic parameter family, KMS mixing and symmetric improper correlation limits. (hard)
  • §5.9: stability under local KMS perturbation of reference: Normalized bounded local Araki KMS perturbation of an infinite modular state, with conditional NESS existence. (hard)
  • §5.10: FCS means, covariances and response comparison: Explicit transient-to-stationary covariance interchange hypotheses, a corrected factor, and actual infinite FCS/current interfaces. (source-ambiguity)
  • Proposition 6.3: CAR representations on an arbitrary Hilbert space, including infinite-dimensional representation spaces and unitary intertwiners. (hard)
  • Proposition 6.4: Generated CAR star-algebra in all bounded operators on a potentially infinite representation Hilbert space. (hard)
  • §6.2: specified infinite-dimensional extensions: An explicit corrected infinite-Fock statement distinguishing bounded trace-class differential second quantization from genuinely unbounded generators, with domains and cores. (source-ambiguity)
  • Exercise 6.4 (explicit main result): Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • Proposition 6.7: Actual doubled fermionic Araki–Wyss representation with conjugate second factor and cyclic vector. (hard)
  • Proposition 6.8: Doubled AW modular conjugation/parity, logarithmic determinant scalar and generator with conjugation bars. (hard)
  • §6.4: infinite-dimensional AW extension: Infinite CAR state representation, unbounded modular domains and trace-class logarithmic differences. (hard)
  • Exercise 6.6 (explicit main result): Finite spin–fermion tensor GNS model and explicit n-reservoir deformed generator (6.11). (hard)
  • §6.5: Spin–Fermion large-time claims (outline, not SF1–SF3): Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • §6.6.2–3: EBB current and entropy identities: Actual EBB lead/sample CAR dynamics, finite-rank commutator fluxes and one-particle reference evolution. (hard)
  • §6.6.4: finite EBB determinant pressures: Actual finite EBB sample/lead matrices and their many-body pressures, at all finite p and infinity. (hard)
  • Exercise 6.10 (explicit main result): EBB finite approximants, infinite AW relative-modular logarithm and characteristic Fredholm determinants. (hard)
  • Exercise 6.9 (explicit main result): Embedded EBB one-particle finite approximants and trace-norm convergence of determinant perturbations. (hard)
  • §6.6.6: EBB NESS via complete wave operators: Concrete EBB finite sample with half-line leads, complete incoming wave operators and limiting CAR state. (hard)
  • Exercise 6.11(1,2,4,5): explicit large-time pressure formulas: All scattering pressure functionals of the actual EBB model, including multiparameter symbols. (hard)
  • §6.6.6: EBB large-time pressure formula (6.19): Concrete EBB ordered-volume/time pressure limit with the same complete on-shell scattering matrix. (hard)
  • §6.6.6: NESS entropy production and open-channel criterion: EBB NESS, outgoing entropy/current observables and positive-measure open-channel criterion. (hard)
  • Exercise 6.13 (explicit main result): Concrete EBB NESS currents and lead-indexed scattering transmission integrals. (hard)
  • Exercise 6.15 (explicit main result): Joint reservoir charge-transfer two-measurement law, its Fredholm determinant and scattering limit. (hard)
  • Exercise 6.16: reflectionless equivalences: Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • Exercise 6.17 (explicit main result): Reflectionless two-lead finite intervals, their Fourier phase conventions and actual limiting CAR pressures. (hard)
  • Exercise 6.18(1): on-shell scattering for general EBB reservoirs: General contact spectral-measure leads, cyclic reduction and limiting resolvent boundary values. (hard)
  • §6.6.7: locally interacting EBB dynamics and Dyson expansion: Infinite CAR norm dynamics for a sample-local quartic interaction and its ordered Dyson series. (hard)
  • §6.7.2: finite XY chain is unitarily equivalent to free fermions: Actual finite XY tensor-spin Hamiltonian, Jordan–Wigner strings and unitary Fock intertwiner. (hard)
  • §6.7.3: XY-chain finite/thermodynamic determinant formula: Coupled XY Gibbs reference and even-algebra thermodynamic inclusions with trace-class determinant perturbation. (hard)
  • §6.7.3: explicit XY pressure, heat current and positive production: Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • Exercise 6.21 (explicit main result): Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • §6.7.3: dilogarithm expression and analytic strip: Resolved source hypotheses, conventions and consistent full compound statement (source-ambiguity)
  • §6.7.3: Jordan–Wigner survives on the even algebra: Compatible even spin/CAR local inclusions and the thermodynamic Jordan–Wigner isomorphism. (hard)
  • Theorem A.6: Varying multidimensional probability experiments, extended cumulant domain, gradient steepness and Borel LDP bounds. (hard)
  • Theorem A.7: Distinct limsup finite domain and actual-convergence/differentiability set, gradient image and local Borel LDP. (hard)
  • Theorem A.8: Analytic multidimensional cumulant family with actual finite-time means and a possibly singular Gaussian probability law. (hard)

Selected references

  • Entropic Fluctuations in Quantum Statistical Mechanics. An Introduction, arXiv:1106.3786v1, https://arxiv.org/abs/1106.3786v1
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Quantum Information·Captain: Goku

Reductive and unipotent actions of affine groupsResearch Paper

Motivation

We present a generalized version of classical geometric invariant theory `a la Mumford where we consider an affine algebraic group GGG acting on a specific affine algebraic variety XXX. We define the notions of linearly reductive and of unipotent action in terms of the GGG fixed point functor in the category of (G,k[X])(G,\Bbbk [X])(G,k[X])--modules. In the case that X={⋆}X=\{\star\}X={⋆} we recuperate the concept of lineraly reductive and of unipotent group. We prove in our "relative" context some of the classical results of GIT such as: existence of quotients, finite generation of invariants, Kostant--Rosenlicht's theorem and Matsushima's criterion. We also present a partial description of the geometry of such linearly reductive actions.

This mission is the statement-level formalization of Reductive and unipotent actions of affine groups (arXiv:1406.4446v1). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

1 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Extensions of subgroup translation actions and normalized twisted cocycles (Observation 5.4, equations (4)–(7), PDF p. 12). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

45 results of the paper are not stated yet, because the library lacks an interface they need:

  • Introduction, unnumbered main result: transitivity of linearly reductive actions: Closed affine subgroup restriction of the specified regular right action; coordinate rational (G,k[X])-modules and Definition 2.1. (hard)
  • Theorem 2.4: six equivalent characterizations: The full arbitrary-dimensional rational relative-module category, rational fixed-part right derived functors, total integrals, and injectivity in rational G-modules. (hard)
  • Observation 2.5: further equivalent conditions: Diagonal rational tensor coactions, rational cohomology, and G-linear splitting of arbitrary relative inclusions. (hard)
  • Observation 2.6(1–2): Reynolds characterization and identity: Natural linear projections on every rational relative module, with naturality for all R-linear colinear morphisms, and commutative relative algebra objects. (hard)
  • Observation 2.6(3): claimed G-equivariance of a K-Reynolds operator: Printed assertion fails on permitted inputs or has an unpinned essential hypothesis. (source-ambiguity)
  • Example 2.7(1): classical group reductivity and trivial actions: Classical exactness on all rational G-modules and Definition 2.1 for arbitrary rational commutative algebras, with nonzero R in the trivial-action converse. (hard)
  • Example 2.7(2–3): change of scalars and products: Rational scalar extension and diagonal tensor algebra coactions, together with the contravariant coordinate action of geometric equivariant maps and products. (hard)
  • Example 2.7(4): quotient action with a trivially acting normal subgroup: Printed assertion fails on permitted inputs or has an unpinned essential hypothesis. (source-ambiguity)
  • Example 2.7(5): translation action is linearly reductive: The actual coordinate Hopf algebra of the affine group, its regular translation coaction, and all relative Hopf modules. (hard)
  • Example 2.7(6): equivariant cross-section gives a trivial product: The closed algebraic fiber Phi inverse of 1, its regular equivariant product isomorphism with the specified inverse, and relative linear reductivity. (hard)
  • Corollary 2.8: equivariant augmentation and fixed points: Equivariant algebra augmentation, classical rational group reductivity, and the geometric fixed-point/augmentation correspondence. (hard)
  • Corollary 2.9: reductive stabilizers of affine orbits: Affine algebraic homogeneous spaces and affine orbit/stabilizer isomorphisms, including separability and characteristic-zero reductivity only in the final consequence. (hard)
  • Lemma 2.10: multiplicative integrals for unipotent groups: Unipotent affine groups, multiplicative R-valued total integrals, and the associated Sweedler contraction Reynolds operators on every relative module. (hard)
  • Theorem 2.11: product characterization for unipotent groups: A closed subvariety of X and a regular U-equivariant isomorphism X to L times U, with the specified translation/trivial action. (hard)
  • Observation 2.12: algebra version, orbits and quotient: Rational tensor algebra decompositions without finite-generation restriction and their geometric orbit and quotient realizations for affine varieties. (hard)
  • Theorem 3.3: subgroup translations, exact induction and affine quotient: Rational induction as the K-fixed submodule of k[G] tensor M, its exactness, algebraic homogeneous space affineness, observability, and ideal extension in k[G]^K. (hard)
  • Example 3.4: observability cannot be omitted: Printed assertion fails on permitted inputs or has an unpinned essential hypothesis. (source-ambiguity)
  • Theorem 3.5: linear reductivity on a homogeneous space: Affine homogeneous-space translations, rational subgroup induction and classical linear reductivity, with affineness in the left-hand conjunction. (hard)
  • Theorem 4.1: generalized Matsushima criterion: Restriction along a closed subgroup and the canonical quotient-group action on R^K for normal K, on full rational relative-module categories. (hard)
  • Observation 4.2(2): converse to restriction fails: The algebraic product Gm times Ga, its closed Gm subgroup, and trivial actions on the base field in rational relative categories. (hard)
  • Observation 4.2(3): affineness through subgroup towers: Algebraic homogeneous-space quotient constructions and affineness in a tower of closed subgroup inclusions. (hard)
  • Observation 4.2(4): geometric transitivity: Canonical affine quotient with coordinate ring k[X]^K and its induced G/K-action, together with restriction along closed subgroups. (hard)
  • Observation 4.2(5): extension criterion for groups: Closed normal algebraic subgroup quotients and classical exact fixed-part functors on rational modules. (hard)
  • Corollary 4.3: classical Matsushima criterion: Unipotent radicals, reductive affine groups and algebraic homogeneous quotients over an algebraically closed characteristic-zero field. (hard)
  • Corollary 4.4: tower with a normal first inclusion: Homogeneous spaces L/K and L/G and the descended G/K-action on L/K, requiring K normal only in G. (hard)
  • Theorem 4.5: descent from induced algebras: The actual rational induction functor on arbitrary K-modules and K-module algebras, and exactness of that functor. (hard)
  • Theorem 4.6: induced affine varieties: The contracted product as the algebraic geometric quotient of X times G under a*(x,g)=(xa,a inverseg), including its induced right G-action. (hard)
  • Theorem 4.7: claimed reduction to the unipotent radical: Printed assertion fails on permitted inputs or has an unpinned essential hypothesis. (source-ambiguity)
  • Theorem 5.2: claimed Levi decomposition criterion: Printed assertion fails on permitted inputs or has an unpinned essential hypothesis. (source-ambiguity)
  • Observation 5.3 and its continuation: claimed conjugation-compatible splitting: Printed assertion fails on permitted inputs or has an unpinned essential hypothesis. (source-ambiguity)
  • Theorem 5.5: claimed explicit cocycle classification: Printed assertion fails on permitted inputs or has an unpinned essential hypothesis. (source-ambiguity)
  • Theorem 6.1: finite generation of invariants: The rational relative commutative-algebra category, invariant subalgebra construction, and algebra finite generation over R and k. The printed self-base S typo also needs a recorded correction. (hard)
  • Theorem 6.2: invariant ideals: The invariant subalgebra and actual ideal extension/comap between S^G and S for arbitrary commutative relative algebra objects and G-stable ideals. (hard)
  • Theorem 6.3 and unnumbered quotient-existence claim: The precise source-cited semi-geometric quotient predicate, and the canonical Spec of the invariant algebra with the map induced by inclusion. (source-ambiguity)
  • Section 6, unnumbered conclusion: quotient in two stages: Canonical affine invariant quotients, the geometric quotient by the actual unipotent radical, and the canonical comparison morphism for invariants in stages. (hard)
  • Section 7, unnumbered result: the translation action is unipotent: The coordinate algebra coaction of left translation as a right action xg=g inversex and its inversion identification with right translation, on all relative modules. (hard)
  • Lemma 7.2: unipotency under change of scalars: Definition 7.1 on all nonzero rational relative modules, scalar restriction, and the induced coordinate map for a regular equivariant morphism. (hard)
  • Theorem 7.3: unipotent restriction and observability: Rational induction and algebraic observability, plus rational restriction for both module algebras and affine varieties. (hard)
  • Observation 7.4: point, augmentation and exact-subgroup consequences: Unipotent affine groups via rational modules, equivariant augmentations/fixed points, rational exact induction and normal subgroup restriction. (hard)
  • Theorem 7.5: generalized Kostant–Rosenlicht theorem: Zariski orbit closedness and regular homogeneous-space isomorphisms with closed unipotent stabilizers, conditional on separability only for the latter. (hard)
  • Observation 7.6: classical orbit and subgroup consequences: Classical unipotent affine algebraic groups, their closed subgroups, and orbit closedness for affine algebraic actions. (hard)
  • Lemma 7.8: every character extends for a unipotent action: Regular algebraic character group and nonzero semi-invariants of the specified coordinate coaction, with nonempty X. (hard)
  • Observation 7.10(1) and consequence in (2): unipotency versus observability: Observable actions as nonzero fixed vectors in stable ideals, observable subgroups, unipotent relative modules, orbit closedness, and extendible rational characters. (hard)
  • Observation 7.10(3): observable but non-unipotent example: The affine Gm-action on A2 with weights 1 and minus 1, stable coordinate ideals, observable action and unipotent action predicates. (hard)
  • Theorem 7.11: simultaneous unipotency and linear reductivity: Separable regular affine actions, relative linear reductivity and unipotency, and closed-orbit algebraic isomorphisms with G (without a global product conclusion). (hard)

Selected references

  • Reductive and unipotent actions of affine groups, arXiv:1406.4446v1, https://arxiv.org/abs/1406.4446v1
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Quantum Information·Captain: Goku

Abelian von Neumann algebras, measure algebras and L^\infty-spacesResearch Paper

Motivation

We give a fresh account of the astonishing interplay between abelian von Neumann algebras, L^\infty-spaces and measure algebras, including an exposition of Maharam's theorem from the von Neumann algebra perspective.

This mission is the statement-level formalization of Abelian von Neumann algebras, measure algebras and L^\infty-spaces (arXiv:2108.06406v2). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

14 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Lemma 4.1: Existence of measure support (Lemma 4.1, PDF p. 12). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

55 results of the paper are not stated yet, because the library lacks an interface they need:

  • Theorem 2.1: Canonical multiplication representation and monotone convergence: Canonical complex L-infinity C*-algebra, characteristic projection measure, canonical L1 duality and L2 multiplication maps (hard)
  • Lemma 2.2: Measure-preserving transfer of dualizability: Canonical complex L-infinity C*-algebra, characteristic projection measure, canonical L1 duality and L2 multiplication maps (hard)
  • Theorem 2.3: Four equivalent descriptions of localizable measure spaces: Bounded L-infinity products, measured star decompositions and the canonical L1 duality interface (hard)
  • Lemma 3.1: Complete additivity is normality: Positive cone of actual L-infinity, extended-valued weights, integration descent, normality and complete additivity on that cone (hard)
  • Lemma 3.2: Measures and sigma-normal weights: Positive cone of actual L-infinity, extended-valued weights, integration descent, normality and complete additivity on that cone (hard)
  • Corollary 3.3: Integration is normal for localizable measures: Positive cone of actual L-infinity, extended-valued weights, integration descent, normality and complete additivity on that cone (hard)
  • Lemma 4.3: Support, complete additivity and relative semifiniteness: Positive cone of actual L-infinity, extended-valued weights, integration descent, normality and complete additivity on that cone (hard)
  • Theorem 4.4: Radon–Nikodym for localizable measures I: Positive cone of actual L-infinity, extended-valued weights, integration descent, normality and complete additivity on that cone (hard)
  • Corollary 4.5: Radon–Nikodym for localizable measures II: Positive cone of actual L-infinity, extended-valued weights, integration descent, normality and complete additivity on that cone (hard)
  • Corollary 4.6: Absolutely continuous measures give normal weights on a sigma-finite base: Positive cone of actual L-infinity, extended-valued weights, integration descent, normality and complete additivity on that cone (hard)
  • Theorem 4.7: Radon–Nikodym for Dedekind measures: Positive cone of actual L-infinity, extended-valued weights, integration descent, normality and complete additivity on that cone (hard)
  • Lemma 5.1: Monotone completeness and order separation in abelian W*-algebras: Abstract W*-algebra with a chosen Banach predual, induced weak-star topology and its spectral/projection order (hard)
  • Theorem 5.2: Six equivalent descriptions of localizability: Canonical L-infinity-to-B(L2) star representation, faithful von Neumann/masa range, chosen predual and RN-weight interfaces (hard)
  • Proposition 5.4: Stonean spectrum and projection density: Abstract W*-algebra with a chosen Banach predual, induced weak-star topology and its spectral/projection order (hard)
  • Theorem 7.15: Loomis–Sikorski representation: Concrete measurable-sets-mod-null Boolean quotient with measure and its actual isomorphisms (hard)
  • Proposition 7.16: Extending Boolean maps to star homomorphisms: Projection Boolean lattice of an abstract W*-algebra, canonical extension of Boolean maps to star homomorphisms, normality and trace comparison (hard)
  • Theorem 7.17: Localizable measure algebras have Radon realizations: Concrete measurable-sets-mod-null Boolean quotient with measure and its actual isomorphisms (hard)
  • Theorem 7.19: Dixmier characterization: Chosen Banach predual and isometric complex star realization of C(K) as an actual L-infinity algebra (hard)
  • Corollary 7.20: Abelian W*-algebras admit classical L-infinity realizations: Abstract W*-algebra, faithful normal semifinite trace, concrete normal realization and localizable L-infinity star realization (hard)
  • Theorem 7.21: Trace-preserving classical realization: State/trace-preserving classical L-infinity star realization with characteristic projections and genuine measure atomlessness (hard)
  • Theorem 7.22: Normal functionals and the unique predual: All Banach preduals with dual-identification compatibility and positive normal bounded functionals on abstract abelian W*-algebras (hard)
  • Proposition 7.23: Normality and trace preservation of extended Boolean maps: Projection Boolean lattice of an abstract W*-algebra, canonical extension of Boolean maps to star homomorphisms, normality and trace comparison (hard)
  • Theorem 7.24: Equivalence of measured Boolean and abelian von Neumann categories: Actual measured/unmeasured categories and the constructed projection functor, morphism restrictions and direct-sum comparison (hard)
  • Theorem 7.25: Equivalence without a chosen measure: Actual measured/unmeasured categories and the constructed projection functor, morphism restrictions and direct-sum comparison (hard)
  • Theorem 7.27: Concrete commutative von Neumann subalgebras: Embedded L-infinity for a sub-sigma-algebra, canonical weak-star topology and ambient order-closed projection operations (hard)
  • Corollary 7.28: Bade characterization: Concrete C*-generation, order-closed Boolean projection closure and generated von Neumann algebra in the same Hilbert representation (hard)
  • Corollary 7.29: Generators of an algebra and of its projection lattice: Concrete C*-generation, order-closed Boolean projection closure and generated von Neumann algebra in the same Hilbert representation (hard)
  • Theorem 8.1: L-infinity is W* without semifiniteness: L-infinity W*-structure, sum/supremum of actual normal positive functionals, trivialized measure on the constructed anti-semifinite part (hard)
  • Theorem 9.2: Homogeneous measure-algebra uniqueness: Exact Maharam cardinal for complete Boolean algebras and principal ideals, homogeneity, measured Boolean isomorphism (hard)
  • Lemma 9.3: Atomic–diffuse decomposition: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Proposition 9.4: Infinite diffuse trace splits into probability summands: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Lemma 9.6: Decomposability cardinal of a direct sum: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Theorem 9.8: Projection generators and countable dependence: Actual projection-generator cardinal, strong topology on Boolean projections and complete generated closure membership (hard)
  • Lemma 9.10: Homogeneous types are zero or infinite: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Lemma 9.11: Every projection contains a homogeneous part: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Lemma 9.12: Homogeneous decomposition: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Lemma 9.14: Disjoint homogeneous components sum to one: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Lemma 9.15: A sigma-finite component of infinite type is homogeneous: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Theorem 9.16: Finite-trace homogeneous uniqueness: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Theorem 9.17: Classification by magnitudes and weighted atom counts: Canonical concrete corners, bounded/measured direct products, projection generator and decomposability cardinals, homogeneous components and magnitude (hard)
  • Proposition 9.18: Bernoulli tensor building blocks: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Corollary 9.19: Normalized homogeneous algebras are Bernoulli tensor blocks: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Theorem 9.20: Maharam theorem for von Neumann measure algebras: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Corollary 9.21: Classification after forgetting trace: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Proposition 9.23: C*-tensor block is continuous functions on a Cantor cube: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Corollary 9.24: Von Neumann tensor block is the classical Bernoulli L-infinity: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Corollary 9.25: Countable Bernoulli block is the Lebesgue interval algebra: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Example 2.4: a semifinite nonlocalizable measure: Countable/co-countable counting L-infinity inclusion and measure-preserving conditional expectation (hard)
  • Remark 4.8(3): unproved set-theoretic equivalence: The paper-specific meaning of measurable cardinal in the announced equivalence (source-ambiguity)
  • Remark 7.26: projection functor preserves direct sums: Actual measured/unmeasured categories and the constructed projection functor, morphism restrictions and direct-sum comparison (hard)
  • Unnumbered Dedekind and Stonean characterization: Canonical L-infinity Gelfand spectrum, norm-preserving real/complex Banach injectivity, and concrete 0/infinity-measure realization (hard)
  • Example 8.2: normal integration does not imply W*: The specific Dixmier quotient of bounded Borel functions by the meager-support ideal, its norm/order and trivial integration weight (hard)
  • Remark 9.22: probability-algebra refinement: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Unnumbered extension after Corollary 9.25: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)
  • Remark 9.22: classical description of L-infinity star isomorphisms: Specified Bernoulli C*-inductive-limit block, incomplete tensor Hilbert space, GNS representation, von Neumann closure and product state; compatible measured direct sums (hard)

Selected references

  • Abelian von Neumann algebras, measure algebras and L^\infty-spaces, arXiv:2108.06406v2, https://arxiv.org/abs/2108.06406v2
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Quantum Information·Captain: Goku

Generalised Coupling and An Elementary Algorithm for the Quantum Schur TransformResearch Paper

Motivation

The quantum Schur transform is a fundamental building block that maps the computational basis to a coupled basis consisting of irreducible representations of the unitary and symmetric groups. Equivalently, it may be regarded as a change of basis from the computational basis to a simultaneous spin eigenbasis of Permutational Quantum Computing (PQC) [Quantum Inf. Comput., 10, 470-497 (2010)]. By adopting the latter perspective, we present a transparent algorithm for implementing the qubit quantum Schur transform which uses O(log⁡(n))O(\log(n))O(log(n)) ancillas and can be decomposed into a sequence of O(n3log⁡(n)log⁡(nϵ))O(n^3\log(n)\log(\frac{n}{\epsilon}))O(n3log(n)log(ϵn​)) Clifford + T gates, where ϵ\epsilonϵ is the accuracy of the algorithm in terms of the trace norm. We discuss the necessity for some applications of implementing this operation as a unitary rather than an isometry, as is often presented. By studying the associated Schur states, which consist of qubits coupled via Clebsch-Gordan coefficients, we introduce the notion of generally coupled quantum states. We present six conditions, which in different combinations ensure the efficient preparation of these states on a quantum computer or their classical simulability (in the sense of computational tractability). It is shown that Wigner 6-j symbols and SU(N) Clebsch-Gordan coefficients naturally fit our framework. Finally, we investigate unitary transformations which preserve the class of computationally tractable states.

This mission is the statement-level formalization of Generalised Coupling and An Elementary Algorithm for the Quantum Schur Transform (arXiv:2305.04069v3). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

5 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is General coupling preserves normalization and orthogonality in M (§IV.A, unnumbered: generally coupled states are normalized and M-orthogonal, PDF p. 13). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

17 results of the paper are not stated yet, because the library lacks an interface they need:

  • Abstract / §V / Appendix A: clean qubit Schur transform with logarithmic ancillas: The compared maps and nuclear-norm error after spectator padding (source-ambiguity)
  • §III.A, Algorithms 1–2: original clean Schur synthesis: Total padded local pre-mapping permutations and original clean source gate schedule (hard)
  • §III.B: PQC bases block diagonalize the two Schur–Weyl actions: All-tree SU(2) coupled basis with both irreducible Schur–Weyl actions (hard)
  • §III.C, Algorithms 3–4: general clean PQC synthesis: Complete general-tree CG blocks, total local ordering, routing and gate lists (hard)
  • §IV.B, unnumbered: GC1–GC6 imply computational tractability: Uniform exact amplitude-evaluation and exact classical-sampling model for GC3–GC6 (source-ambiguity)
  • §IV.C, unnumbered: efficient preparation without GC4: Computational model for coefficient-driven unitary synthesis and its target accuracy (source-ambiguity)
  • §IV.C, unnumbered: logarithmic ancillas for sequential individual-state preparation: Live-register schedule attached to sequential individual-state preparation (hard)
  • §IV.D, unnumbered: fixed-rank SU(N) coupling conditions and preparation: Phase-fixed GT CG coefficients and uniform exact evaluation/preparation semantics (source-ambiguity)
  • §IV.D, unnumbered: fixed-dimensional qudit extension of PQC circuits: All-label fixed-d qudit pre-mapping and clean SU(d)-CG gate schedule (hard)
  • §IV.E, Eqs.(52)–(57): Wigner coupling conditions and failure of GC4: Normalized finite Racah tensor with admissible triangular domains and uniform coefficient programs (hard)
  • §V, Eqs.(77)–(78): exact total qubit counts: Concrete original and compressed register-liveness traces whose maxima are counted (hard)
  • Lemma VI.2: one Hadamard preserves a CT family: Operational CT-family model with exact amplitudes and conditional sampling (source-ambiguity)
  • §VI, unnumbered: one arbitrary computable single-qubit gate preserves CT: Uniform computable one-qubit coefficient access in an exact CT-family model (source-ambiguity)
  • Claim A.1: O(n^3) two-level rotations for Schur transform: Concrete source-compatible sequential pre-map and clean two-level circuit target (hard)
  • Claim A.3: O(n^6) two-level rotations for general PQC unitary: Concrete arbitrary-tree source unitary and active two-level decomposition interface (hard)
  • Claim A.4: O(n^3) two-level rotations for individual PQC state: Concrete general PQC state amplitudes and their active two-level preparation interface (hard)
  • Appendix A, Table II: remaining Clifford+T bounds: Separate nuclear-norm circuit-map and prepared-state approximation conventions (source-ambiguity)

Selected references

  • Generalised Coupling and An Elementary Algorithm for the Quantum Schur Transform, arXiv:2305.04069v3, https://arxiv.org/abs/2305.04069v3
7 thms2 active usersReviewed
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Quantum Information·Captain: Goku

Efficient Clifford+T approximation of single-qubit operatorsResearch Paper

Motivation

We give an efficient randomized algorithm for approximating an arbitrary element of SU(2)SU(2)SU(2) by a product of Clifford+TTT operators, up to any given error threshold ϵ>0\epsilon>0ϵ>0. Under a mild hypothesis on the distribution of primes, the algorithm's expected runtime is polynomial in log⁡(1/ϵ)\log(1/\epsilon)log(1/ϵ). If the operator to be approximated is a zzz-rotation, the resulting gate sequence has TTT-count K+4log⁡2(1/ϵ)K+4\log_2(1/\epsilon)K+4log2​(1/ϵ), where KKK is approximately equal to 101010. We also prove a worst-case lower bound of K+4log⁡2(1/ϵ)K+4\log_2(1/\epsilon)K+4log2​(1/ϵ), where K=−9K=-9K=−9, so that our algorithm is within an additive constant of optimal for certain zzz-rotations. For an arbitrary member of SU(2)SU(2)SU(2), we achieve approximations with TTT-count K+12log⁡2(1/ϵ)K+12\log_2(1/\epsilon)K+12log2​(1/ϵ). By contrast, the Solovay-Kitaev algorithm achieves TTT-count O(log⁡c(1/ϵ))O(\log^c(1/\epsilon))O(logc(1/ϵ)), where ccc is approximately 3.973.973.97.

This mission is the statement-level formalization of Efficient Clifford+T approximation of single-qubit operators (arXiv:1212.6253v2). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

12 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Units and totally nonnegative unit squares (Lemma 10: units and totally nonnegative unit squares, PDF p. 4). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

9 results of the paper are not stated yet, because the library lacks an interface they need:

  • Abstract and §1: efficient ancilla-free Clifford+T approximation (seed): Uniform prime-candidate/seed law and a real-input access convention for the bundled efficient compiler. (source-ambiguity)
  • Theorem 12: relative norm equation and randomized construction: Binary-cost randomized norm-solver execution on coefficient-encoded Z[omega]. (hard)
  • Theorem 22: candidate count and efficient sampling: Full printed epsilon>0 candidate enumerator and efficient uniform sampler outside the intended small-error geometry. (source-ambiguity)
  • Algorithm 23: returned-circuit correctness: Successful finite execution trace joining candidate generation, norm-solver attempts, checked acceptance and exact gate synthesis. (hard)
  • §8.1: successful-output T-count bounds: Source exact-synthesis/normalization output interface with denominator exponent and actual returned-word T-count. (hard)
  • Remark 28: fixed Clifford+T conjugation: Uniform slope-four z-rotation approximation family before fixed-word conjugation. (source-ambiguity)
  • §8.2 and Hypothesis 29: expected runtime: Uniform pointwise probability law for Hypothesis 29 and source-specific bit-runtime input model. (source-ambiguity)
  • §8.3: seeding and T-count overhead: Specified angle-seed and bounded-T-count-word seed laws with successful-seed probability and restart cutoff. (source-ambiguity)
  • §10: additive overclocking claim: Quantified admissible additive decrement and successful-beta fraction for overclocked sampling. (source-ambiguity)

Selected references

  • Efficient Clifford+T approximation of single-qubit operators, arXiv:1212.6253v2, https://arxiv.org/abs/1212.6253v2
14 thms2 active usersReviewed
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Quantum Information·Captain: Goku

Entropic Fluctuations in Statistical Mechanics II. Quantum Dynamical SystemsResearch Paper

Motivation

The celebrated Evans-Searles, respectively Gallavotti-Cohen, fluctuation theorem concerns certain universal statistical features of the entropy production rate of a classical system in a transient, respectively steady, state. In this paper, we consider and compare several possible extensions of these fluctuation theorems to quantum systems. In addition to the direct two-time measurement approach whose discussion is based on (LMP 114:32 (2024)), we discuss a variant where measurements are performed indirectly on an auxiliary system called ancilla, and which allows to retrieve non-trivial statistical information using ancilla state tomography. We also show that modular theory provides a way to extend the classical notion of phase space contraction rate to the quantum domain, which leads to a third extension of the fluctuation theorems. We further discuss the quantum version of the principle of regular entropic fluctuations, introduced in the classical context in (Nonlinearity 24, 699 (2011)). Finally, we relate the statistical properties of these various notions of entropy production to spectral resonances of quantum transfer operators. The obtained results shed a new light on the nature of entropic fluctuations in quantum statistical mechanics.

This mission is the statement-level formalization of Entropic Fluctuations in Statistical Mechanics II. Quantum Dynamical Systems (arXiv:2409.15485v2). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

8 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Finite ancilla tomography recovers the entropic functional (finite-ancilla-tomography, PDF p. 13). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

28 results of the paper are not stated yet, because the library lacks an interface they need:

  • theorem-2.1: An unambiguous corrected source statement for theorem-2.1 (source-ambiguity)
  • continuous-2tm-extension: An unambiguous corrected source statement for continuous-2tm-extension (source-ambiguity)
  • proposition-2.2: The actual open-system Mat2(O) perturbed dynamics and Araki–Connes ancilla functional (hard)
  • proposition-2.3: Canonical modular spectral probability and extended Araki entropy (hard)
  • theorem-2.5: An unambiguous corrected source statement for theorem-2.5 (source-ambiguity)
  • proposition-2.6: The quantum 2tm law and its speed-t full/local LDP (hard)
  • weak-quantum-rigidity: Canonical extension of 2tm laws to singular NESS and reservoir modular averaging (hard)
  • proposition-2.9: Unique O-valued analytic Connes extension and the NESS characteristic law for every real t (hard)
  • ancilla-analytic-continuation: Conjugated-parameter cocycle product evaluated in an arbitrary singular algebraic state (hard)
  • proposition-2.11: The analytic perturbation orbit and constructed Dyson cocycle of an open system (hard)
  • proposition-2.12: Natural-cone comparison for the actual evolved reference and ancilla functional (hard)
  • two-time-comparison: Modular Cesaro definition of 2tm statistics on evolved reference states (hard)
  • ancilla-exchange-of-limits: Actual real-alpha ancilla functionals in a general NESS open system (hard)
  • classical-exchange-of-limits: An unambiguous corrected source statement for classical-exchange-of-limits (source-ambiguity)
  • classical-discrete-analogue: An unambiguous corrected source statement for classical-discrete-analogue (source-ambiguity)
  • proposition-2.17: Relative Tomita construction and time-covariant Connes cocycle in the GNS algebra (hard)
  • theorem-2.18: Unbounded modular logarithms and bounded relative Hamiltonian derived from Qu1; canonical quantum Cesaro NESS production positivity (p.26 Remark 1) (hard)
  • open-system-entropy-observable: Unbounded logarithmic modular perturbation formula and modular derivation domain (hard)
  • reference-functional-identity: The three canonically defined algebraic quantum functionals (hard)
  • proposition-3.1: Canonical standard Liouvillean and quantum transfer group from the actual Connes family (hard)
  • ancilla-generator-similarity: Modular spectral powers acting on the actual closed ancilla generator domain (hard)
  • proposition-3.2: The distinct physical quantum transfer groups and finite-time functionals (hard)
  • theorem-4.3: Canonical 1/2-Liouvillean weighted resolvent, D inverse domain, and algebraic NESS state (hard)
  • theorem-4.4: Deform2 on actual quantum groups, singular NESS evaluations, and modular-power domains (hard)
  • theorem-4.5: Deform3 and the physical cumulants of the genuine modular resonance family (hard)
  • alternative-ancilla-resonance: An unambiguous corrected source statement for alternative-ancilla-resonance (source-ambiguity)
  • proposition-4.6: Bounded weighted composition on L2 classes for a non-measure-preserving flow (hard)
  • classical-transfer-representations: The L2 weighted transfer group and its actual generator, not just its scalar integral (hard)

Selected references

  • Entropic Fluctuations in Statistical Mechanics II. Quantum Dynamical Systems, arXiv:2409.15485v2, https://arxiv.org/abs/2409.15485v2
10 thms2 active usersReviewed
🏆Completed
Quantum Information·Captain: Goku

Sufficiency in quantum statistical inferenceResearch Paper

Motivation

This paper attempts to develop a theory of sufficiency in the setting of non-commutative algebras parallel to the ideas in classical mathematical statistics. Sufficiency of a coarse-graining means that all information is extracted about the mutual relation of a given family of states. In the paper sufficient coarse-grainings are characterized in several equivalent ways and the non-commutative analogue of the factorization theorem is obtained. Among the applications the equality case for the strong subadditivity of the von Neumann entropy, the Imoto-Koashi theorem and exponential families are treated. The setting of the paper allows the underlying Hilbert space to be infinite dimensional.

This mission is the statement-level formalization of Sufficiency in quantum statistical inference (arXiv:math-ph/0412093v1). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

10 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Lemma 1: countable common-null-set subfamily (Lemma 1: countable common-null-set subfamily, PDF p. 4). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

29 results of the paper are not stated yet, because the library lacks an interface they need:

  • Unnumbered: classical minimal sufficient sigma-algebra: An additional reference-measure condition for the claimed minimal sufficient density algebra (source-ambiguity)
  • Theorem 1: five criteria for a sufficient subalgebra: Natural-cone PA, Connes cocycles, and the computed generalized conditional expectation (hard)
  • Lemma 3: monotonicity of canonical cone overlap: Positive trace-class square-root realization and trace of the product of Hilbert-Schmidt square roots (hard)
  • Lemma 4 and its concluding converse: equality in overlap monotonicity: Support-zero spectral imaginary powers, the actual Heisenberg adjoint, and canonical cone overlap (hard)
  • Proposition 3: classical factorization criterion: Common-reference almost-everywhere factorization (source-ambiguity)
  • Unnumbered: sufficiency survives support compression: Computed support-corner algebras and their normal 2-positive compression/extension maps (hard)
  • Unnumbered: modular characterization of recovery fixed algebras: Constructed generalized dual, modular group, and fixed-point star algebras (hard)
  • Theorem 2: six equivalent sufficiency and canonical-recovery conditions: Canonical generalized dual and computed multiplicative-domain image and recovery fixed algebras (hard)
  • Remark after Theorem 2: relative-entropy equality iff canonical recovery: Computed canonical normal recovery channel for finite extended-relative-entropy equality (hard)
  • Unnumbered: noncommuting experiments have no sufficient measurement: The computed minimal sufficient algebra and general commutative-domain sufficient measurements (hard)
  • Equation (16): modular-invariant reference-state factorization: Canonical type-I trace densities and an affiliated positive central correction (hard)
  • Theorem 3: noncommutative factorization criterion (seed main result): Canonical modular-invariant density factorization with a computed central affiliated factor (hard)
  • Unnumbered: cocycle-generated minimal sufficient algebra: Cocycle-generated minimal sufficient algebra, modular invariance, and its canonical S-factorization (hard)
  • Theorem 4: maximality of canonical S-factorization: The canonical S-factors and a possibly unbounded affiliated maximality correction R0 (hard)
  • Unnumbered: canonical direct-sum tensor S-decomposition and uniqueness: A canonical countable orthogonal completed-Hilbert tensor decomposition and compatible uniqueness relation (hard)
  • Theorem 5: sufficient channel block structure and induced S-decomposition: Canonical source and target S-blocks, their normal right maps, and the extended positive-weight dual (hard)
  • Corollary 1: Kraus block criterion for finite-dimensional sufficiency: The actual finite-dimensional canonical S-decomposition and operator transport into its tensor blocks (hard)
  • Corollary 2: claimed nondisturbance Kraus commutation criterion: A correct condition for right-factor stationary states (source-ambiguity)
  • Example 4: claimed local unique entropy-constrained exponential fit: Nondegeneracy for the local moment map and multiplier uniqueness (source-ambiguity)
  • Equations (23)–(25): variationally perturbed state, Gibbs formula and support: Normalized modularly perturbed states and their unnormalized positive vector functionals (hard)
  • Theorem 6 [15]: sufficiency criteria for an exponential family: Normalized modularly perturbed states and their unnormalized positive vector functionals (hard)
  • Unnumbered: minimum free energy, analyticity and derivative identity: Normalized modularly perturbed states and their unnormalized positive vector functionals (hard)
  • Theorem 7: sufficient coarse-grainings of exponential families: Normalized modularly perturbed states and their unnormalized positive vector functionals (hard)
  • Remark after Theorem 7: matrix logarithmic sufficiency criterion: A computed faithful quantum exponential family and correctly typed logarithmic trace-dual criterion (hard)
  • Corollary 3: commutative sufficient subalgebras of exponential families: Normalized modularly perturbed states and their unnormalized positive vector functionals (hard)
  • Theorem 8: countable Markov block structure at SSA equality: A countable completed-Hilbert Markov block decomposition of B and transported density equality (hard)
  • Unnumbered: pure-state SSA equality yields a single product block: Support-level tensor splitting of a purification, with actual transported product vectors (hard)
  • Equation (30): claimed non-atomic direct-integral normal density: A valid normal-state model for a non-atomic direct integral (source-ambiguity)
  • Appendix: support-corner dual and generalized conditional expectation (31): Canonical support-corner generalized dual and its normal predual, including the CP specialization (hard)

Selected references

  • Sufficiency in quantum statistical inference, arXiv:math-ph/0412093v1, https://arxiv.org/abs/math-ph/0412093v1
14 thms2 active usersReviewed
🏆Completed
Quantum Information·Captain: Goku

Quantum mechanics in phase space: The Schr"odinger and the Moyal representationsResearch Paper

Motivation

We present a phase space formulation of quantum mechanics in the Schr"odinger representation and derive the associated Weyl pseudo-differential calculus. We prove that the resulting theory is unitarily equivalent to the standard "configuration space" formulation and show that it allows for a uniform treatment of both pure and mixed quantum states. In the second part of the paper we determine the unitary transformation (and its infinitesimal generator) that maps the phase space Schr"odinger representation into another (called Moyal) representation, where the wave function is the cross-Wigner function familiar from deformation quantization. Some features of this representation are studied, namely the associated pseudo-differential calculus and the main spectral and dynamical results. Finally, the relation with deformation quantization is discussed.

This mission is the statement-level formalization of Quantum mechanics in phase space: The Schr"odinger and the Moyal representations (arXiv:1209.1850v1). Every numbered result of the paper was scanned. The results stated here are open sorry goals, ready for solvers to prove.

What is formalized

16 statements of the paper are in Lean 4 (Mathlib 0df444a). The goal theorem is Unitary equivalence with the window subspace (Abstract main result: unitary equivalence with configuration space, PDF p. 1). The milestones are the other numbered results, in the paper's order, each quoting the paper.

The paper's own objects are the definition item of this mission. Objects shared across papers come from the prove2quantum library and from earlier papers' missions, listed as references.

Provenance: prove2quantum's pipeline drafted each statement (gpt-6.1-sol on Stanford's Sherlock cluster). A blind read-back, written without seeing the paper, is attached to every item. An independent moderator compared each statement against the paper and passed the whole paper without edits.

Deferred results

9 results of the paper are not stated yet, because the library lacks an interface they need:

  • §3, unnumbered: mixed-state eigenspace and convex probability claim: Consistent mixed-state observable and projection conventions (source-ambiguity)
  • §3, unnumbered: projection and collapse probability claim: Consistent mixed-state observable and projection conventions (source-ambiguity)
  • Theorem 4.6: Continuous exterior distribution product, partial-Fourier Weyl kernel, and independent double-phase symbol transform (hard)
  • Theorem 4.7: Schwartz tensor/contraction maps and their correct distribution transposes (hard)
  • Corollary 4.8: Kernel-defined Weyl operators on Schwartz inputs and nonzero window-contraction interface (hard)
  • Theorem 5.7: Independent Weyl-symbol submersion pullback and Schwartz/distribution U conjugation (hard)
  • Corollary 5.10: Distribution-valued time derivative and independent star-product Hamiltonian action (hard)
  • Corollary 5.11: Generic tempered Weyl/star operators and Schwartz-domain U/window intertwiners (hard)
  • Corollary 5.9: Independently constructed tempered-symbol Moyal product and Moyal operator (hard)

Selected references

  • Quantum mechanics in phase space: The Schr"odinger and the Moyal representations, arXiv:1209.1850v1, https://arxiv.org/abs/1209.1850v1
20 thms2 active usersReviewed
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Convex OptimizationOperations ResearchOptimization·Captain: Shuze Chen

Interior Point Algorithms VII: The Homogeneous Model for Monotone Complementarity and Its Central StepTextbook

Motivation

The monotone complementarity problem (MCP) asks, for a continuous monotone map fff on the positive orthant, for a pair (x,s)(x,s)(x,s) with s=f(x)s=f(x)s=f(x), x≥0x\ge0x≥0, s≥0s\ge0s≥0 and xjsj=0x_js_j=0xj​sj​=0 for every jjj. It contains linear programming, convex quadratic programming, the monotone linear complementarity problem and the Karush–Kuhn–Tucker conditions of smooth convex programs. Interior-point methods solve it in polynomially many Newton-type steps when a strictly positive, well-centered starting point is known, but in general no such point is known, and the problem may be infeasible or feasible without any positive feasible point.

Andersen and Ye (Mathematical Programming 84, 1999) extended the homogeneous self-dual embedding of linear programming to the nonlinear MCP: one embeds the problem into a homogeneous problem that always has a trivial interior starting point, solves it by a path-following method, and reads off either a solution or a certificate of infeasibility. Section 8.4 of Ye, Interior Point Algorithms: Theory and Analysis (Wiley 1997, doi:10.1002/9781118032701) presents this construction, the convexity property of the residual set it rests on, and the analysis of one step of the algorithm. This mission formalizes that section.

Setting

Vectors are compared componentwise; eee is the all-ones vector, X=diag⁡(x)X=\operatorname{diag}(x)X=diag(x), S=diag⁡(s)S=\operatorname{diag}(s)S=diag(s), ∥⋅∥\|\cdot\|∥⋅∥ the Euclidean norm and R++N={x∈RN:x>0}\mathbb R^N_{++}=\{x\in\mathbb R^N: x>0\}R++N​={x∈RN:x>0} the open orthant. A map f:R++n→Rnf:\mathbb R^n_{++}\to\mathbb R^nf:R++n​→Rn is monotone if (x1−x2)T(f(x1)−f(x2))≥0(x^1-x^2)^T(f(x^1)-f(x^2))\ge0(x1−x2)T(f(x1)−f(x2))≥0 for all x1,x2>0x^1,x^2>0x1,x2>0; ∇f(x)\nabla f(x)∇f(x) denotes its Jacobian.

The residual set is R++={s−f(x):x>0, s>0}R_{++}=\{s-f(x): x>0,\ s>0\}R++​={s−f(x):x>0, s>0}, and for r∈Rnr\in\mathbb R^nr∈Rn the slice S++(r)={(x,s)∈R++2n:s=f(x)+r}S_{++}(r)=\{(x,s)\in\mathbb R^{2n}_{++}: s=f(x)+r\}S++​(r)={(x,s)∈R++2n​:s=f(x)+r}.

The homogeneous map on R++n+1\mathbb R^{n+1}_{++}R++n+1​, with points written (x;τ)(x;\tau)(x;τ), is

ψ(x,τ)=(τf(x/τ)−xTf(x/τ)).\psi(x,\tau)=\begin{pmatrix}\tau f(x/\tau)\\-x^Tf(x/\tau)\end{pmatrix}.ψ(x,τ)=(τf(x/τ)−xTf(x/τ)​).

The homogeneous model (HMCP) asks for (x,τ,s,κ)≥0(x,\tau,s,\kappa)\ge0(x,τ,s,κ)≥0 with (s;κ)=ψ(x,τ)(s;\kappa)=\psi(x,\tau)(s;κ)=ψ(x,τ) minimizing xTs+τκx^Ts+\tau\kappaxTs+τκ.

A map ggg on R++N\mathbb R^N_{++}R++N​ is scaled Lipschitz with a monotone increasing v:(0,1)→(1,∞)v:(0,1)\to(1,\infty)v:(0,1)→(1,∞) if

∥X(g(x+d)−g(x)−∇g(x)d)∥1≤v(α) dT∇g(x)dwhenever x>0, ∥X−1d∥∞≤α<1.\|X(g(x+d)-g(x)-\nabla g(x)d)\|_1\le v(\alpha)\,d^T\nabla g(x)d\quad\text{whenever }x>0,\ \|X^{-1}d\|_\infty\le\alpha<1.∥X(g(x+d)−g(x)−∇g(x)d)∥1​≤v(α)dT∇g(x)dwhenever x>0, ∥X−1d∥∞​≤α<1.

The algorithm works in Rn+1\mathbb R^{n+1}Rn+1 with x:=(x;τ)x:=(x;\tau)x:=(x;τ) and s:=(s;κ)s:=(s;\kappa)s:=(s;κ). At an iterate (xk,sk)>0(x^k,s^k)>0(xk,sk)>0 let rk=sk−ψ(xk)r^k=s^k-\psi(x^k)rk=sk−ψ(xk) and μk=(xk)Tsk/(n+1)\mu^k=(x^k)^Ts^k/(n+1)μk=(xk)Tsk/(n+1). The direction (dx,ds)(d_x,d_s)(dx​,ds​) solves

ds−∇ψ(xk)dx=−ηrk,Xkds+Skdx=γμke−Xksk,d_s-\nabla\psi(x^k)d_x=-\eta r^k,\qquad X^kd_s+S^kd_x=\gamma\mu^ke-X^ks^k,ds​−∇ψ(xk)dx​=−ηrk,Xkds​+Skdx​=γμke−Xksk,

and for a step size α\alphaα the new iterate is x+=xk+αdxx^+=x^k+\alpha d_xx+=xk+αdx​ and s+=sk+αds+ψ(x+)−ψ(xk)−α∇ψ(xk)dxs^+=s^k+\alpha d_s+\psi(x^+)-\psi(x^k)-\alpha\nabla\psi(x^k)d_xs+=sk+αds​+ψ(x+)−ψ(xk)−α∇ψ(xk)dx​.

Formalization targets

Goal: one step of the homogeneous algorithm stays centered (Theorem 8.29)

Assume ψ\psiψ is scaled Lipschitz with vψv_\psivψ​, and let

β=13+4vψ(2/2),η=βn+1,γ=1−η,α=1.\beta=\frac1{3+4v_\psi(\sqrt2/2)},\qquad\eta=\frac\beta{\sqrt{n+1}},\qquad\gamma=1-\eta,\qquad\alpha=1.β=3+4vψ​(2​/2)1​,η=n+1​β​,γ=1−η,α=1.

If (xk,sk)>0(x^k,s^k)>0(xk,sk)>0 and ∥Xksk−μke∥≤βμk\|X^ks^k-\mu^ke\|\le\beta\mu^k∥Xksk−μke∥≤βμk, then β≤1/3\beta\le1/3β≤1/3 and

x+>0,s+=ψ(x+)+(1−η)rk>0,∥X+s+−μ+e∥≤βμ+,μ+=(x+)Ts+n+1.x^+>0,\qquad s^+=\psi(x^+)+(1-\eta)r^k>0,\qquad\|X^+s^+-\mu^+e\|\le\beta\mu^+,\quad\mu^+=\frac{(x^+)^Ts^+}{n+1}.x+>0,s+=ψ(x+)+(1−η)rk>0,∥X+s+−μ+e∥≤βμ+,μ+=n+1(x+)Ts+​.

Milestones

  • Lemma 8.20: for (xi,si)∈S++(θir)(x^i,s^i)\in S_{++}(\theta^ir)(xi,si)∈S++​(θir), (θ2−θ1)rT(x2−x1)≤(x2−x1)T(s2−s1)(\theta^2-\theta^1)r^T(x^2-x^1)\le(x^2-x^1)^T(s^2-s^1)(θ2−θ1)rT(x2−x1)≤(x2−x1)T(s2−s1).
  • Lemma 8.21: for θ0≤θ1\theta^0\le\theta^1θ0≤θ1 with nonempty slices, ⋃θ∈[θ0,θ1]{(x,y)∈S++(θr):xTy≤δ}\bigcup_{\theta\in[\theta^0,\theta^1]}\{(x,y)\in S_{++}(\theta r): x^Ty\le\delta\}⋃θ∈[θ0,θ1]​{(x,y)∈S++​(θr):xTy≤δ} is bounded.
  • Theorem 8.23: R++R_{++}R++​ is open and convex.
  • Lemma 8.24: if ∇f\nabla f∇f is positive semi-definite, so is ∇ψ\nabla\psi∇ψ.
  • Theorem 8.25 (i): ψ\psiψ is continuous and homogeneous of degree 1, (x;τ)Tψ(x,τ)=0(x;\tau)^T\psi(x,\tau)=0(x;τ)Tψ(x,τ)=0 and (x;τ)T∇ψ(x,τ)=−ψ(x,τ)T(x;\tau)^T\nabla\psi(x,\tau)=-\psi(x,\tau)^T(x;τ)T∇ψ(x,τ)=−ψ(x,τ)T.
  • Theorem 8.25 (ii): ψ\psiψ is continuous and monotone when fff is.
  • Lemma 8.27: dxTds=dxT∇ψ(xk)dx+η(1−η−γ)(n+1)μkd_x^Td_s=d_x^T\nabla\psi(x^k)d_x+\eta(1-\eta-\gamma)(n+1)\mu^kdxT​ds​=dxT​∇ψ(xk)dx​+η(1−η−γ)(n+1)μk.
  • Lemma 8.28: r+=(1−αη)rkr^+=(1-\alpha\eta)r^kr+=(1−αη)rk and (x+)Ts+=(xk)Tsk(1−α(1−γ))+α2η(1−η−γ)(n+1)μk(x^+)^Ts^+=(x^k)^Ts^k(1-\alpha(1-\gamma))+\alpha^2\eta(1-\eta-\gamma)(n+1)\mu^k(x+)Ts+=(xk)Tsk(1−α(1−γ))+α2η(1−η−γ)(n+1)μk.

Stronger: monotonicity of fff is not needed

The goal remains true when fff is only assumed differentiable on R++n\mathbb R^n_{++}R++n​ and ψ\psiψ scaled Lipschitz; this variant is posed beside the goal (see Formalization scope).

Significance

Theorem 8.29 is the one-step invariant behind the O(nlog⁡(1/ϵ))O(\sqrt n\log(1/\epsilon))O(n​log(1/ϵ)) iteration bound of the homogeneous algorithm: combined with Lemma 8.28 it shows that the infeasibility residual and the complementarity gap shrink by the same factor 1−β/n+11-\beta/\sqrt{n+1}1−β/n+1​ per step while the iterates stay in a fixed neighborhood of the central path. Together with Theorem 8.25 (v)–(vi), which identifies solutions and infeasibility certificates of the MCP from maximal complementary solutions of the homogeneous model, this yields an algorithm that needs no feasible starting point and detects infeasibility. The convexity of R++R_{++}R++​ (Theorem 8.23) is the structural fact that makes the homotopy path of the homogeneous model well defined.

These results are proved in the book (with Theorem 8.19, which Theorem 8.23 uses, left as an exercise). None of them is formalized on the platform: the existing Andersen–Ye material covers only the linear complementarity case, as a set-valued operator, and no statement about scaled Lipschitz maps or the nonlinear homogeneous map exists. A completed development gives a machine-checked analysis of an interior-point step for a nonlinear problem, with every constant explicit.

Difficulty

The algebra of Lemmas 8.27 and 8.28 depends on the identities xTψ(x)=0x^T\psi(x)=0xTψ(x)=0 and xT∇ψ(x)=−ψ(x)Tx^T\nabla\psi(x)=-\psi(x)^TxT∇ψ(x)=−ψ(x)T, which require differentiating a map defined through x/τx/\taux/τ; working with ∇ψ\nabla\psi∇ψ as a Fréchet derivative rather than a hand-written block matrix is part of the work. In Theorem 8.29 the nonlinearity enters only through the scaled Lipschitz bound, which applies only when ∥X−1dx∥∞<1\|X^{-1}d_x\|_\infty<1∥X−1dx​∥∞​<1, so the step must first be shown to stay well inside the orthant before the neighborhood estimate can be closed; the norms involved (ℓ1\ell_1ℓ1​, ℓ2\ell_2ℓ2​, ℓ∞\ell_\inftyℓ∞​ and the scaled norms ∥D−1dx∥\|D^{-1}d_x\|∥D−1dx​∥, ∥Dds∥\|Dd_s\|∥Dds​∥) must be chained in the right order. Theorem 8.23 is of a different kind: its proof passes through the homeomorphism of Theorem 8.19, whose proof the book leaves as an exercise, and a compactness argument along a homotopy.

Two points in the printed text need care. The block matrix (8.24) displayed for ∇ψ\nabla\psi∇ψ has a spurious transpose in its lower-left block; with the printed matrix Lemma 8.24 would fail for nonsymmetric ∇f\nabla f∇f, and the statements here use the true Jacobian. The printed proof of Theorem 8.25 (iii) (the scaled Lipschitz constant of ψ\psiψ from that of fff) establishes the bound with vf(2α/(1−α))v_f(2\alpha/(1-\alpha))vf​(2α/(1−α)), not the stated vf(2α/(1+α))v_f(2\alpha/(1+\alpha))vf​(2α/(1+α)), and only for α<1/3\alpha<1/3α<1/3; that part is not in this mission, and Theorem 8.29 assumes the scaled Lipschitz property of ψ\psiψ directly, as the book does.

Formalization scope

Vectors are Fin N → ℝ, a point (x;τ)(x;\tau)(x;τ) of Rn+1\mathbb R^{n+1}Rn+1 is z : Fin (n+1) → ℝ with Fin.init z =x=x=x and z (Fin.last n) =τ=\tau=τ, and every Jacobian is Mathlib's fderiv. The standing assumptions of §8.4 are explicit hypotheses of every theorem: fff continuous and monotone on the open orthant R++n\mathbb R^n_{++}R++n​, and differentiable there wherever ∇f\nabla f∇f or ∇ψ\nabla\psi∇ψ appears (the section assumes this silently). In the scaled Lipschitz condition, "monotone increasing" is read as nondecreasing on (0,1)(0,1)(0,1) and α\alphaα ranges over (0,1)(0,1)(0,1). The direction is any solution of (8.42)–(8.43). The new dual iterate is defined by the first line of (8.47); its closed form is a conclusion. The book writes no O(⋅)O(\cdot)O(⋅) in the statements of this mission: β\betaβ, η\etaη, γ\gammaγ and α=1\alpha=1α=1 are the book's explicit constants. ψ\psiψ is a total function in Lean (division by τ=0\tau=0τ=0 returns 000), but every statement evaluates it and its derivative only at τ>0\tau>0τ>0.

A trivializing formalization is excluded: the scaled Lipschitz hypothesis is not vacuous (it holds, for instance, for f≡0f\equiv0f≡0 with any admissible vvv), the direction is not assumed to have any property beyond (8.42)–(8.43), and the monotonicity of ψ\psiψ and the identities (8.41) are proved milestones, not hypotheses of the goal.

Beyond the paper. One variant is posed: central_step_strong (strengthened) drops the continuity and monotonicity of fff from Theorem 8.29, because the scaled Lipschitz condition already forces dT∇ψ(x)d≥0d^T\nabla\psi(x)d\ge0dT∇ψ(x)d≥0, the only use the proof makes of monotonicity.

The development needs Fréchet derivatives of compositions with x↦x/τx\mapsto x/\taux↦x/τ, finite-dimensional norm inequalities, and, for Theorem 8.23, invariance of domain or a direct proof of Theorem 8.19. The definitions (monotone maps on the orthant, the scaled Lipschitz condition, the homogeneous map) are reusable for the remaining parts of Theorems 8.25 and 8.26. Contributions of a proof of Theorem 8.19, and of the faithful Theorem 8.25 (iii), are welcome.

Selected references

  • Y. Ye, Interior Point Algorithms: Theory and Analysis, Wiley-Interscience, 1997, §8.4. doi:10.1002/9781118032701
  • E. D. Andersen and Y. Ye, On a homogeneous algorithm for the monotone complementarity problem, Mathematical Programming 84 (1999), 375–399.
  • O. Güler, Existence of interior points and interior paths in nonlinear monotone complementarity problems, Mathematics of Operations Research 18 (1993), 128–147. doi:10.1287/moor.18.1.128
  • Y. Ye, M. J. Todd and S. Mizuno, An O(nL)O(\sqrt nL)O(n​L)-iteration homogeneous and self-dual linear programming algorithm, Mathematics of Operations Research 19 (1994), 53–67. doi:10.1287/moor.19.1.53
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Interior Point Algorithms VI: Predictor Directions Are O(μ) on the Optimal PartitionTextbook

Why the tail of an interior-point method matters

Primal-dual interior-point methods for linear programming come with two kinds of guarantee. The worst-case one is a bound of O(nlog⁡(R/ϵ))O(\sqrt n\log(R/\epsilon))O(n​log(R/ϵ)) iterations to reduce the duality gap below ϵ\epsilonϵ. The asymptotic one says what happens to the duality gap near the end of the run, where practical codes spend their final iterations. For the predictor-corrector method of Mizuno, Todd and Ye (1993), Ye, Güler, Tapia and Zhang (1993) showed that the same algorithm that achieves the best known worst-case bound also drives the duality gap to zero quadratically. The proof needs neither nondegeneracy nor convergence of the iterates. Chapter 7 of Yinyu Ye's Interior Point Algorithms: Theory and Analysis (Wiley 1997, DOI 10.1002/9781118032701) gives a self-contained account. This mission formalizes its LP core.

Timeline, as attributed in the chapter's notes (§7.5): Zhang, Tapia and Dennis first obtained superlinear convergence of a primal-dual method assuming the iterates converge, and quadratic convergence assuming nondegeneracy. Quadratic convergence for general LP, with neither assumption, was first established by Ye, Güler, Tapia and Zhang, and independently by Mehrotra and by Tsuchiya. The monotone LCP version (§7.3) is due to Ye and Anstreicher, who also showed that the predictor step cannot converge superlinearly when no strictly complementary solution exists (Proposition 7.13). Monteiro and Wright extended that negative result to Newton-like methods in general.

Setting

Fix a real m×nm\times nm×n matrix AAA of rank mmm, b∈Rmb\in\mathbb R^mb∈Rm and c∈Rnc\in\mathbb R^nc∈Rn. Consider the standard-form pair

(LP) min⁡{cTx:Ax=b, x≥0},(LD) max⁡{bTy:ATy+s=c, s≥0}.(\mathrm{LP})\ \min\{c^Tx: Ax=b,\ x\ge0\},\qquad(\mathrm{LD})\ \max\{b^Ty: A^Ty+s=c,\ s\ge0\}.(LP) min{cTx:Ax=b, x≥0},(LD) max{bTy:ATy+s=c, s≥0}.

An interior point is a triple (x,y,s)(x,y,s)(x,y,s) with Ax=bAx=bAx=b, x>0x>0x>0, ATy+s=cA^Ty+s=cATy+s=c, s>0s>0s>0. Write X=diag⁡(x)X=\operatorname{diag}(x)X=diag(x), S=diag⁡(s)S=\operatorname{diag}(s)S=diag(s), eee for the all-ones vector and μ=xTs/n\mu=x^Ts/nμ=xTs/n. The neighbourhood of the central path is

N2(η)={(x,y,s) interior: ∥Xs−μe∥≤ημ}.\mathcal N_2(\eta)=\{(x,y,s)\text{ interior}:\ \|Xs-\mu e\|\le\eta\mu\}.N2​(η)={(x,y,s) interior: ∥Xs−μe∥≤ημ}.

The strict complementarity partition (P,Z)=(P∗,Z∗)(P,Z)=(P^*,Z^*)(P,Z)=(P∗,Z∗) collects the indices on which some optimal x∗x^*x∗ is positive (PPP) and those on which some optimal slack s∗s^*s∗ is positive (ZZZ). The predictor direction at (x,s)(x,s)(x,s) is the solution of

Xds+Sdx=−Xs,Adx=0,−ATdy−ds=0.(7.4)Xd_s+Sd_x=-Xs,\qquad Ad_x=0,\qquad -A^Td_y-d_s=0. \tag{7.4}Xds​+Sdx​=−Xs,Adx​=0,−ATdy​−ds​=0.(7.4)

It is the case γ=0\gamma=0γ=0 of the direction d(x,s,γ)d(x,s,\gamma)d(x,s,γ) defined by Sdx+Xds=γμe−XsSd_x+Xd_s=\gamma\mu e-XsSdx​+Xds​=γμe−Xs, Adx=0Ad_x=0Adx​=0, −ATdy−ds=0-A^Td_y-d_s=0−ATdy​−ds​=0. The scaling is D=X1/2S−1/2D=X^{1/2}S^{-1/2}D=X1/2S−1/2, and ∥vS∥\|v_S\|∥vS​∥ denotes the Euclidean norm of the subvector of vvv indexed by SSS. Algorithm 4.5 (predictor-corrector, η=1/4\eta=1/4η=1/4) starts in N2(1/4)\mathcal N_2(1/4)N2​(1/4). At each iteration it takes the largest step θk\theta^kθk along d(xk,sk,0)d(x^k,s^k,0)d(xk,sk,0) that keeps the iterate in N2(1/2)\mathcal N_2(1/2)N2​(1/2), then a full step along d(⋅,⋅,1)d(\cdot,\cdot,1)d(⋅,⋅,1) back towards the central path.

The order of convergence of a sequence rk→0r^k\to0rk→0 (Definition 1.1) is the supremum of the p≥0p\ge0p≥0 with lim sup⁡k∣rk+1∣/∣rk∣p<∞\limsup_k|r^{k+1}|/|r^k|^p<\inftylimsupk​∣rk+1∣/∣rk∣p<∞. Quadratic convergence (Definition 1.2) means lim sup⁡k∣rk+1∣/∣rk∣2<∞\limsup_k|r^{k+1}|/|r^k|^2<\inftylimsupk​∣rk+1∣/∣rk∣2<∞.

Formalization targets

Goal: Theorem 7.5

For every μ0>0\mu^0>0μ0>0 there is a constant C>0C>0C>0, depending only on (A,b,c)(A,b,c)(A,b,c) and μ0\mu^0μ0, such that at every (x,y,s)∈N2(1/4)(x,y,s)\in\mathcal N_2(1/4)(x,y,s)∈N2​(1/4) with μ≤μ0\mu\le\mu^0μ≤μ0 the predictor direction satisfies

∥(dx)P∥≤Cμand∥(ds)Z∥≤Cμ.\|(d_x)_P\|\le C\mu\qquad\text{and}\qquad\|(d_s)_Z\|\le C\mu .∥(dx​)P​∥≤Cμand∥(ds​)Z​∥≤Cμ.

Milestones

  • Lemma 7.2. If Xds+Sdx=−XsXd_s+Sd_x=-XsXds​+Sdx​=−Xs and dxTds≥0d_x^Td_s\ge0dxT​ds​≥0, then ∥D−1dx∥2+∥Dds∥2≤xTs\|D^{-1}d_x\|^2+\|Dd_s\|^2\le x^Ts∥D−1dx​∥2+∥Dds​∥2≤xTs.
  • Relation (7.6). There is ξ∈(0,1)\xi\in(0,1)ξ∈(0,1), independent of the point, with ξ≤xj≤1/ξ\xi\le x_j\le1/\xiξ≤xj​≤1/ξ on PPP and ξ≤sj≤1/ξ\xi\le s_j\le1/\xiξ≤sj​≤1/ξ on ZZZ throughout N2(1/4)∩{μ≤μ0}\mathcal N_2(1/4)\cap\{\mu\le\mu^0\}N2​(1/4)∩{μ≤μ0}.
  • Lemma 7.3. ∥(dx)Z∥≤Cμ\|(d_x)_Z\|\le C\mu∥(dx​)Z​∥≤Cμ and ∥(ds)P∥≤Cμ\|(d_s)_P\|\le C\mu∥(ds​)P​∥≤Cμ, with CCC uniform as in the goal.
  • Lemma 7.4. (dx)P(d_x)_P(dx​)P​ is the unique minimizer of 12∥DP−1u∥2\tfrac12\|D_P^{-1}u\|^221​∥DP−1​u∥2 subject to APu=−AZ(dx)ZA_Pu=-A_Z(d_x)_ZAP​u=−AZ​(dx​)Z​. dyd_ydy​ minimizes 12∥DZAZTv∥2\tfrac12\|D_ZA_Z^Tv\|^221​∥DZ​AZT​v∥2 subject to APTv=−(ds)PA_P^Tv=-(d_s)_PAPT​v=−(ds​)P​, and every minimizer gives (ds)Z=−AZTv(d_s)_Z=-A_Z^Tv(ds​)Z​=−AZT​v.
  • Proposition 7.1. For positive rk→0r^k\to0rk→0, the order of convergence equals lim inf⁡klog⁡rk+1/log⁡rk\liminf_k\log r^{k+1}/\log r^kliminfk​logrk+1/logrk.
  • Theorem 7.6. For Algorithm 4.5: (i) (xk)Tsk≤ϵ(x^k)^Ts^k\le\epsilon(xk)Tsk≤ϵ once k≥1+1+42n2log⁡(x0)Ts0ϵk\ge\frac{1+\sqrt{1+4\sqrt2n}}{2}\log\frac{(x^0)^Ts^0}{\epsilon}k≥21+1+42​n​​logϵ(x0)Ts0​; (ii) 1−θk≤C(xk)Tsk1-\theta^k\le C(x^k)^Ts^k1−θk≤C(xk)Tsk; (iii) (xk)Tsk→0(x^k)^Ts^k\to0(xk)Tsk→0 quadratically.
  • Lemmas 7.9 and 7.10 (monotone LCP linear algebra). For M=(MPPMPZMZPMZZ)M=\begin{pmatrix}M_{PP}&M_{PZ}\\M_{ZP}&M_{ZZ}\end{pmatrix}M=(MPP​MZP​​MPZ​MZZ​​) with vTMv≥0v^TMv\ge0vTMv≥0: MPPxP=0  ⟺  MPPTxP=0M_{PP}x_P=0\iff M_{PP}^Tx_P=0MPP​xP​=0⟺MPPT​xP​=0, which implies (MZP+MPZT)xP=0(M_{ZP}+M_{PZ}^T)x_P=0(MZP​+MPZT​)xP​=0; and R(MPPMPZ0I)=R(MPPTMZPT0−I)\mathcal R\begin{pmatrix}M_{PP}&M_{PZ}\\0&I\end{pmatrix}=\mathcal R\begin{pmatrix}M_{PP}^T&M_{ZP}^T\\0&-I\end{pmatrix}R(MPP​0​MPZ​I​)=R(MPPT​0​MZPT​−I​).

The goal fixes no value of CCC: it asserts only that one constant serves every point, which is the content the later results use.

Significance

Theorem 7.5, together with Lemma 7.3, says the whole predictor direction is O(μ)O(\mu)O(μ). Through the step-size bound of Lemma 4.17 this gives 1−θk=O(μk)1-\theta^k=O(\mu^k)1−θk=O(μk) and hence μk+1=O((μk)2)\mu^{k+1}=O((\mu^k)^2)μk+1=O((μk)2): quadratic convergence of the duality gap (Theorem 7.6). The same least-squares structure yields the LCP version (Theorem 7.12) and the variants of §7.4. Bounds of this kind on the directions near the optimal face also underlie finite termination and basis identification schemes.

On the formal side, no machine-checked development of interior-point asymptotics is known to the authors of this mission. Mathlib has the linear algebra (ranges, block matrices, least squares through orthogonal projection) but no linear-programming duality theory beyond what other missions supply. A formal proof would supply the Goldman–Tucker partition, the uniform bounds (7.6) and a Hoffman-type bound for least-squares solutions in reusable form.

Difficulty

Everything rests on a constant being uniform. For a single interior point, any bound ∥(dx)P∥≤Cμ\|(d_x)_P\|\le C\mu∥(dx​)P​∥≤Cμ holds with some CCC depending on the point, so the content is entirely in choosing CCC before the point. The natural first attempt bounds the direction through ∥D∥\|D\|∥D∥ and ∥D−1∥\|D^{-1}\|∥D−1∥ on all indices, and this fails: on PPP the entries Dj2=xj/sjD_j^2=x_j/s_jDj2​=xj​/sj​ grow like 1/μ1/\mu1/μ, and on ZZZ they shrink like μ\muμ. Only the split along the optimal partition, with the least-squares characterization of the "large" components and the uniform bounds (7.6), balances the two. Relation (7.6) itself requires the partition to be strict (Goldman–Tucker) and an a priori bound on the iterates. For Theorem 7.6 the algorithm's step-size rule has to be carried as defined: a largest step over an interval of neighbourhood memberships.

Formalization scope

Vectors are Fin n → ℝ and matrices Matrix (Fin m) (Fin n) ℝ. All norms are Euclidean. P∗P^*P∗ and Z∗Z^*Z∗ are defined as unions of supports of optimal solutions; that they partition the indices is a consequence solvers must prove, not an assumption. The standing assumption rank⁡A=m\operatorname{rank}A=mrankA=m appears as a hypothesis.

Instantiations of the book's O(⋅)O(\cdot)O(⋅) (p. 213: "may depend on nnn and/or the original problem data, but … independent of the iteration kkk"):

  • Theorem 7.5, Lemma 7.3: ∃C>0\exists C>0∃C>0 quantified before the point; CCC depends on (A,b,c)(A,b,c)(A,b,c) and on a bound μ0\mu^0μ0 for μ\muμ.
  • Relation (7.6): ∃ξ∈(0,1)\exists\xi\in(0,1)∃ξ∈(0,1) quantified before the point, depending on (A,b,c)(A,b,c)(A,b,c) and μ0\mu^0μ0. The book states it for the iterates; here it covers every point of N2(1/4)\mathcal N_2(1/4)N2​(1/4) with μ≤μ0\mu\le\mu^0μ≤μ0.
  • Theorem 7.6(i): the constant of Theorem 4.18's proof, threshold 1+1+42n2log⁡((x0)Ts0/ϵ)\frac{1+\sqrt{1+4\sqrt2n}}{2}\log((x^0)^Ts^0/\epsilon)21+1+42​n​​log((x0)Ts0/ϵ), with R=(x0)Ts0R=(x^0)^Ts^0R=(x0)Ts0. (ii): ∃C>0\exists C>0∃C>0 before kkk. (iii): Definition 1.2.

A constant chosen after the point would make the goal trivially true; the statements quantify ∃C\exists C∃C first to rule this out. Algorithm 4.5 is run for every kkk (no termination test), and its step is the greatest admissible θ\thetaθ, not a supremum. The order of convergence and all limits superior take values in EReal, so infinite values are not collapsed to junk reals. Lemma 7.4's second problem is stated as corrected (the printed objective 12∥DZv∥2\tfrac12\|D_Zv\|^221​∥DZ​v∥2 is ill-typed and the sign of (ds)Z=AZTv(d_s)_Z=A_Z^Tv(ds​)Z​=AZT​v is inconsistent with (7.4)); see the findings.

Beyond the paper. One strengthened variant: Theorem 7.5 on N2(η)\mathcal N_2(\eta)N2​(η) for every η∈(0,1)\eta\in(0,1)η∈(0,1) and without the rank condition. The book's proof uses the neighbourhood only through (1−η)μ≤xjsj≤(1+η)μ(1-\eta)\mu\le x_js_j\le(1+\eta)\mu(1−η)μ≤xj​sj​≤(1+η)μ, and the rank of AAA enters nowhere.

Needed infrastructure: LP duality and strict complementarity (Goldman–Tucker), a Hoffman-type bound for minimum-norm solutions, and weighted least squares. The definitions of the neighbourhood, the direction d(x,s,γ)d(x,s,\gamma)d(x,s,γ) and Algorithm 4.5 are shared with the chapter 4 analysis and are reusable for other interior-point results. Contributions proving any milestone independently are welcome.

Selected references

  • Y. Ye, Interior Point Algorithms: Theory and Analysis, Wiley-Interscience, 1997. https://doi.org/10.1002/9781118032701
  • S. Mizuno, M. J. Todd, Y. Ye, On adaptive-step primal-dual interior-point algorithms for linear programming, Mathematics of Operations Research 18 (1993) 964–981. https://doi.org/10.1287/moor.18.4.964
  • Y. Ye, O. Güler, R. A. Tapia, Y. Zhang, A quadratically convergent O(nL)O(\sqrt n L)O(n​L)-iteration algorithm for linear programming, Mathematical Programming 59 (1993) 151–162. https://doi.org/10.1007/BF01582144
  • A. J. Goldman, A. W. Tucker, Theory of linear programming, in Linear Inequalities and Related Systems, Annals of Mathematics Studies 38, Princeton, 1956. https://doi.org/10.1515/9781400881987-005
  • A. J. Hoffman, On approximate solutions of systems of linear inequalities, Journal of Research of the National Bureau of Standards 49 (1952) 263–265. https://doi.org/10.6028/jres.049.027
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Interior Point Algorithms V: Random Gaussian Feasibility Problems Are Strictly Feasible on One SideTextbook

Motivation

Worst-case analysis of interior-point methods gives iteration bounds of order n L\sqrt n\,Ln​L or nlog⁡(1/ϵ)\sqrt n\log(1/\epsilon)n​log(1/ϵ), while practical codes finish in a number of iterations that barely grows with the problem size. Chapter 6 of Yinyu Ye's Interior Point Algorithms: Theory and Analysis (Wiley 1997, doi:10.1002/9781118032701) addresses the gap probabilistically. Its rigorous half (§6.2) takes the simplest nontrivial problem, the homogeneous linear feasibility problem with a random Gaussian matrix, and shows that it is solved in O(nlog⁡n)O(\sqrt n\log n)O(n​logn) iterations with high probability, then extends this to random linear programs through the homogeneous self-dual model.

That analysis rests on a geometric fact about random cones: the probability that nnn random vectors lie in a common open half-space through the origin. The answer is Wendel's formula (J. G. Wendel, A problem in geometric probability, Math. Scand. 11 (1962), 109–111); Ye restates it as Corollaries 6.11–6.12 and draws from it Theorem 6.13: with probability one a random homogeneous system is strictly feasible on exactly one side of its theorem of the alternative. Strict feasibility is what makes the condition number of the problem finite, which is what an interior-point termination procedure needs.

Setting

Let AAA be a real m×nm\times nm×n matrix with columns a1,…,ana_1,\dots,a_na1​,…,an​; vector inequalities are componentwise and eee is the all-ones vector. The homogeneous system (6.5) is

X={x∈Rn: Ax=0, x≥0, x≠0},\mathcal X=\{x\in\mathbb R^n:\ Ax=0,\ x\ge0,\ x\ne0\},X={x∈Rn: Ax=0, x≥0, x=0},

and its alternative (6.11) is the system s=ATy≥0s=A^Ty\ge0s=ATy≥0, s≠0s\ne0s=0 in y∈Rmy\in\mathbb R^my∈Rm. An interior feasible point of (6.5) is an xxx with Ax=0Ax=0Ax=0 and x>0x>0x>0; of (6.11), a yyy with ATy>0A^Ty>0ATy>0.

In the standard Gaussian model (§6.2.2) the mnmnmn entries of AAA are independent N(0,1)N(0,1)N(0,1) random variables. With d=n−md=n-md=n−m the book defines

pnd=(n−10)+⋯+(n−1d−1)2n−1,p_{nd}=\frac{\binom{n-1}{0}+\cdots+\binom{n-1}{d-1}}{2^{n-1}},pnd​=2n−1(0n−1​)+⋯+(d−1n−1​)​,

and pnmp_{nm}pnm​ is the same sum with ddd replaced by mmm.

A partition A=(AB,AN)A=(A_B,A_N)A=(AB​,AN​) chooses mmm columns for the m×mm\times mm×m matrix ABA_BAB​ and leaves the other n−mn-mn−m in ANA_NAN​. System (6.12) asks for x=(xB,xm+1)∈Rm+1x=(x_B,x_{m+1})\in\mathbb R^{m+1}x=(xB​,xm+1​)∈Rm+1 with

ABxB+(ANe) xm+1=0,x≠0,x≥0.A_Bx_B+(A_Ne)\,x_{m+1}=0,\qquad x\ne0,\qquad x\ge0.AB​xB​+(AN​e)xm+1​=0,x=0,x≥0.

For the one-step analysis of §6.1, F=[g,H]F=[g,H]F=[g,H] is an orthogonal n×nn\times nn×n matrix with first column ggg, and vvv is uniformly distributed on the unit sphere of Rn−1\mathbb R^{n-1}Rn−1.

The Lean development lives in the namespace YeIPA.Random: the module YeIPA.Random.Setting defines IsFeasible65, IsInterior65, IsFeasible611, IsInterior611, IsFeasible612, gaussianMatrix, pnd and uniformSphere.

Formalization targets

Goal: Theorem 6.13

For 1≤n1\le n1≤n, m≤nm\le nm≤n and standard Gaussian AAA,

Pr⁡[(X=∅ ∧ ∃y: ATy>0) ∨ ((6.11) infeasible ∧ ∃x>0: Ax=0)]=1.\Pr\Big[\big(\mathcal X=\emptyset\ \wedge\ \exists y:\ A^Ty>0\big)\ \vee\ \big(\text{(6.11) infeasible}\ \wedge\ \exists x>0:\ Ax=0\big)\Big]=1 .Pr[(X=∅ ∧ ∃y: ATy>0) ∨ ((6.11) infeasible ∧ ∃x>0: Ax=0)]=1.

Milestones

  1. Lemma 6.14 (p. 194): for AAA of full row rank, (6.5) is feasible if and only if some partition A=(AB,AN)A=(A_B,A_N)A=(AB​,AN​) makes (6.12) feasible.
  2. Corollary 6.11 (p. 193): Pr⁡[X≠∅]=pnd\Pr[\mathcal X\ne\emptyset]=p_{nd}Pr[X=∅]=pnd​.
  3. Corollary 6.12 (p. 193): Pr⁡[∃x>0: Ax=0]=pnd\Pr[\exists x>0:\ Ax=0]=p_{nd}Pr[∃x>0: Ax=0]=pnd​.
  4. Lemma 6.5 (p. 185): for every sequence of orthogonal F=[g,H]F=[g,H]F=[g,H],
Pr⁡(∥Hv∥∞≤3log⁡n/n)→1(n→∞).\Pr\Big(\|Hv\|_\infty\le\sqrt{3\log n/n}\Big)\to1\qquad(n\to\infty).Pr(∥Hv∥∞​≤3logn/n​)→1(n→∞).

Significance

Theorem 6.13 is the input to the chapter's main results: Theorem 6.15 (a feasible point of a random system satisfies the scaling condition (6.9) with high probability) and the O(nlog⁡n)O(\sqrt n\log n)O(n​logn) iteration bounds of Theorems 6.16 and 6.23 for random feasibility and random LP problems. It turns a deterministic theorem of the alternative, which allows degenerate configurations, into a statement that such configurations have probability zero. Corollaries 6.11–6.12 are Wendel's formula in the form used for linear programming, and also give the exact probability that a random homogeneous system is feasible, a quantity that recurs in the analysis of random linear programs and of perceptron capacity. Lemma 6.5 is the concentration estimate behind the one-step analysis of predictor steps in §6.1.

None of these results is formalized on Prove2Me or, as far as a search shows, in Mathlib. Wendel's formula has no machine-checked proof that this mission could reference; the platform's PoissonDirichlet.Wendel.* items concern an unrelated identity. A complete development adds Gaussian random matrices, conic theorems of the alternative and an exact geometric probability to the platform.

Difficulty

The book cites Wendel's formula ("It is shown that …") and leaves its proof as Exercise 6.3, so Corollaries 6.11–6.12 must be proved from scratch. The natural first idea, conditioning on the column space or on a single column and integrating, does not close: the count of half-spaces is combinatorial, not an integral that evaluates in closed form. The probability-one statements need general position of Gaussian columns, which is the property that rules out the degenerate cases of the theorem of the alternative. Corollary 6.12 is not a consequence of Corollary 6.11 alone: the gap between "feasible" and "strictly feasible" is an event that must be shown to be null, and the book's own argument passes through the dual system and the identity pnd+pnm=1p_{nd}+p_{nm}=1pnd​+pnm​=1 for m+d=nm+d=nm+d=n, which is itself part of what must be proved. Lemma 6.5 is a uniform statement over all orthogonal completions HHH of ggg, with an explicit threshold 3log⁡n/n\sqrt{3\log n/n}3logn/n​ rather than an unspecified constant.

Formalization scope

Vectors are functions on Fin n (indices 0,…,n−10,\dots,n-10,…,n−1), matrices are Matrix (Fin m) (Fin n) ℝ, and the Gaussian model is the product measure Measure.pi of gaussianReal 0 1 over the entries, a sample being read as Matrix.of A. Probabilities are values of this measure in [0,∞][0,\infty][0,∞], compared with pndp_{nd}pnd​ through ENNReal.ofReal. "With probability one" in Theorem 6.13 is an almost-everywhere statement for that explicit measure; the failure set is null. The section's standing assumption that AAA has full row rank (p. 187) is carried as m≤nm\le nm≤n in the probabilistic statements, under which a Gaussian AAA has rank mmm almost surely, and as rank⁡A=m\operatorname{rank}A=mrankA=m in Lemma 6.14. Every probabilistic statement assumes n≥1n\ge1n≥1; for n=0n=0n=0 both systems are empty and pndp_{nd}pnd​ is undefined. In Lemma 6.14 a partition is an injection σ\sigmaσ from {1,…,m}\{1,\dots,m\}{1,…,m} into the columns; the variable xm+1x_{m+1}xm+1​ is the last coordinate of a vector on Fin (m+1). In Lemma 6.5 the sequence is indexed by k=n−1k=n-1k=n−1, the matrices FkF_kFk​ are arbitrary with FkTFk=IF_k^TF_k=IFkT​Fk​=I, HkH_kHk​ deletes the first column, the uniform law is Lebesgue surface measure on the sphere normalized to mass one, log⁡\loglog is natural, and ∥Hv∥∞≤t\|Hv\|_\infty\le t∥Hv∥∞​≤t is written coordinatewise. No result of this chapter uses an O(⋅)O(\cdot)O(⋅) bound in the stated items, so there are no constants to instantiate; Lemma 6.5's threshold 3log⁡n/n\sqrt{3\log n/n}3logn/n​ is the book's own.

A trivializing formalization is ruled out: the probabilities are those of the explicit product Gaussian measure (not an arbitrary measure, nor "for almost every AAA" against an unspecified measure), interior points are required to be strictly positive in every coordinate, and Theorem 6.13 asserts both the emptiness of one system and an interior point of the other.

Beyond the paper. One variant is posed. YeIPA.Random.lemma_6_14_corrected (corrected, finding F1) adds m<nm<nm<n to Lemma 6.14: as printed the lemma fails for n=mn=mn=m, where ANA_NAN​ is empty and x=(0,…,0,1)x=(0,\dots,0,1)x=(0,…,0,1) solves (6.12) for every AAA, e.g. A=(1)A=(1)A=(1). The faithful Lemma 6.14 stays the milestone.

Infrastructure that a complete development needs and that is reusable beyond this mission: general position of Gaussian matrices (every m×mm\times mm×m minor nonzero almost surely), Gordan's theorem of the alternative for Ax=0, x≥0, x≠0Ax=0,\ x\ge0,\ x\ne0Ax=0, x≥0, x=0, Wendel's formula, and tail bounds for coordinates of a uniform point on the sphere. Proofs of any milestone, and of these supporting facts as separate lemmas, are welcome.

Selected references

  • Yinyu Ye, Interior Point Algorithms: Theory and Analysis, Wiley-Interscience, 1997, Chapter 6. doi:10.1002/9781118032701
  • J. G. Wendel, A problem in geometric probability, Mathematica Scandinavica 11 (1962), 109–111.
  • S. I. Resnick, Extreme Values, Regular Variation, and Point Processes, Springer, 1987, pp. 42 and 71 (cited in the proof of Lemma 6.5).
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Interior Point Algorithms III: The Primal-Dual Potential Function Falls by a Constant Every StepTextbook

Motivation

Karmarkar's 1984 algorithm solved linear programs in polynomial time by reducing a logarithmic potential function by a constant at every step (doi:10.1007/BF02579150). Its successors replaced Karmarkar's projective transformations with a primal-dual potential that measures the duality gap and the distance from the central path together. Tanabe, and Todd and Ye (1990, doi:10.1287/moor.15.3.508), introduced this potential. Ye (1991) and Kojima, Mizuno and Yoshise (1991) showed that one fixed reduction per step, for the parameter ρ=n\rho=\sqrt nρ=n​, gives the best known iteration bound O(nlog⁡(1/ϵ))O(\sqrt n\log(1/\epsilon))O(n​log(1/ϵ)). The potential-reduction method needs no step that stays close to the central path. It became one of the standard interior-point methods, and its analysis was later carried over to semidefinite and conic programming.

Chapter 4 of Yinyu Ye's Interior Point Algorithms: Theory and Analysis (Wiley, 1997, doi:10.1002/9781118032701) proves this one-step estimate for two algorithms. The first is the primal-dual potential-reduction algorithm (§4.3), which alternates a primal projected-gradient step with an update of the dual lower bound. The second is the symmetric primal-dual algorithm (§4.4), which moves along the Newton direction toward a point of the central path. This mission formalizes both estimates and the lemmas they rest on.

Setting

Let AAA be a real m×nm\times nm×n matrix of rank mmm, b∈Rmb\in\mathbb R^mb∈Rm with b≠0b\ne0b=0, c∈Rnc\in\mathbb R^nc∈Rn, n≥1n\ge1n≥1. The standard-form pair is

(LP) min⁡{cTx:Ax=b, x≥0},(LD) max⁡{bTy:ATy+s=c, s≥0},(\mathrm{LP})\ \min\{c^Tx: Ax=b,\ x\ge0\},\qquad(\mathrm{LD})\ \max\{b^Ty: A^Ty+s=c,\ s\ge0\},(LP) min{cTx:Ax=b, x≥0},(LD) max{bTy:ATy+s=c, s≥0},

and both are assumed to have interior feasible points. Vector inequalities are componentwise, eee is the all-ones vector, X=diag⁡(x)X=\operatorname{diag}(x)X=diag(x), S=diag⁡(s)S=\operatorname{diag}(s)S=diag(s), ∥⋅∥\|\cdot\|∥⋅∥ is the Euclidean norm and min⁡(v)=min⁡jvj\min(v)=\min_jv_jmin(v)=minj​vj​. The interior F˚\mathring{\mathcal F}F˚ consists of the triples (x,y,s)(x,y,s)(x,y,s) with Ax=bAx=bAx=b, x>0x>0x>0, ATy+s=cA^Ty+s=cATy+s=c, s>0s>0s>0. For ρ≥0\rho\ge0ρ≥0, the primal-dual potential and the primal potential with lower bound zzz are

ψn+ρ(x,s)=(n+ρ)log⁡(xTs)−∑j=1nlog⁡(xjsj),Pn+ρ(x,z)=(n+ρ)log⁡(cTx−z)−∑j=1nlog⁡xj.\psi_{n+\rho}(x,s)=(n+\rho)\log(x^Ts)-\sum_{j=1}^n\log(x_js_j),\qquad\mathcal P_{n+\rho}(x,z)=(n+\rho)\log(c^Tx-z)-\sum_{j=1}^n\log x_j.ψn+ρ​(x,s)=(n+ρ)log(xTs)−j=1∑n​log(xj​sj​),Pn+ρ​(x,z)=(n+ρ)log(cTx−z)−j=1∑n​logxj​.

The potential-reduction step (§4.3, ρ=n\rho=\sqrt nρ=n​). At (xk,yk,sk)∈F˚(x^k,y^k,s^k)\in\mathring{\mathcal F}(xk,yk,sk)∈F˚ with zk=bTykz^k=b^Ty^kzk=bTyk, let

pk=p(zk)=(I−XkAT(A(Xk)2AT)−1AXk) Xk(n+ρcTxk−zkc−(Xk)−1e),xk+1=xk−αXkpk∥pk∥.p^k=p(z^k)=(I-X^kA^T(A(X^k)^2A^T)^{-1}AX^k)\,X^k\Big(\frac{n+\rho}{c^Tx^k-z^k}c-(X^k)^{-1}e\Big),\qquad x^{k+1}=x^k-\alpha\frac{X^kp^k}{\|p^k\|}.pk=p(zk)=(I−XkAT(A(Xk)2AT)−1AXk)Xk(cTxk−zkn+ρ​c−(Xk)−1e),xk+1=xk−α∥pk∥Xkpk​.

The dual update is s(zk)=c−ATy(zk)s(z^k)=c-A^Ty(z^k)s(zk)=c−ATy(zk) with y(zk)=y2−cTxk−zkn+ρy1y(z^k)=y_2-\frac{c^Tx^k-z^k}{n+\rho}y_1y(zk)=y2​−n+ρcTxk−zk​y1​, y1=(A(Xk)2AT)−1by_1=(A(X^k)^2A^T)^{-1}by1​=(A(Xk)2AT)−1b and y2=(A(Xk)2AT)−1A(Xk)2cy_2=(A(X^k)^2A^T)^{-1}A(X^k)^2cy2​=(A(Xk)2AT)−1A(Xk)2c.

The symmetric step (§4.4). With μ=xTs/n\mu=x^Ts/nμ=xTs/n and γ=n/(n+ρ)\gamma=n/(n+\rho)γ=n/(n+ρ), the direction (dx,dy,ds)(d_x,d_y,d_s)(dx​,dy​,ds​) solves Sdx+Xds=γμe−XsSd_x+Xd_s=\gamma\mu e-XsSdx​+Xds​=γμe−Xs, Adx=0Ad_x=0Adx​=0, −ATdy−ds=0-A^Td_y-d_s=0−ATdy​−ds​=0. The step size is θ=αmin⁡(Xs)/∥(XS)−1/2(xTsn+ρe−Xs)∥\theta=\alpha\sqrt{\min(Xs)}/\|(XS)^{-1/2}(\frac{x^Ts}{n+\rho}e-Xs)\|θ=αmin(Xs)​/∥(XS)−1/2(n+ρxTs​e−Xs)∥ for a constant 0<α<10<\alpha<10<α<1.

Formalization targets

Goal: Theorem 4.9

There is a constant δ>1/20\delta>1/20δ>1/20 such that for every such LP and every (xk,yk,sk)∈F˚(x^k,y^k,s^k)\in\mathring{\mathcal F}(xk,yk,sk)∈F˚, with ρ=n\rho=\sqrt nρ=n​, α=0.3\alpha=0.3α=0.3 and sk+1=s(zk)s^{k+1}=s(z^k)sk+1=s(zk),

ψn+ρ(xk+1,sk)≤ψn+ρ(xk,sk)−δorψn+ρ(xk,sk+1)≤ψn+ρ(xk,sk)−δ.\psi_{n+\rho}(x^{k+1},s^k)\le\psi_{n+\rho}(x^k,s^k)-\delta\qquad\text{or}\qquad\psi_{n+\rho}(x^k,s^{k+1})\le\psi_{n+\rho}(x^k,s^k)-\delta.ψn+ρ​(xk+1,sk)≤ψn+ρ​(xk,sk)−δorψn+ρ​(xk,sk+1)≤ψn+ρ​(xk,sk)−δ.

Milestones

  • Lemma 3.1: for ∥d∥∞<1\|d\|_\infty<1∥d∥∞​<1, eTd≥∑ilog⁡(1+di)≥eTd−∥d∥2/(2(1−∥d∥∞))e^Td\ge\sum_i\log(1+d_i)\ge e^Td-\|d\|^2/(2(1-\|d\|_\infty))eTd≥∑i​log(1+di​)≥eTd−∥d∥2/(2(1−∥d∥∞​)).
  • The display after (4.10): for 0<α<10<\alpha<10<α<1, Pn+ρ(xk+1,zk)−Pn+ρ(xk,zk)≤−α∥pk∥+α2/(2(1−α))\mathcal P_{n+\rho}(x^{k+1},z^k)-\mathcal P_{n+\rho}(x^k,z^k)\le-\alpha\|p^k\|+\alpha^2/(2(1-\alpha))Pn+ρ​(xk+1,zk)−Pn+ρ​(xk,zk)≤−α∥pk∥+α2/(2(1−α)).
  • Lemma 4.8: with μk=(xk)Tsk/n\mu^k=(x^k)^Ts^k/nμk=(xk)Tsk/n and μ=(xk)Ts(zk)/n\mu=(x^k)^Ts(z^k)/nμ=(xk)Ts(zk)/n, if ∥p(zk)∥<min⁡(ηn/(n+η2),1−η)\|p(z^k)\|<\min(\eta\sqrt{n/(n+\eta^2)},1-\eta)∥p(zk)∥<min(ηn/(n+η2)​,1−η) then s(zk)>0s(z^k)>0s(zk)>0, ∥Xks(zk)−μe∥<ημ\|X^ks(z^k)-\mu e\|<\eta\mu∥Xks(zk)−μe∥<ημ and μ<(1−.5η/n)μk\mu<(1-.5\eta/\sqrt n)\mu^kμ<(1−.5η/n​)μk.
  • Lemma 4.11: the symmetric step stays in F˚\mathring{\mathcal F}F˚ and
ψn+ρ(x+,s+)−ψn+ρ(x,s)≤−αmin⁡(Xs)∥(XS)−1/2(e−n+ρxTsXs)∥+α22(1−α).\psi_{n+\rho}(x^+,s^+)-\psi_{n+\rho}(x,s)\le-\alpha\sqrt{\min(Xs)}\Big\|(XS)^{-1/2}\Big(e-\frac{n+\rho}{x^Ts}Xs\Big)\Big\|+\frac{\alpha^2}{2(1-\alpha)}.ψn+ρ​(x+,s+)−ψn+ρ​(x,s)≤−αmin(Xs)​​(XS)−1/2(e−xTsn+ρ​Xs)​+2(1−α)α2​.
  • Lemma 4.12: for v>0v>0v>0 and ρ≥n\rho\ge\sqrt nρ≥n​, min⁡(v) ∥V−1/2(e−n+ρeTvv)∥≥3/4\sqrt{\min(v)}\,\|V^{-1/2}(e-\frac{n+\rho}{e^Tv}v)\|\ge\sqrt{3/4}min(v)​∥V−1/2(e−eTvn+ρ​v)∥≥3/4​.

Significance

Theorem 4.9 says that the potential can be lowered by a fixed amount no matter where the iterate is. Either the primal step makes progress, or the dual update produces a feasible, nearly central dual point with a smaller gap. Since ψn+n(x,s)≥nlog⁡(xTs)+nlog⁡n\psi_{n+\sqrt n}(x,s)\ge\sqrt n\log(x^Ts)+n\log nψn+n​​(x,s)≥n​log(xTs)+nlogn, a constant decrease per step yields the O(nlog⁡((x0)Ts0/ϵ))O(\sqrt n\log((x^0)^Ts^0/\epsilon))O(n​log((x0)Ts0/ϵ)) bound of Corollary 4.10 without any neighborhood of the central path. Lemmas 4.11 and 4.12 give the same conclusion for the symmetric algorithm, with decrease α3/4−α2/(2(1−α))\alpha\sqrt{3/4}-\alpha^2/(2(1-\alpha))α3/4​−α2/(2(1−α)). These are the per-step facts behind practical potential-reduction codes and their conic extensions.

The platform has no statement of these results. LinearOptimization.interior_point_potential_reduction (proved) turns a constant decrease of a different potential into an iteration count. The Mizuno–Todd–Ye bounds on the second-order term of the Newton step, which are Ye's Lemmas 4.14, 4.15 and 4.19, are already formalized as AdaptiveStepIPM.PredCorr.lemma_1a, AdaptiveStepIPM.WideNbhd.lemma_1b, lemma_1c and lemma_2c. They belong to the path-following part of the chapter and are not repeated here. Formalizing Theorem 4.9 checks the case split and its numerical constants, and produces a reusable layer: the weighted projection I−XAT(AX2AT)−1AXI-XA^T(AX^2A^T)^{-1}AXI−XAT(AX2AT)−1AX, the dual estimates y(z)y(z)y(z), s(z)s(z)s(z), and the potential identities.

Difficulty

The case split is driven by ∥p(zk)∥\|p(z^k)\|∥p(zk)∥, which is defined through a projection, but Lemma 4.8 reasons about it through the identity p(zk)=n+ρcTxk−zkXks(zk)−ep(z^k)=\frac{n+\rho}{c^Tx^k-z^k}X^ks(z^k)-ep(zk)=cTxk−zkn+ρ​Xks(zk)−e (4.11). The book states that identity without proof. It needs the normal equations for y1y_1y1​, y2y_2y2​, and so the invertibility of AX2ATAX^2A^TAX2AT. The primal case needs xk+1>0x^{k+1}>0xk+1>0 and cTxk+1−zk>0c^Tx^{k+1}-z^k>0cTxk+1−zk>0 before any logarithm is evaluated. It also needs the first-order term ∇PTdx\nabla\mathcal P^Td_x∇PTdx​ to equal −α∥pk∥-\alpha\|p^k\|−α∥pk∥ exactly, which uses that pkp^kpk is an orthogonal projection. The dual case combines Lemma 3.1, applied to Xksk+1/μ−eX^ks^{k+1}/\mu-eXksk+1/μ−e, with the arithmetic–geometric mean inequality and the bound nlog⁡(1−.5η/n)≤−η/2\sqrt n\log(1-.5\eta/\sqrt n)\le-\eta/2n​log(1−.5η/n​)≤−η/2. The constant must be checked for every n≥1n\ge1n≥1: with η=.43\eta=.43η=.43 and α=.3\alpha=.3α=.3 the two decreases are about .054.054.054 and .053.053.053, so the margin over 1/201/201/20 is small. Lemma 4.11's proof is omitted in the book. Unlike its Chapter 3 analogue it needs no normalization xTs=nx^Ts=nxTs=n, but the exact cancellation of its first-order term has to be verified. Lemma 4.12 is left as an exercise.

Formalization scope

Vectors are Fin n → ℝ and matrices Matrix (Fin m) (Fin n) ℝ; indices are 000-based. The Euclidean norm is written out as norm2. The rank of AAA, the existence of interior points (p. 63) and b≠0b\ne0b=0 (Chapter 4 preamble, p. 109) are bundled as StandingAssumptions A b c and are a hypothesis of every theorem, together with n≥1n\ge1n≥1. The vector p(z)p(z)p(z) is defined by its projection formula, not by (4.11), so (4.11) is something to prove. Matrix inverses are Mathlib's ⁻¹, which is 000 on singular matrices and never reached under the rank assumption.

Explicit constants: Theorem 4.9 does not fix the step length α\alphaα, and the proof chooses α=.3\alpha=.3α=.3 (with η=.43\eta=.43η=.43 in the case split). The goal is stated for the step (4.10) with α=.3\alpha=.3α=.3. Its δ\deltaδ is quantified before mmm, nnn, AAA, bbb, ccc and the iterate, so one number δ>1/20\delta>1/20δ>1/20 serves every instance. A formalization in which δ\deltaδ depends on the instance, or in which the step is any point that happens to lower the potential, would be trivial; both are ruled out by stating the theorem for the book's step (4.10) and dual update (4.12)–(4.13) with a uniform δ\deltaδ. No result in this mission uses O(⋅)O(\cdot)O(⋅).

Beyond the paper. One variant is posed: a strengthened Theorem 4.9 for every ρ≥n\rho\ge\sqrt nρ≥n​ with the same δ\deltaδ (finding F1). The proof uses ρ=n\rho=\sqrt nρ=n​ only through (n+ρ)/n≥1+1/n(n+\rho)/n\ge1+1/\sqrt n(n+ρ)/n≥1+1/n​ and ρlog⁡(1−.5η/n)≤−η/2\rho\log(1-.5\eta/\sqrt n)\le-\eta/2ρlog(1−.5η/n​)≤−η/2, and both hold for ρ≥n\rho\ge\sqrt nρ≥n​.

Not in scope: the iteration-count statements (Theorems 4.4, 4.7, 4.13, 4.20, Corollary 4.10), which the book gives with O(⋅)O(\cdot)O(⋅); Karmarkar's algorithm and the path-following analysis of §§4.1–4.2 (Proposition 4.2, Lemmas 4.5–4.6), which need the max-potential of the dual level set; and the predictor-corrector Lemmas 4.16–4.17 and Theorem 4.18, which are statements about an algorithm's sequence.

Selected references

  • Y. Ye, Interior Point Algorithms: Theory and Analysis, Wiley-Interscience, 1997, Chapter 4. doi:10.1002/9781118032701
  • N. Karmarkar, A new polynomial-time algorithm for linear programming, Combinatorica 4 (1984) 373–395. doi:10.1007/BF02579150
  • M. J. Todd and Y. Ye, A centered projective algorithm for linear programming, Mathematics of Operations Research 15 (1990) 508–529. doi:10.1287/moor.15.3.508
  • Y. Ye, An O(n³L) potential reduction algorithm for linear programming, Mathematical Programming 50 (1991) 239–258.
  • M. Kojima, S. Mizuno and A. Yoshise, An O(√n L) iteration potential reduction algorithm for linear complementarity problems, Mathematical Programming 50 (1991) 331–342.
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Interior Point Algorithms II: Proximity Measures and Newton Steps for the Analytic CenterTextbook

Motivation

Interior-point methods for linear programming do not move along the boundary of the feasible region; they follow a curve of points that stay "central". The basic notion of centrality is the analytic center of a polytope, the interior point that maximizes the product of the slacks of its defining inequalities (Sonnevend, 1986, doi:10.1007/BFb0043914). Every path-following and potential-reduction algorithm repeatedly computes an approximate analytic center, so it matters how "approximately centered" is measured and what one Newton step does to that measure.

Chapter 3 of Yinyu Ye's Interior Point Algorithms: Theory and Analysis (Wiley, 1997, doi:10.1002/9781118032701) answers both questions for a polytope given by linear inequalities. It introduces three measures of proximity to the analytic center (dual, primal and primal-dual), proves that they are equivalent, and shows that the corresponding Newton steps square them. The same estimates reappear, with minor changes, in the complexity analysis of Karmarkar's algorithm and of the primal-dual potential-reduction methods of the later chapters. This mission formalizes those statements.

Setting

Let AAA be a real m×nm\times nm×n matrix and c∈Rnc\in\mathbb R^nc∈Rn. Vector inequalities are componentwise, eee is the all-ones vector, X=diag⁡(x)X=\operatorname{diag}(x)X=diag(x), S=diag⁡(s)S=\operatorname{diag}(s)S=diag(s), and ∥⋅∥\|\cdot\|∥⋅∥ is the Euclidean norm. The chapter works under standing assumptions: AAA has rank mmm, n>mn>mn>m, and the polytope

Ω={y∈Rm: s=c−ATy≥0}\Omega=\{y\in\mathbb R^m:\ s=c-A^Ty\ge 0\}Ω={y∈Rm: s=c−ATy≥0}

is bounded with nonempty interior. An interior point (y,s)(y,s)(y,s) of Ω\OmegaΩ has s=c−ATy>0s=c-A^Ty>0s=c−ATy>0. The homogeneous primal set is XΩ={x:Ax=0, x≥0}\mathcal X_\Omega=\{x: Ax=0,\ x\ge 0\}XΩ​={x:Ax=0, x≥0}; its interior points have Ax=0Ax=0Ax=0, x>0x>0x>0.

Three potential functions are used:

B(y)=∑jlog⁡sj,P(x)=nlog⁡(cTx)−∑jlog⁡xj,ψn(x,s)=nlog⁡(xTs)−∑jlog⁡(xjsj).\mathcal B(y)=\sum_j\log s_j,\qquad \mathcal P(x)=n\log(c^Tx)-\sum_j\log x_j,\qquad \psi_n(x,s)=n\log(x^Ts)-\sum_j\log(x_js_j).B(y)=j∑​logsj​,P(x)=nlog(cTx)−j∑​logxj​,ψn​(x,s)=nlog(xTs)−j∑​log(xj​sj​).

The analytic center (ya,sa)(y^a,s^a)(ya,sa) is the interior point of Ω\OmegaΩ maximizing B\mathcal BB, and the primal potential minimizer xax^axa is the interior point of XΩ\mathcal X_\OmegaXΩ​ with cTxa=nc^Tx^a=ncTxa=n minimizing P\mathcal PP.

The three proximity measures are

ηd(s)=∥S−1AT(AS−2AT)−1AS−1e∥,ηp(x)=∥(I−XAT(AX2AT)−1AX)(Xc−e)∥,η(x,s)=∥Xs−e∥.\eta_d(s)=\|S^{-1}A^T(AS^{-2}A^T)^{-1}AS^{-1}e\|,\qquad \eta_p(x)=\|(I-XA^T(AX^2A^T)^{-1}AX)(Xc-e)\|,\qquad \eta(x,s)=\|Xs-e\|.ηd​(s)=∥S−1AT(AS−2AT)−1AS−1e∥,ηp​(x)=∥(I−XAT(AX2AT)−1AX)(Xc−e)∥,η(x,s)=∥Xs−e∥.

The dual Newton step is s+=s+AT(AS−2AT)−1AS−1es^+=s+A^T(AS^{-2}A^T)^{-1}AS^{-1}es+=s+AT(AS−2AT)−1AS−1e; the primal Newton step is x+=x−X(I−XAT(AX2AT)−1AX)(Xc−e)x^+=x-X(I-XA^T(AX^2A^T)^{-1}AX)(Xc-e)x+=x−X(I−XAT(AX2AT)−1AX)(Xc−e); the primal-dual Newton directions (dx,dy,ds)(d_x,d_y,d_s)(dx​,dy​,ds​) solve Sdx+Xds=e−XsSd_x+Xd_s=e-XsSdx​+Xds​=e−Xs, Adx=0Ad_x=0Adx​=0, −ATdy−ds=0-A^Td_y-d_s=0−ATdy​−ds​=0, and are taken with step size θ=αmin⁡(Xs)/∥(XS)−1/2(e−Xs)∥\theta=\alpha\sqrt{\min(Xs)}/\|(XS)^{-1/2}(e-Xs)\|θ=αmin(Xs)​/∥(XS)−1/2(e−Xs)∥ for a constant 0<α<10<\alpha<10<α<1.

Formalization targets

Goal: Theorem 3.2

For an interior point (y,s)(y,s)(y,s) of Ω\OmegaΩ, an interior point xxx of XΩ\mathcal X_\OmegaXΩ​, the analytic center (ya,sa)(y^a,s^a)(ya,sa) and the primal minimizer xax^axa:

  1. ηd(s)≤η(x,s)\eta_d(s)\le\eta(x,s)ηd​(s)≤η(x,s) and ηp(x)≤η(x,s)\eta_p(x)\le\eta(x,s)ηp​(x)≤η(x,s); if ηd(s)<1\eta_d(s)<1ηd​(s)<1 some interior x′x'x′ has η(x′,s)≤ηd(s)\eta(x',s)\le\eta_d(s)η(x′,s)≤ηd​(s), and if ηp(x)<1\eta_p(x)<1ηp​(x)<1 some interior (y′,s′)(y',s')(y′,s′) has η(x,s′)≤ηp(x)\eta(x,s')\le\eta_p(x)η(x,s′)≤ηp​(x).
  2. If η(x,s)<1\eta(x,s)<1η(x,s)<1, some x^≥0\hat x\ge0x^≥0 with Ax^=0A\hat x=0Ax^=0, cTx^=sTx^=nc^T\hat x=s^T\hat x=ncTx^=sTx^=n has η(x^,s)≤η(x,s)/1−η(x,s)2/n\eta(\hat x,s)\le\eta(x,s)/\sqrt{1-\eta(x,s)^2/n}η(x^,s)≤η(x,s)/1−η(x,s)2/n​.
  3. If η(x,s)<1\eta(x,s)<1η(x,s)<1 and cTx=nc^Tx=ncTx=n,
ψn(x,s)−ψn(xa,sa),P(x)−P(xa),B(ya)−B(y) ≤ η(x,s)22(1−η(x,s)).\psi_n(x,s)-\psi_n(x^a,s^a),\quad \mathcal P(x)-\mathcal P(x^a),\quad \mathcal B(y^a)-\mathcal B(y)\ \le\ \frac{\eta(x,s)^2}{2(1-\eta(x,s))}.ψn​(x,s)−ψn​(xa,sa),P(x)−P(xa),B(ya)−B(y) ≤ 2(1−η(x,s))η(x,s)2​.
  1. If η(x,s)<1\eta(x,s)<1η(x,s)<1,
∥S−1sa−e∥≤η(x,s)1−η(x,s),∥X−1xa−e∥≤η(x,s)1−η(x,s).\|S^{-1}s^a-e\|\le\frac{\eta(x,s)}{1-\eta(x,s)},\qquad \|X^{-1}x^a-e\|\le\frac{\eta(x,s)}{1-\eta(x,s)}.∥S−1sa−e∥≤1−η(x,s)η(x,s)​,∥X−1xa−e∥≤1−η(x,s)η(x,s)​.

Milestones

  • Lemma 3.1: for ∥d∥∞<1\|d\|_\infty<1∥d∥∞​<1, eTd≥∑ilog⁡(1+di)≥eTd−∥d∥2/(2(1−∥d∥∞))e^Td\ge\sum_i\log(1+d_i)\ge e^Td-\|d\|^2/(2(1-\|d\|_\infty))eTd≥∑i​log(1+di​)≥eTd−∥d∥2/(2(1−∥d∥∞​)).
  • Theorem 3.2 (i): part 1 of the goal on its own.
  • Theorem 3.3: if ηd(s)<1\eta_d(s)<1ηd​(s)<1, then s+>0s^+>0s+>0 and ηd(s+)≤ηd(s)2\eta_d(s^+)\le\eta_d(s)^2ηd​(s+)≤ηd​(s)2.
  • Corollary 3.8: if ηp(x)<1\eta_p(x)<1ηp​(x)<1, then x+>0x^+>0x+>0, Ax+=0Ax^+=0Ax+=0 and ηp(x+)≤ηp(x)2\eta_p(x^+)\le\eta_p(x)^2ηp​(x+)≤ηp​(x)2.
  • Lemma 3.12: under sTx=ns^Tx=nsTx=n, ψn(x+θdx,s+θds)−ψn(x,s)≤−αmin⁡(Xs)∥(XS)−1/2(e−Xs)∥+α2/(2(1−α))\psi_n(x+\theta d_x,s+\theta d_s)-\psi_n(x,s)\le-\alpha\sqrt{\min(Xs)}\|(XS)^{-1/2}(e-Xs)\|+\alpha^2/(2(1-\alpha))ψn​(x+θdx​,s+θds​)−ψn​(x,s)≤−αmin(Xs)​∥(XS)−1/2(e−Xs)∥+α2/(2(1−α)).
  • Theorem 3.13: for each 0<η<10<\eta<10<η<1 there are 0<α<10<\alpha<10<α<1 and δ>0\delta>0δ>0 such that η(x,s)≥η\eta(x,s)\ge\etaη(x,s)≥η and sTx=ns^Tx=nsTx=n imply ψn(x+θdx,s+θds)−ψn(x,s)≤−δ\psi_n(x+\theta d_x,s+\theta d_s)-\psi_n(x,s)\le-\deltaψn​(x+θdx​,s+θds​)−ψn​(x,s)≤−δ, uniformly over all instances.

Significance

Theorem 3.2 is what makes an "approximate analytic center" a well-defined notion: whichever of the three measures an algorithm controls, the other two and the potential gaps follow. Parts 3 and 4 convert a small residual ∥Xs−e∥\|Xs-e\|∥Xs−e∥ into a bound on the logarithmic-volume gap and on the relative distance to the true center. These are the quantities used to start the Newton procedures of Theorem 3.3 and Corollary 3.8, and to bound the number of central-section and potential-reduction steps elsewhere in the book. Lemma 3.12 and Theorem 3.13 are the per-step estimate of the primal-dual potential-reduction method, the template for the linear-programming algorithms of Chapter 4.

None of these statements is formalized on Prove2Me. The platform has self-concordant Newton analysis (ConvexOptimization.newton_quadratic_phase_contraction) and proximity measures for self-scaled cones (SelfScaledIPM.FuncProx.*, open), but neither works with Ye's measures or steps. The results are classical and proved in the book. Formalizing them yields a reusable linear-algebraic layer (projections I−XAT(AX2AT)−1AXI-XA^T(AX^2A^T)^{-1}AXI−XAT(AX2AT)−1AX, scaled least squares, the primal-dual Newton system) for any later interior-point mission. It also checks hypotheses the book leaves implicit: Lemma 3.12 is false without the normalization sTx=ns^Tx=nsTx=n that §3.4.2 states only once.

Difficulty

Most of the chapter's arguments are short on paper but depend on facts that are only described in words. In (i), ηd(s)\eta_d(s)ηd​(s) and ηp(x)\eta_p(x)ηp​(x) are the optimal values of weighted least-squares problems, and the domination needs that variational characterization, which goes through the invertibility of AS−2ATAS^{-2}A^TAS−2AT and AX2ATAX^2A^TAX2AT (rank mmm). Part 4 is not a one-step estimate: it is obtained by running the Newton procedures of Theorem 3.3 and Corollary 3.8 indefinitely, summing a geometric series of quadratically shrinking corrections, and identifying the limit with the analytic center. That identification requires uniqueness of the center (strict concavity on a bounded polytope) and the correspondence between sas^asa and xax^axa through Xasa=eX^as^a=eXasa=e. Part 3 needs that correspondence as well. The positivity of the damped iterates in Lemma 3.12, which the book asserts "can be verified", has to be established before any logarithm is evaluated. Theorem 3.13 requires constants independent of nnn, which rules out every argument that loses a dimension factor.

Formalization scope

Vectors are Fin n → ℝ and matrices Matrix (Fin m) (Fin n) ℝ; indices are 000-based. The Euclidean norm is written out as norm2, since Mathlib's norm on Fin n → ℝ is the sup norm. Matrix inverses are Mathlib's ⁻¹, which is 000 on singular matrices. The rank hypothesis makes every inverted matrix positive definite at the points where the theorems use it, so the junk value is never reached. The standing assumptions (rank A=mA=mA=m, n>mn>mn>m, Ω\OmegaΩ bounded with nonempty interior) are bundled as StandingAssumptions A c and are a hypothesis of every theorem. The normalization sTx=ns^Tx=nsTx=n of §3.4.2 is a hypothesis of Lemma 3.12 and Theorem 3.13. The analytic center and the primal minimizer are defined by their extremal properties, not by the optimality system Xs=eXs=eXs=e, so the goal cannot be obtained by unfolding a definition. The theorems are stated for the steps exactly as the book defines them, never for an arbitrary "good" step.

Explicit constants: no result in this mission uses O(⋅)O(\cdot)O(⋅). Theorem 3.13's constants are existential in the book ("we can choose α\alphaα … for a positive constant δ\deltaδ"). They are stated as ∀η∈(0,1) ∃α∈(0,1) ∃δ>0\forall\eta\in(0,1)\ \exists\alpha\in(0,1)\ \exists\delta>0∀η∈(0,1) ∃α∈(0,1) ∃δ>0 quantified before nnn, AAA, ccc and (x,s)(x,s)(x,s), which is what the proof establishes. A formalization in which α\alphaα or δ\deltaδ depends on the instance would be trivial and is ruled out by this quantifier order.

Beyond the paper. One variant is posed: a sharpness probe of Lemma 3.12 with the normalization sTx=ns^Tx=nsTx=n removed (findings F1, F3). With A=(1,−1)A=(1,-1)A=(1,−1), c=(1,1)c=(1,1)c=(1,1), s=(1,1)s=(1,1)s=(1,1), x=(10,10)x=(10,10)x=(10,10), α=1/2\alpha=1/2α=1/2 the step only rescales xxx, so the potential is unchanged while the claimed bound is about −6.11-6.11−6.11. A disproof is the expected outcome.

Not in scope: Theorem 3.11 (the primal-dual Newton step), whose positivity claim fails as printed (finding F2); the iteration-count theorems 3.4, 3.7, 3.9, stated with O(⋅)O(\cdot)O(⋅); the central-section Lemmas 3.5–3.6; and Theorem 3.10 (affine scaling). Contributions welcome: the least-squares characterization of ηd\eta_dηd​, ηp\eta_pηp​ as reusable lemmas, and the uniqueness of the analytic center.

Selected references

  • Y. Ye, Interior Point Algorithms: Theory and Analysis, Wiley-Interscience, 1997, Chapter 3. doi:10.1002/9781118032701
  • G. Sonnevend, An "analytical centre" for polyhedrons and new classes of global algorithms for linear (smooth, convex) programming, Lecture Notes in Control and Information Sciences 84, Springer, 1986, pp. 866–875. doi:10.1007/BFb0043914
  • N. Karmarkar, A new polynomial-time algorithm for linear programming, Combinatorica 4 (1984) 373–395. doi:10.1007/BF02579150
  • M. J. Todd and Y. Ye, A centered projective algorithm for linear programming, Mathematics of Operations Research 15 (1990) 508–529. doi:10.1287/moor.15.3.508
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Linear OptimizationNumerical AnalysisOperations Research+1·Captain: Shuze Chen

Primal-Dual Interior-Point Methods V: Below a Threshold μ̄ the Projection Step Returns a Strictly Complementary SolutionTextbook

Why exact solutions from interior-point iterates matter

Primal-dual interior-point methods for linear programming generate strictly positive iterates that approach the solution set but never reach it: the duality measure μ\muμ tends to zero, yet no iterate is ever an exact solution. Many applications need more than an approximate point — an exact optimal basis, a vertex, or the optimal partition of the variables into those that are positive and those that vanish at optimality. Chapter 7 of Stephen J. Wright's Primal-Dual Interior-Point Methods (SIAM 1997) studies the end game of these methods: how fast μ\muμ decreases once the iterates are close to the solution set, and how to jump from an interior iterate to an exact solution.

The finite termination procedure formalized here is due to Ye (Math. Programming 1992): guess the optimal partition from the current iterate and project onto the face of the feasible set it determines. The superlinear analysis of the affine-scaling step goes back to Ye, Güler, Tapia and Zhang (Math. Programming 1993); its key uniform bound on weighted least-squares solutions was proved independently by Stewart (Linear Algebra Appl. 1989) and Todd (Oper. Res. 1990). The bound on solutions of linear systems used throughout is Hoffman's (J. Res. Nat. Bur. Standards 1952).

Setting

Fix A∈Rm×nA \in \mathbb{R}^{m\times n}A∈Rm×n, b∈Rmb \in \mathbb{R}^mb∈Rm, c∈Rnc \in \mathbb{R}^nc∈Rn, with no assumption on the rank of AAA. The primal problem is min⁡cTx\min c^{\mathsf T}xmincTx subject to Ax=bAx = bAx=b, x≥0x \ge 0x≥0; the dual is max⁡bTλ\max b^{\mathsf T}\lambdamaxbTλ subject to ATλ+s=cA^{\mathsf T}\lambda + s = cATλ+s=c, s≥0s \ge 0s≥0. Let ΩP\Omega_PΩP​ and ΩD\Omega_DΩD​ be their sets of optimal solutions and Ω=ΩP×ΩD\Omega = \Omega_P \times \Omega_DΩ=ΩP​×ΩD​. The optimal partition is

B={j:xj∗≠0 for some x∗∈ΩP},N={j:sj∗≠0 for some (λ∗,s∗)∈ΩD}.\mathcal B = \{j : x^*_j \ne 0 \text{ for some } x^* \in \Omega_P\},\qquad \mathcal N = \{j : s^*_j \ne 0 \text{ for some } (\lambda^*, s^*) \in \Omega_D\}.B={j:xj∗​=0 for some x∗∈ΩP​},N={j:sj∗​=0 for some (λ∗,s∗)∈ΩD​}.

A strictly complementary solution is (x∗,λ∗,s∗)∈Ω(x^*, \lambda^*, s^*) \in \Omega(x∗,λ∗,s∗)∈Ω with xi∗>0x^*_i > 0xi∗​>0 for i∈Bi \in \mathcal Bi∈B and si∗>0s^*_i > 0si∗​>0 for i∈Ni \in \mathcal Ni∈N. The quantity

ϵ(A,b,c)=min⁡(min⁡i∈Bsup⁡x∗∈ΩPxi∗, min⁡i∈Nsup⁡(λ∗,s∗)∈ΩDsi∗)\epsilon(A,b,c) = \min\Bigl(\min_{i\in\mathcal B}\sup_{x^*\in\Omega_P}x^*_i,\ \min_{i\in\mathcal N}\sup_{(\lambda^*,s^*)\in\Omega_D}s^*_i\Bigr)ϵ(A,b,c)=min(i∈Bmin​x∗∈ΩP​sup​xi∗​, i∈Nmin​(λ∗,s∗)∈ΩD​sup​si∗​)

measures how far the solution set reaches into each coordinate, and C3=ϵ(A,b,c)/nC_3 = \epsilon(A,b,c)/nC3​=ϵ(A,b,c)/n.

The strictly feasible set F∘\mathcal F^\circF∘ consists of triples (x,λ,s)(x,\lambda,s)(x,λ,s) with Ax=bAx = bAx=b, ATλ+s=cA^{\mathsf T}\lambda + s = cATλ+s=c, x>0x > 0x>0, s>0s > 0s>0. The duality measure is μ=xTs/n\mu = x^{\mathsf T}s/nμ=xTs/n. The wide neighborhood is N−∞(γ)={(x,λ,s)∈F∘:xisi≥γμ for all i}\mathcal N_{-\infty}(\gamma) = \{(x,\lambda,s) \in \mathcal F^\circ : x_is_i \ge \gamma\mu \text{ for all } i\}N−∞​(γ)={(x,λ,s)∈F∘:xi​si​≥γμ for all i} for γ∈(0,1)\gamma \in (0,1)γ∈(0,1), and the 2-norm neighborhood is N2(θ)={(x,λ,s)∈F∘:∥XSe−μe∥≤θμ}\mathcal N_2(\theta) = \{(x,\lambda,s) \in \mathcal F^\circ : \|XSe - \mu e\| \le \theta\mu\}N2​(θ)={(x,λ,s)∈F∘:∥XSe−μe∥≤θμ}, where XSeXSeXSe is the vector of products xisix_is_ixi​si​ and ∥⋅∥\|\cdot\|∥⋅∥ is Euclidean.

The affine-scaling direction at (x,λ,s)(x,\lambda,s)(x,λ,s) is any solution of

ATΔλ+Δs=0,AΔx=0,siΔxi+xiΔsi=−xisi (i=1,…,n).A^{\mathsf T}\Delta\lambda + \Delta s = 0,\qquad A\Delta x = 0,\qquad s_i\Delta x_i + x_i\Delta s_i = -x_is_i\ (i = 1,\dots,n).ATΔλ+Δs=0,AΔx=0,si​Δxi​+xi​Δsi​=−xi​si​ (i=1,…,n).

The partition estimate at (x,s)(x,s)(x,s) is B(x,s)={i:xi≥si}\mathcal B(x,s) = \{i : x_i \ge s_i\}B(x,s)={i:xi​≥si​}, N(x,s)={1,…,n}∖B(x,s)\mathcal N(x,s) = \{1,\dots,n\}\setminus\mathcal B(x,s)N(x,s)={1,…,n}∖B(x,s), and the projection problem of Ye's procedure is

min⁡(x∗,λ∗,s∗) 12∥x∗−x∥2+12∥s∗−s∥2  s.t.  Ax∗=b, ATλ∗+s∗=c, xN(x,s)∗=0, sB(x,s)∗=0.\min_{(x^*,\lambda^*,s^*)}\ \tfrac12\|x^*-x\|^2 + \tfrac12\|s^*-s\|^2\ \text{ s.t. }\ Ax^* = b,\ A^{\mathsf T}\lambda^* + s^* = c,\ x^*_{\mathcal N(x,s)} = 0,\ s^*_{\mathcal B(x,s)} = 0.(x∗,λ∗,s∗)min​ 21​∥x∗−x∥2+21​∥s∗−s∥2  s.t.  Ax∗=b, ATλ∗+s∗=c, xN(x,s)∗​=0, sB(x,s)∗​=0.

Formalization targets

Goal: Theorem 7.12

For γ∈(0,1)\gamma \in (0,1)γ∈(0,1) there is a threshold μˉ>0\bar\mu > 0μˉ​>0 such that every (x,λ,s)∈N−∞(γ)(x,\lambda,s) \in \mathcal N_{-\infty}(\gamma)(x,λ,s)∈N−∞​(γ) with

0<μ≤μˉ0 < \mu \le \bar\mu0<μ≤μˉ​

satisfies (i) B(x,s)=B\mathcal B(x,s) = \mathcal BB(x,s)=B and N(x,s)=N\mathcal N(x,s) = \mathcal NN(x,s)=N, and (ii) the projection problem has a solution, and every solution is a strictly complementary solution. The threshold is existential, as printed; the goal does not fix its value.

Milestones on the way

  1. Lemma 5.13: for μ0>0\mu_0 > 0μ0​>0 there is C0C_0C0​ with ∥(x,s)∥≤C0\|(x,s)\| \le C_0∥(x,s)∥≤C0​ on N−∞(γ)∩{μ≤μ0}\mathcal N_{-\infty}(\gamma) \cap \{\mu \le \mu_0\}N−∞​(γ)∩{μ≤μ0​}, and there 0<xi≤μ/C30 < x_i \le \mu/C_30<xi​≤μ/C3​ (i∈Ni \in \mathcal Ni∈N), 0<si≤μ/C30 < s_i \le \mu/C_30<si​≤μ/C3​ (i∈Bi \in \mathcal Bi∈B), si≥C3γs_i \ge C_3\gammasi​≥C3​γ (i∈Ni \in \mathcal Ni∈N), xi≥C3γx_i \ge C_3\gammaxi​≥C3​γ (i∈Bi \in \mathcal Bi∈B).
  2. Lemma A.3 (Hoffman): for matrices G,HG, HG,H there is L≥0L \ge 0L≥0 such that for every (g,h)∈Range⁡([G;H])(g,h) \in \operatorname{Range}([G;H])(g,h)∈Range([G;H]) the system Gw≤gGw \le gGw≤g, Hw=hHw = hHw=h has a solution with ∥w∥≤L∥(g,h)∥\|w\| \le L\|(g,h)\|∥w∥≤L∥(g,h)∥.

Further results on the same objects

  1. Lemma 7.1: for the affine-scaling direction on N−∞(γ)\mathcal N_{-\infty}(\gamma)N−∞​(γ), ∥ΔxN∥≤C4μ\|\Delta x_{\mathcal N}\| \le C_4\mu∥ΔxN​∥≤C4​μ and ∥ΔsB∥≤C4μ\|\Delta s_{\mathcal B}\| \le C_4\mu∥ΔsB​∥≤C4​μ with C4=n/(γ1/2C3)C_4 = n/(\gamma^{1/2}C_3)C4​=n/(γ1/2C3​).
  2. Lemma A.4 (Stewart–Todd): for HHH of full row rank, sup⁡Σ∥ΣHT(HΣHT)−1∥\sup_{\Sigma}\|\Sigma H^{\mathsf T}(H\Sigma H^{\mathsf T})^{-1}\|supΣ​∥ΣHT(HΣHT)−1∥ over positive diagonal Σ\SigmaΣ is finite.
  3. Lemma 7.2: there is C^≥0\hat C \ge 0C^≥0 depending only on HHH such that for h∈Range⁡(H)h \in \operatorname{Range}(H)h∈Range(H) and nonsingular diagonal Σ\SigmaΣ, the unique solution of min⁡12∥Σw∥2\min\frac12\|\Sigma w\|^2min21​∥Σw∥2 s.t. Hw=hHw = hHw=h has norm at most C^∥h∥\hat C\|h\|C^∥h∥.
  4. Theorem 7.4: there is C5C_5C5​, depending only on A,b,c,γA, b, c, \gammaA,b,c,γ, with ∥(Δx,Δs)∥≤C5μ\|(\Delta x,\Delta s)\| \le C_5\mu∥(Δx,Δs)∥≤C5​μ for the affine-scaling direction at every point of N−∞(γ)\mathcal N_{-\infty}(\gamma)N−∞​(γ).
  5. Lemma 7.6: with C5C_5C5​ from Theorem 7.4 at γ=0.5\gamma = 0.5γ=0.5, from N2(0.25)\mathcal N_2(0.25)N2​(0.25) the affine-scaling step stays in N2(0.5)\mathcal N_2(0.5)N2​(0.5) for every α∈[0,1−4C52μ]\alpha \in [0, 1 - 4C_5^2\mu]α∈[0,1−4C52​μ], so the largest such step satisfies α≥1−4C52μ\alpha \ge 1 - 4C_5^2\muα≥1−4C52​μ.

Significance

Theorem 7.12 turns any path-following method into a finite algorithm: once μ\muμ falls below μˉ\bar\muμˉ​, one linear least-squares problem, costing about one interior-point iteration, returns an exact strictly complementary solution, and with it the optimal partition. For integer data the threshold is bounded below by 2−O(L)2^{-O(L)}2−O(L), which is how interior-point methods are made to terminate exactly within their polynomial iteration bounds (Chapter 3 of the book). Theorem 7.4 and Lemma 7.6 give the competing route — the affine-scaling step is O(μ)O(\mu)O(μ), so predictor steps approach full length and the predictor-corrector method converges two-step quadratically.

None of these results is formalized on the platform. Hoffman's error bound exists there for a single consistent system with a constant chosen after the right-hand side; Lemma A.3 and its variant need one constant for all right-hand sides. The optimal partition, ϵ(A,b,c)\epsilon(A,b,c)ϵ(A,b,c), the uniform weighted least-squares bound and the projection step are reusable for any formal treatment of interior-point methods, degenerate linear programs or error bounds.

Difficulty

Near a degenerate solution set the linear systems defining the affine-scaling step become arbitrarily ill-conditioned: the diagonal scaling X1/2S−1/2X^{1/2}S^{-1/2}X1/2S−1/2 has entries tending to 000 and to ∞\infty∞, and the coefficient matrix has no nonsingular limit unless the solution is unique. Any bound that passes through the conditioning of that matrix degrades as μ→0\mu \to 0μ→0; the constants of Theorem 7.4 and Lemma 7.2 must be independent of the scaling. For Theorem 7.12 the difficulty is uniformity in the other direction: the threshold must be one number for the whole neighborhood, which needs a bound on the distance from an iterate to a face of the feasible set that does not depend on the iterate.

Formalization scope

Vectors are Fin n → ℝ, matrices Matrix (Fin m) (Fin n) ℝ, points are triples (x, lam, s); the Euclidean norm is defined explicitly (Mathlib's default on Fin n → ℝ is the sup norm). Directions and the projection problem are defined by their equations and optimality, never by closed-form formulas. ϵ(A,b,c)\epsilon(A,b,c)ϵ(A,b,c) uses real suprema, which agree with the book's whenever F∘≠∅\mathcal F^\circ \ne \emptysetF∘=∅; every theorem is vacuous otherwise. No rank assumption on AAA is made except in Lemma A.4, where the page states it. Ω≠∅\Omega \ne \emptysetΩ=∅ is not a hypothesis: membership in a neighborhood implies it.

Explicit constants: the book writes "there is a constant" in Lemmas 5.13 and 7.1; their proofs give C3=ϵ(A,b,c)/nC_3 = \epsilon(A,b,c)/nC3​=ϵ(A,b,c)/n (5.32) and C4=n/(γ1/2C3)C_4 = n/(\gamma^{1/2}C_3)C4​=n/(γ1/2C3​) (7.6), which are used. The constants C0C_0C0​, LLL, C^\hat CC^, C5C_5C5​, μˉ\bar\muμˉ​ have no closed form in the book (they come from Lemma 2.5, Hoffman's lemma and Lemma 7.2) and stay existential, but each is quantified before the point, the right-hand side or the scaling it must work for; a constant chosen after the point would make Theorem 7.4 trivial. A trivializing form of the goal — a threshold allowed to be 000, or a conclusion about some minimizer chosen by the solver — is ruled out by requiring μˉ>0\bar\mu > 0μˉ​>0, existence of a minimizer, and strict complementarity of every minimizer (λ∗\lambda^*λ∗ is not unique). In Lemma 7.6 the largest step is a hypothesis (IsGreatest), and the statement also asserts that the whole interval [0,1−4C52μ][0, 1-4C_5^2\mu][0,1−4C52​μ] is admissible.

Beyond the paper: one variant is posed. lemma_A_3_strong (strengthened, finding F1) is Lemma A.3 for every consistent system Gw≤gGw \le gGw≤g, Hw=hHw = hHw=h instead of (g,h)∈Range⁡([G;H])(g,h) \in \operatorname{Range}([G;H])(g,h)∈Range([G;H]); this is Hoffman's 1952 theorem. Finding F2 records that the printed proof of Theorem 7.4 covers only full-row-rank AAA; the statement is kept as printed.

Needed infrastructure: LP optimal solution sets and the optimal partition, matrix inverses and ranks, finite sums. A uniform Hoffman bound and the Stewart–Todd bound are reusable well beyond this mission; contributions of either are welcome.

Selected references

  • S. J. Wright, Primal-Dual Interior-Point Methods, SIAM, Philadelphia, 1997. https://doi.org/10.1137/1.9781611971453
  • Y. Ye, On the finite convergence of interior-point algorithms for linear programming, Mathematical Programming 57 (1992), 325–335. https://doi.org/10.1007/BF01581087
  • Y. Ye, O. Güler, R. A. Tapia, Y. Zhang, A quadratically convergent O(nL)O(\sqrt n L)O(n​L)-iteration algorithm for linear programming, Mathematical Programming 59 (1993), 151–162. https://doi.org/10.1007/BF01581242
  • A. J. Hoffman, On approximate solutions of systems of linear inequalities, Journal of Research of the National Bureau of Standards 49 (1952), 263–265. https://doi.org/10.6028/jres.049.027
  • G. W. Stewart, On scaled projections and pseudoinverses, Linear Algebra and its Applications 112 (1989), 189–193. https://doi.org/10.1016/0024-3795(89)90594-6
  • M. J. Todd, A Dantzig–Wolfe-like variant of Karmarkar's interior-point linear programming algorithm, Operations Research 38 (1990), 1006–1018. https://doi.org/10.1287/opre.38.6.1006
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Primal-Dual Interior-Point Methods VI: The Simplified Homogeneous Self-Dual Form Decides Whether an LP Has a SolutionTextbook

Motivation

Interior-point methods for linear programming are usually analysed under the assumption that the primal-dual pair has a strictly feasible point, or at least a solution. A solver used in practice cannot assume this: it must either return a solution or report that the primal or the dual problem is infeasible. The homogeneous self-dual approach embeds the linear program into one larger problem that always has a solution and reads the answer off that solution. Ye, Todd and Mizuno introduced the HSD form (Ye, Todd, Mizuno 1994); Xu, Hung and Ye simplified it (Xu, Hung, Ye 1996). The approach is used in production interior-point codes. This mission formalizes Chapter 9 of Wright's textbook (Wright 1997), together with the parts of Appendix A on which its proofs rest.

Setting

Let A∈Rm×nA \in \mathbb R^{m\times n}A∈Rm×n, b∈Rmb \in \mathbb R^mb∈Rm, c∈Rnc \in \mathbb R^nc∈Rn. No assumption is made on the rank of AAA. Vector inequalities are componentwise. The primal-dual pair is

min⁡cTx  s.t. Ax=b, x≥0,(9.3a)max⁡bTλ  s.t. ATλ+s=c, s≥0.(9.3b)\min c^{\mathsf T}x \ \text{ s.t. } Ax = b,\ x \ge 0, \qquad (9.3a) \qquad\qquad \max b^{\mathsf T}\lambda \ \text{ s.t. } A^{\mathsf T}\lambda + s = c,\ s \ge 0. \qquad (9.3b)mincTx  s.t. Ax=b, x≥0,(9.3a)maxbTλ  s.t. ATλ+s=c, s≥0.(9.3b)

The primal solution set ΩP\Omega_PΩP​ consists of the feasible xxx that attain the minimum of cTxc^{\mathsf T}xcTx. The dual solution set ΩD\Omega_DΩD​ consists of the feasible (λ,s)(\lambda, s)(λ,s) that attain the maximum of bTλb^{\mathsf T}\lambdabTλ. The primal-dual solution set is Ω=ΩP×ΩD\Omega = \Omega_P \times \Omega_DΩ=ΩP​×ΩD​.

The simplified HSD form introduces scalars τ,κ\tau, \kappaτ,κ and asks for (τ,x,λ,κ,s)(\tau, x, \lambda, \kappa, s)(τ,x,λ,κ,s) with

[κs0]=[0−cTbTc0−AT−bA0][τxλ],(τ,x,κ,s)≥0,τκ+xTs=0.(9.5)\begin{bmatrix}\kappa\\ s\\ 0\end{bmatrix} = \begin{bmatrix} 0 & -c^{\mathsf T} & b^{\mathsf T}\\ c & 0 & -A^{\mathsf T}\\ -b & A & 0\end{bmatrix}\begin{bmatrix}\tau\\ x\\ \lambda\end{bmatrix}, \qquad (\tau, x, \kappa, s) \ge 0, \qquad \tau\kappa + x^{\mathsf T}s = 0. \qquad (9.5)​κs0​​=​0c−b​−cT0A​bT−AT0​​​τxλ​​,(τ,x,κ,s)≥0,τκ+xTs=0.(9.5)

Here λ\lambdaλ is free. This is a mixed monotone linear complementarity problem (mLCP) with a skew-symmetric matrix. It always has the zero solution. A solution is strictly complementary if τ+κ>0\tau + \kappa > 0τ+κ>0 and x+s>0x + s > 0x+s>0.

More generally, a self-dual linear program (9.1) has the form min⁡fTu+gTw\min f^{\mathsf T}u + g^{\mathsf T}wminfTu+gTw subject to M11u+M12w≥−fM_{11}u + M_{12}w \ge -fM11​u+M12​w≥−f, −M12Tu+M22w=−g-M_{12}^{\mathsf T}u + M_{22}w = -g−M12T​u+M22​w=−g, u≥0u \ge 0u≥0, where M11M_{11}M11​ and M22M_{22}M22​ are skew-symmetric. Its mLCP (9.2) asks for (u,v,w)(u, v, w)(u,v,w) with v=M11u+M12w+fv = M_{11}u + M_{12}w + fv=M11​u+M12​w+f, 0=−M12Tu+M22w+g0 = -M_{12}^{\mathsf T}u + M_{22}w + g0=−M12T​u+M22​w+g, (u,v)≥0(u, v) \ge 0(u,v)≥0 and uTv=0u^{\mathsf T}v = 0uTv=0. A solution is strictly complementary if u+v>0u + v > 0u+v>0.

The HSD form of Ye, Todd and Mizuno starts from an initial point (x0,λ0,s0)(x^0, \lambda^0, s^0)(x0,λ0,s0) with x0>0x^0 > 0x0>0, s0>0s^0 > 0s0>0. It sets bˉ=b−Ax0\bar b = b - Ax^0bˉ=b−Ax0, cˉ=c−ATλ0−s0\bar c = c - A^{\mathsf T}\lambda^0 - s^0cˉ=c−ATλ0−s0 and zˉ=cTx0+1−bTλ0\bar z = c^{\mathsf T}x^0 + 1 - b^{\mathsf T}\lambda^0zˉ=cTx0+1−bTλ0, and adds a free variable θ\thetaθ. The result is the mLCP (9.8):

[κs00]=[0−cTbTzˉc0−AT−cˉ−bA0bˉ−zˉcˉT−bˉT0][τxλθ]+[000(x0)Ts0+1],(τ,x,κ,s)≥0,τκ+xTs=0.\begin{bmatrix}\kappa\\ s\\ 0\\ 0\end{bmatrix} = \begin{bmatrix} 0 & -c^{\mathsf T} & b^{\mathsf T} & \bar z\\ c & 0 & -A^{\mathsf T} & -\bar c\\ -b & A & 0 & \bar b\\ -\bar z & \bar c^{\mathsf T} & -\bar b^{\mathsf T} & 0\end{bmatrix}\begin{bmatrix}\tau\\ x\\ \lambda\\ \theta\end{bmatrix} + \begin{bmatrix}0\\0\\0\\ (x^0)^{\mathsf T}s^0 + 1\end{bmatrix}, \qquad (\tau, x, \kappa, s) \ge 0, \quad \tau\kappa + x^{\mathsf T}s = 0.​κs00​​=​0c−b−zˉ​−cT0AcˉT​bT−AT0−bˉT​zˉ−cˉbˉ0​​​τxλθ​​+​000(x0)Ts0+1​​,(τ,x,κ,s)≥0,τκ+xTs=0.

Formalization targets

Goal: Theorem 9.2

Ω≠∅  ⟺  every strictly complementary solution (τ∗,x∗,λ∗,κ∗,s∗) of (9.5) has τ∗>0 and κ∗=0.\Omega \ne \emptyset \iff \text{every strictly complementary solution } (\tau^*, x^*, \lambda^*, \kappa^*, s^*) \text{ of (9.5) has } \tau^* > 0 \text{ and } \kappa^* = 0.Ω=∅⟺every strictly complementary solution (τ∗,x∗,λ∗,κ∗,s∗) of (9.5) has τ∗>0 and κ∗=0.

The right-hand side quantifies over all strictly complementary solutions. It is not vacuous, because such solutions always exist.

Milestones

In attack order:

  1. Theorem A.1. The KKT conditions are necessary at a local solution of a linearly constrained problem.
  2. Theorem A.2. The KKT conditions are sufficient for convex programs, and under strict convexity a local solution is the unique global one.
  3. Theorem 2.4 (Goldman–Tucker). A solvable LP pair has a strictly complementary solution.
  4. Theorem 9.1. If a self-dual linear program is feasible, its mLCP (9.2) has a strictly complementary solution.
  5. Theorem 9.3. If (9.5) has a strictly complementary solution with κ∗>0\kappa^* > 0κ∗>0, then cTx∗<0c^{\mathsf T}x^* < 0cTx∗<0 or −bTλ∗<0-b^{\mathsf T}\lambda^* < 0−bTλ∗<0. The first case shows that (9.3b) is infeasible; the second shows that (9.3a) is infeasible.
  6. Theorem 9.4. Every solution of (9.8) has θ∗=0\theta^* = 0θ∗=0.
  7. Theorems 9.5 and 9.6. These are the analogues of Theorems 9.2 and 9.3 for the HSD form (9.8).

Significance

Theorems 9.2 and 9.3 make the simplified HSD form a complete decision procedure for linear programming. Any strictly complementary solution of (9.5) has one of two forms. Either τ∗>0\tau^* > 0τ∗>0, and then (x∗,λ∗,s∗)/τ∗(x^*, \lambda^*, s^*)/\tau^*(x∗,λ∗,s∗)/τ∗ solves the original pair. Or κ∗>0\kappa^* > 0κ∗>0, and then the solution certifies that the primal or the dual problem is infeasible. Path-following methods converge to strictly complementary points. An algorithm applied to a problem that always has a solution therefore settles the original problem, without any feasibility assumption. Theorems 9.4–9.6 transfer this to the HSD form, which also admits the strictly feasible starting point (τ,x,λ,θ,κ,s)=(1,x0,λ0,1,1,s0)(\tau, x, \lambda, \theta, \kappa, s) = (1, x^0, \lambda^0, 1, 1, s^0)(τ,x,λ,θ,κ,s)=(1,x0,λ0,1,1,s0) (p. 184) and so can be solved by a feasible interior-point method.

On the formalization side, nothing about the simplified HSD form is on the platform yet. The Ye–Todd–Mizuno form appears in other missions as open statements about a single strictly self-complementary solution, under a hypothesis of the form τ∗=0\tau^* = 0τ∗=0. Theorems 9.5 and 9.6 here instead quantify over all strictly complementary solutions and use the hypothesis κ∗>0\kappa^* > 0κ∗>0, so they are separate statements. Appendix A's Theorem A.1 is the classical KKT theorem for linear constraints, and Theorem 9.1 is a structural fact about skew-symmetric complementarity problems. Both can be reused beyond this chapter.

Difficulty

The "if" direction of Theorem 9.2 is short once strictly complementary solutions of (9.5) are known to exist. That existence is Theorem 9.1, which is the substance of the chapter. Monotone LCPs in general need not have strictly complementary solutions. A solver has to show that the self-dual structure rules out the failure seen for general monotone LCPs, for a problem in a nonstandard form with free variables and equality rows.

The "only if" direction needs more than one well-chosen solution. It needs the fact that all strictly complementary solutions of (9.5) have the same support. The book asserts this without proof (finding F3 in FINDINGS.yaml), so a solver must supply it. Theorem 9.5 also requires a strictly complementary solution of (9.8) with τ∗>0\tau^* > 0τ∗>0 whenever Ω≠∅\Omega \ne \emptysetΩ=∅. Here the normalisation row of (9.8) has to be satisfied, which the simplified form does not ask for.

Formalization scope

Vectors are Fin n → ℝ and matrices are Matrix (Fin m) (Fin n) ℝ; mmm and nnn may be zero. The definitions file WrightPDIPM.SelfDual.Setup writes out the systems (9.2), (9.5) and (9.8) row by row, exactly as printed, together with their solution and strict-complementarity predicates. ΩP\Omega_PΩP​ and ΩD\Omega_DΩD​ are defined by optimality against every feasible point, with no infimum, so they are empty when the optimum is not attained. The gradient in Appendix A is the vector of Fréchet partial derivatives, and every statement that uses it assumes differentiability. The Euclidean norm is defined explicitly.

Several standing hypotheses of the book are made explicit:

  • Ω≠∅\Omega \ne \emptysetΩ=∅ for Goldman–Tucker. The page states the theorem without it, but its section presupposes it, and it is false without it.
  • x0>0x^0 > 0x0>0 and s0>0s^0 > 0s0>0 for Theorems 9.4–9.6 (p. 183).
  • u∗∈Uu^* \in Uu∗∈U and differentiability at u∗u^*u∗ in Theorem A.2(i).

The page writes z∗z^*z∗ for s∗s^*s∗ in Theorem 9.6. This is a misprint, and the formal statement uses sss. No statement in this selection carries an O(⋅)O(\cdot)O(⋅) constant.

Theorem 9.2 must not be formalized with "strictly complementary" weakened to "solution", since the zero solution would then falsify the right-hand side. Its right-hand side must not be read as an existence claim either. The statement keeps the book's universal quantifier over strictly complementary solutions.

Beyond the paper. Two variants are posed beside the faithful statements. theorem_9_3_strong (strengthened, finding F1) is Theorem 9.3 for every solution of (9.5) with κ∗>0\kappa^* > 0κ∗>0, since the book's proof never uses strict complementarity. theorem_9_6_strong (strengthened, finding F2) is the same strengthening of Theorem 9.6, via Theorem 9.4.

A complete development needs:

  • duality theory for standard-form LPs (strong duality and Goldman–Tucker);
  • the KKT theorem for linear constraints;
  • elementary linear algebra on skew-symmetric matrices.

The KKT and Goldman–Tucker statements, and the mLCP definitions, are reusable for the LCP material of Chapter 8. Contributions of any milestone are welcome. Theorems 9.7–9.8, about Algorithm IPF diverging on infeasible problems, are not included.

Selected references

  • S. J. Wright, Primal-Dual Interior-Point Methods, SIAM, Philadelphia, 1997. doi:10.1137/1.9781611971453
  • Y. Ye, M. J. Todd, S. Mizuno, An O(nL)O(\sqrt{n}L)O(n​L)-iteration homogeneous and self-dual linear programming algorithm, Mathematics of Operations Research 19 (1994) 53–67. doi:10.1287/moor.19.1.53
  • X. Xu, P.-F. Hung, Y. Ye, A simplified homogeneous and self-dual linear programming algorithm and its implementation, Annals of Operations Research 62 (1996) 151–171. doi:10.1007/BF02206815
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Primal-Dual Interior-Point Methods IV: The Infeasible Path-Following Method Drives μ to Zero Q-LinearlyTextbook

Motivation

Primal-dual interior-point methods solve a linear program by following the central path from a point with strictly positive primal variables xxx and dual slacks sss. The classical analysis assumes that the starting point is feasible: it already satisfies the equality constraints Ax=bAx = bAx=b and ATλ+s=cA^{\mathsf T}\lambda + s = cATλ+s=c. In practice such a point is rarely available, and the production codes of the early 1990s (OB1, and later the solvers built on Mehrotra's method) instead start from an arbitrary positive point and let the Newton steps reduce the constraint violation together with the duality gap. These infeasible-interior-point methods worked well long before they were understood. Global convergence was first proved by Kojima, Megiddo and Mizuno (Math. Programming 61, 1993), and polynomial complexity of a long-step variant by Zhang (SIAM J. Optim. 4, 1994).

Chapter 6 of Stephen J. Wright's Primal-Dual Interior-Point Methods (SIAM 1997) gives a self-contained account of this theory for one representative long-step method, Algorithm IPF. This mission formalizes the statements of that chapter.

Setting

Let A∈Rm×nA \in \mathbb{R}^{m\times n}A∈Rm×n (no rank assumption), b∈Rmb \in \mathbb{R}^mb∈Rm, c∈Rnc \in \mathbb{R}^nc∈Rn, and consider the primal min⁡cTx\min c^{\mathsf T}xmincTx s.t. Ax=bAx = bAx=b, x≥0x \ge 0x≥0 and the dual max⁡bTλ\max b^{\mathsf T}\lambdamaxbTλ s.t. ATλ+s=cA^{\mathsf T}\lambda + s = cATλ+s=c, s≥0s \ge 0s≥0. The solution set Ω\OmegaΩ consists of the triples (x,λ,s)(x,\lambda,s)(x,λ,s) with Ax=bAx = bAx=b, ATλ+s=cA^{\mathsf T}\lambda + s = cATλ+s=c, (x,s)≥0(x,s) \ge 0(x,s)≥0 and xisi=0x_is_i = 0xi​si​=0 for all iii. For a triple (x,λ,s)(x,\lambda,s)(x,λ,s) the residuals are rb=Ax−br_b = Ax - brb​=Ax−b and rc=ATλ+s−cr_c = A^{\mathsf T}\lambda + s - crc​=ATλ+s−c, and the duality measure is μ=xTs/n\mu = x^{\mathsf T}s/nμ=xTs/n. Norms are Euclidean unless marked.

Fix a starting point (x0,λ0,s0)(x^0,\lambda^0,s^0)(x0,λ0,s0) with (x0,s0)>0(x^0,s^0) > 0(x0,s0)>0, with residuals (rb0,rc0)(r_b^0, r_c^0)(rb0​,rc0​) and duality measure μ0\mu_0μ0​. For γ∈(0,1)\gamma \in (0,1)γ∈(0,1) and β≥1\beta \ge 1β≥1, the neighborhood is

N−∞(γ,β)={(x,λ,s):∥(rb,rc)∥≤∥(rb0,rc0)∥μ0 βμ, (x,s)>0, xisi≥γμ (i=1,…,n)}.\mathcal N_{-\infty}(\gamma,\beta) = \Big\{(x,\lambda,s) : \|(r_b,r_c)\| \le \frac{\|(r_b^0,r_c^0)\|}{\mu_0}\,\beta\mu,\ (x,s) > 0,\ x_is_i \ge \gamma\mu \ (i = 1,\dots,n)\Big\}.N−∞​(γ,β)={(x,λ,s):∥(rb​,rc​)∥≤μ0​∥(rb0​,rc0​)∥​βμ, (x,s)>0, xi​si​≥γμ (i=1,…,n)}.

Algorithm IPF has parameters γ,β\gamma, \betaγ,β and 0<σmin⁡<σmax⁡≤0.50 < \sigma_{\min} < \sigma_{\max} \le 0.50<σmin​<σmax​≤0.5. At iterate kkk it picks σk∈[σmin⁡,σmax⁡]\sigma_k \in [\sigma_{\min},\sigma_{\max}]σk​∈[σmin​,σmax​] and solves the linear system

ATΔλk+Δsk=−rck,AΔxk=−rbk,SkΔxk+XkΔsk=−XkSke+σkμke,A^{\mathsf T}\Delta\lambda^k + \Delta s^k = -r_c^k, \qquad A\Delta x^k = -r_b^k, \qquad S^k\Delta x^k + X^k\Delta s^k = -X^kS^ke + \sigma_k\mu_k e,ATΔλk+Δsk=−rck​,AΔxk=−rbk​,SkΔxk+XkΔsk=−XkSke+σk​μk​e,

where Xk=diag⁡(xk)X^k = \operatorname{diag}(x^k)Xk=diag(xk), Sk=diag⁡(sk)S^k = \operatorname{diag}(s^k)Sk=diag(sk) and e=(1,…,1)Te = (1,\dots,1)^{\mathsf T}e=(1,…,1)T. It takes the largest αk∈[0,1]\alpha_k \in [0,1]αk​∈[0,1] such that the point (xk,λk,sk)+αk(Δxk,Δλk,Δsk)(x^k,\lambda^k,s^k) + \alpha_k(\Delta x^k,\Delta\lambda^k,\Delta s^k)(xk,λk,sk)+αk​(Δxk,Δλk,Δsk) lies in N−∞(γ,β)\mathcal N_{-\infty}(\gamma,\beta)N−∞​(γ,β) and μ\muμ there is at most (1−0.01αk)μk(1 - 0.01\alpha_k)\mu_k(1−0.01αk​)μk​. That point is the next iterate. The analysis uses νk=∏j<k(1−αj)\nu_k = \prod_{j<k}(1-\alpha_j)νk​=∏j<k​(1−αj​) and the scaling Dk=(Xk)1/2(Sk)−1/2D^k = (X^k)^{1/2}(S^k)^{-1/2}Dk=(Xk)1/2(Sk)−1/2.

A positive sequence βk→0\beta_k \to 0βk​→0 converges Q-linearly if βk+1/βk≤ρ\beta_{k+1}/\beta_k \le \rhoβk+1​/βk​≤ρ for some ρ∈(0,1)\rho \in (0,1)ρ∈(0,1) and all large kkk. It converges R-linearly if it is dominated by a Q-linearly convergent sequence.

Formalization targets

Goal: Theorem 6.1

If Ω≠∅\Omega \neq \emptysetΩ=∅ and the start is infeasible, (rb0,rc0)≠0(r_b^0,r_c^0) \neq 0(rb0​,rc0​)=0, then along Algorithm IPF

{μk}→0 Q-linearly,{∥(rbk,rck)∥}→0 R-linearly.\{\mu_k\} \to 0 \ \text{Q-linearly}, \qquad \{\|(r_b^k,r_c^k)\|\} \to 0 \ \text{R-linearly}.{μk​}→0 Q-linearly,{∥(rbk​,rck​)∥}→0 R-linearly.

The statement fixes no rate. It asserts only the shape of the convergence, so it survives any improvement of the constants.

Milestones

  1. Lemma 6.3: νk∥(xk,sk)∥1≤C1μk\nu_k\|(x^k,s^k)\|_1 \le C_1\mu_kνk​∥(xk,sk)∥1​≤C1​μk​ for a constant C1>0C_1 > 0C1​>0 independent of kkk.
  2. Lemma 6.4: for the start (ζe,0,ζe)(\zeta e, 0, \zeta e)(ζe,0,ζe) with ∥(x∗,s∗)∥∞≤ζ\|(x^*,s^*)\|_\infty \le \zeta∥(x∗,s∗)∥∞​≤ζ for some solution, ζνk∥(xk,sk)∥1≤4βnμk\zeta\nu_k\|(x^k,s^k)\|_1 \le 4\beta n\mu_kζνk​∥(xk,sk)∥1​≤4βnμk​.
  3. Lemma 6.5: ∥(Dk)−1Δxk∥, ∥DkΔsk∥≤C2μk1/2\|(D^k)^{-1}\Delta x^k\|,\ \|D^k\Delta s^k\| \le C_2\mu_k^{1/2}∥(Dk)−1Δxk∥, ∥DkΔsk∥≤C2​μk1/2​ for a constant C2>0C_2 > 0C2​>0 independent of kkk.
  4. Lemma 6.6: for the same special start, the same bounds hold with C2=ωnC_2 = \omega nC2​=ωn, ω=9β/γ1/2\omega = 9\beta/\gamma^{1/2}ω=9β/γ1/2.
  5. Lemma 6.7: there is αˉ∈(0,1)\bar\alpha \in (0,1)αˉ∈(0,1) such that every α∈[0,αˉ]\alpha \in [0,\bar\alpha]α∈[0,αˉ] satisfies the three inequalities (6.37) and both step-length conditions at every iterate.
  6. Lemma 6.7, (6.38): for the special start, αˉ≥δˉ/n2\bar\alpha \ge \bar\delta/n^2αˉ≥δˉ/n2 with δˉ>0\bar\delta > 0δˉ>0 independent of nnn.
  7. Theorem 6.8: eventually xik,sjk≤μk/C3x^k_i, s^k_j \le \mu_k/C_3xik​,sjk​≤μk​/C3​ and sik,xjk≥C3γs^k_i, x^k_j \ge C_3\gammasik​,xjk​≥C3​γ (i∈Ni \in \mathcal Ni∈N, j∈Bj \in \mathcal Bj∈B), and every limit point of (xk,sk)(x^k,s^k)(xk,sk) gives a strictly complementary solution.
  8. Theorem 6.9: if a strictly feasible point exists, {(xk,sk)}\{(x^k,s^k)\}{(xk,sk)} is bounded and has a limit point.

Significance

Theorem 6.1 is the reason infeasible starts are safe. A practitioner may start from any positive point. If the problem has a solution, the method then removes the infeasibility and the duality gap at a linear rate, without a Phase I. Lemma 6.7 is the core estimate: one uniform lower bound on the step length gives both rates, and its nnn-explicit form (6.38) gives the O(n2∣log⁡ϵ∣)O(n^2|\log\epsilon|)O(n2∣logϵ∣) complexity of Theorem 6.2. Theorems 6.8 and 6.9 show that the iterates identify the optimal partition B∪N\mathcal B \cup \mathcal NB∪N. This is the basis of the finite-termination procedures of the next chapter.

These results are proved in the literature and in the book. They have not been machine-checked: the Prove2Me corpus contains a formalized convergence theorem for Vanderbei's path-following method under a boundedness hypothesis, but no infeasible-interior-point theory with the neighborhood N−∞(γ,β)\mathcal N_{-\infty}(\gamma,\beta)N−∞​(γ,β). The mission would produce the first checked proof of global linear convergence for an infeasible primal-dual method. The definitions it introduces (the algorithm as a predicate on sequences, Q-/R-linear convergence) can be reused for the self-dual and termination chapters.

Difficulty

The obvious argument for a feasible method bounds the second-order term ΔxikΔsik\Delta x^k_i\Delta s^k_iΔxik​Δsik​ by a multiple of μk\mu_kμk​ through the orthogonality (Δxk)TΔsk=0(\Delta x^k)^{\mathsf T}\Delta s^k = 0(Δxk)TΔsk=0. With infeasible iterates this orthogonality fails, and the cross terms involve the iterates (xk,sk)(x^k, s^k)(xk,sk) themselves, which are not bounded a priori. The step from "the residuals shrink" to "the step length stays bounded below" therefore needs a bound on νk∥(xk,sk)∥1\nu_k\|(x^k,s^k)\|_1νk​∥(xk,sk)∥1​ in terms of μk\mu_kμk​. That bound has to come from a solution of the linear program. The hypothesis Ω≠∅\Omega \neq \emptysetΩ=∅ is essential. The page prints Theorem 6.1 without it, but on the infeasible program A=(1)A = (1)A=(1), b=−1b = -1b=−1, c=0c = 0c=0 the neighborhood condition keeps μk\mu_kμk​ bounded away from zero. A solver should also note that the explicit αˉ\bar\alphaαˉ displayed in the proof of Lemma 6.7 drops a factor 1/21/21/2 against the bound (6.47) derived just before it, and can equal 111. Only the existential statement survives as printed. The bound (6.38) is left to the reader in the book (Exercise 4).

Formalization scope

Vectors are Fin n → ℝ, matrices Matrix (Fin m) (Fin n) ℝ. Euclidean norms and the 1-norm ∥(x,s)∥1=∑∣xi∣+∑∣si∣\|(x,s)\|_1 = \sum|x_i| + \sum|s_i|∥(x,s)∥1​=∑∣xi​∣+∑∣si​∣ are defined explicitly, because Mathlib's default norm on Fin n → ℝ is the sup norm. "Generated by Algorithm IPF" is a predicate on the sequences (xk,λk,sk)(x^k,\lambda^k,s^k)(xk,λk,sk), σk\sigma_kσk​, αk\alpha_kαk​ and the directions. The direction is given by its linear equations, not by a closed form, and "the largest α\alphaα" is an IsGreatest hypothesis on the admissible set rather than a constructed maximum. The neighborhood is anchored at the run's own start. Every statement carries the hypotheses the chapter states once and then omits: n≥1n \ge 1n≥1; (rb0,rc0)≠0(r_b^0, r_c^0) \neq 0(rb0​,rc0​)=0 (p. 111); Ω≠∅\Omega \neq \emptysetΩ=∅ where the proof starts from a solution (pp. 127, 185); and, for Lemma 6.7, a start inside N−∞(γ,β)\mathcal N_{-\infty}(\gamma,\beta)N−∞​(γ,β) (pp. 109, 120). Constants C1C_1C1​, C2C_2C2​, αˉ\bar\alphaαˉ, C3C_3C3​ are positive and quantified before the iteration index. δˉ\bar\deltaδˉ in (6.38) is quantified after the algorithm's parameters and before the dimension and the instance. That is how "independent of nnn" is rendered.

Explicit constants. Lemma 6.6's "a constant ω\omegaω independent of nnn" is stated with the value its proof sets, ω=9β/γ1/2\omega = 9\beta/\gamma^{1/2}ω=9β/γ1/2. The book gives no explicit δˉ\bar\deltaδˉ for (6.38), and none is invented. Theorem 6.2 (K=O(n2∣log⁡ϵ∣)K = O(n^2|\log\epsilon|)K=O(n2∣logϵ∣)) is not posed, because an explicit iteration count would rest on that unproved exercise.

A trivializing formalization is ruled out. The Q-linear predicate requires μk>0\mu_k > 0μk​>0 and μk→0\mu_k \to 0μk​→0, not only a ratio bound, and R-linear convergence is domination by such a sequence. C3>0C_3 > 0C3​>0 is explicit, since division by zero would make (6.49) vacuous. No hypothesis beyond the book's is added to make a run easier to exhibit, and the greatest step length is required to exist exactly as the algorithm prescribes.

Beyond the paper. One sharpness probe accompanies the goal: Theorem 6.1 with Ω≠∅\Omega \neq \emptysetΩ=∅ removed, expected to be false by the counterexample above; a disproof is the sought outcome.

Infrastructure that would help: the identity (rbk,rck)=νk(rb0,rc0)(r_b^k,r_c^k) = \nu_k(r_b^0,r_c^0)(rbk​,rck​)=νk​(rb0​,rc0​), orthogonality of xˉ\bar xxˉ and sˉ\bar ssˉ when Axˉ=0A\bar x = 0Axˉ=0 and ATλˉ+sˉ=0A^{\mathsf T}\bar\lambda + \bar s = 0ATλˉ+sˉ=0, the Goldman–Tucker theorem (strictly complementary solutions exist; Wright, Theorem 2.4), and Hoffman's bound for Theorem 6.9. Contributions of reusable lemmas on Q-/R-linear sequences are welcome.

Selected references

  • S. J. Wright, Primal-Dual Interior-Point Methods, SIAM, Philadelphia, 1997, Chapter 6 and Appendix A. https://doi.org/10.1137/1.9781611971453
  • M. Kojima, N. Megiddo, S. Mizuno, A primal-dual infeasible-interior-point algorithm for linear programming, Mathematical Programming 61, 1993. https://doi.org/10.1007/BF01582151
  • Y. Zhang, On the convergence of a class of infeasible interior-point methods for the horizontal linear complementarity problem, SIAM Journal on Optimization 4, 1994. https://doi.org/10.1137/0804012
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Primal-Dual Interior-Point Methods III: A Full Short Step Keeps the Iterate in the Neighborhood N₂(θ)Textbook

Why path-following methods are studied

Primal-dual interior-point methods are the standard polynomial-time algorithms for linear programming and the basis of most commercial LP solvers. Among them, path-following methods are the ones with the cleanest complexity theory: they keep every iterate inside a neighborhood of the central path and shrink the duality measure by a fixed factor per step. Chapter 5 of Stephen J. Wright's Primal-Dual Interior-Point Methods (SIAM 1997) presents three such methods — a short-step method, the Mizuno–Todd–Ye predictor-corrector method, and a long-step method — and analyzes each from a small set of one-step estimates.

The short-step method and its O(nlog⁡(1/ϵ))O(\sqrt n\log(1/\epsilon))O(n​log(1/ϵ)) iteration bound go back to Kojima, Mizuno and Yoshise (1989) and Monteiro and Adler (1989); the predictor-corrector method with two nested 2-norm neighborhoods is due to Mizuno, Todd and Ye (Math. Oper. Res. 1993); the long-step method in the wide neighborhood N−∞(γ)\mathcal N_{-\infty}(\gamma)N−∞​(γ) with its O(nlog⁡(1/ϵ))O(n\log(1/\epsilon))O(nlog(1/ϵ)) bound is in the same line of work. Wright's chapter gives a single, uniform treatment of the three, with explicit constants.

Setting

Fix A∈Rm×nA \in \mathbb{R}^{m\times n}A∈Rm×n, b∈Rmb \in \mathbb{R}^mb∈Rm, c∈Rnc \in \mathbb{R}^nc∈Rn, with no assumption on the rank of AAA. The primal problem is min⁡cTx\min c^{\mathsf T}xmincTx subject to Ax=bAx = bAx=b, x≥0x \ge 0x≥0, and the dual is max⁡bTλ\max b^{\mathsf T}\lambdamaxbTλ subject to ATλ+s=cA^{\mathsf T}\lambda + s = cATλ+s=c, s≥0s \ge 0s≥0. A primal-dual point is a triple (x,λ,s)(x, \lambda, s)(x,λ,s). The strictly feasible set F∘\mathcal F^\circF∘ consists of the triples with Ax=bAx = bAx=b, ATλ+s=cA^{\mathsf T}\lambda + s = cATλ+s=c, x>0x > 0x>0 and s>0s > 0s>0 (componentwise).

For vectors x,sx, sx,s write X=diag⁡(x)X = \operatorname{diag}(x)X=diag(x), S=diag⁡(s)S = \operatorname{diag}(s)S=diag(s) and e=(1,…,1)Te = (1,\dots,1)^{\mathsf T}e=(1,…,1)T, so XSeXSeXSe is the vector of pairwise products xisix_i s_ixi​si​. The duality measure is μ=xTs/n\mu = x^{\mathsf T}s/nμ=xTs/n. The 2-norm neighborhood of the central path is

N2(θ)={(x,λ,s)∈F∘:∥XSe−μe∥2≤θμ},θ∈(0,1),\mathcal N_2(\theta) = \{(x,\lambda,s) \in \mathcal F^\circ : \|XSe - \mu e\|_2 \le \theta\mu\}, \qquad \theta \in (0,1),N2​(θ)={(x,λ,s)∈F∘:∥XSe−μe∥2​≤θμ},θ∈(0,1),

and the one-sided ∞\infty∞-norm neighborhood is N−∞(γ)={(x,λ,s)∈F∘:xisi≥γμ for all i}\mathcal N_{-\infty}(\gamma) = \{(x,\lambda,s) \in \mathcal F^\circ : x_i s_i \ge \gamma\mu \text{ for all } i\}N−∞​(γ)={(x,λ,s)∈F∘:xi​si​≥γμ for all i}, γ∈(0,1)\gamma \in (0,1)γ∈(0,1).

Given a centering parameter σ∈[0,1]\sigma \in [0,1]σ∈[0,1], a search direction (Δx,Δλ,Δs)(\Delta x, \Delta\lambda, \Delta s)(Δx,Δλ,Δs) is any solution of the step equations

ATΔλ+Δs=0,AΔx=0,SΔx+XΔs=−XSe+σμe.A^{\mathsf T}\Delta\lambda + \Delta s = 0, \qquad A\Delta x = 0, \qquad S\Delta x + X\Delta s = -XSe + \sigma\mu e .ATΔλ+Δs=0,AΔx=0,SΔx+XΔs=−XSe+σμe.

Along it, (x(α),λ(α),s(α))=(x,λ,s)+α(Δx,Δλ,Δs)(x(\alpha), \lambda(\alpha), s(\alpha)) = (x,\lambda,s) + \alpha(\Delta x,\Delta\lambda,\Delta s)(x(α),λ(α),s(α))=(x,λ,s)+α(Δx,Δλ,Δs) and μ(α)=x(α)Ts(α)/n\mu(\alpha) = x(\alpha)^{\mathsf T}s(\alpha)/nμ(α)=x(α)Ts(α)/n. The vector ΔXΔSe\Delta X\Delta S eΔXΔSe of products ΔxiΔsi\Delta x_i\Delta s_iΔxi​Δsi​ is the second-order term that the linearization ignores.

Formalization targets

Goal: Theorem 5.6

Let θ,σ∈(0,1)\theta, \sigma \in (0,1)θ,σ∈(0,1) satisfy

θ2+n(1−σ)223/2(1−θ)≤σθ.\frac{\theta^2 + n(1-\sigma)^2}{2^{3/2}(1-\theta)} \le \sigma\theta .23/2(1−θ)θ2+n(1−σ)2​≤σθ.

If (x,λ,s)∈N2(θ)(x,\lambda,s) \in \mathcal N_2(\theta)(x,λ,s)∈N2​(θ) and (Δx,Δλ,Δs)(\Delta x,\Delta\lambda,\Delta s)(Δx,Δλ,Δs) solves the step equations with this σ\sigmaσ, then

(x(α),λ(α),s(α))∈N2(θ)for all α∈[0,1].(x(\alpha),\lambda(\alpha),s(\alpha)) \in \mathcal N_2(\theta) \qquad \text{for all } \alpha \in [0,1].(x(α),λ(α),s(α))∈N2​(θ)for all α∈[0,1].

This is a one-step statement with no algorithm loop. It is the invariant of the short-step method (θ=0.4\theta = 0.4θ=0.4, σ=1−0.4/n\sigma = 1 - 0.4/\sqrt nσ=1−0.4/n​).

Milestones on the way

  1. Lemma 5.1: ΔxTΔs=0\Delta x^{\mathsf T}\Delta s = 0ΔxTΔs=0 and μ(α)=(1−α(1−σ))μ\mu(\alpha) = (1 - \alpha(1-\sigma))\muμ(α)=(1−α(1−σ))μ.
  2. Lemma 5.3: for u,v∈Rnu, v \in \mathbb{R}^nu,v∈Rn with uTv≥0u^{\mathsf T}v \ge 0uTv≥0, ∥UVe∥2≤2−3/2∥u+v∥22\|UVe\|_2 \le 2^{-3/2}\|u+v\|_2^2∥UVe∥2​≤2−3/2∥u+v∥22​.
  3. Lemma 5.4: on N2(θ)\mathcal N_2(\theta)N2​(θ), ∥ΔXΔSe∥2≤θ2+n(1−σ)223/2(1−θ)μ\|\Delta X\Delta S e\|_2 \le \frac{\theta^2 + n(1-\sigma)^2}{2^{3/2}(1-\theta)}\mu∥ΔXΔSe∥2​≤23/2(1−θ)θ2+n(1−σ)2​μ.
  4. Lemma 5.5: ∥X(α)S(α)e−μ(α)e∥2≤∣1−α∣ ∥XSe−μe∥2+α2∥ΔXΔSe∥2≤∣1−α∣θμ+α2θ2+n(1−σ)223/2(1−θ)μ\|X(\alpha)S(\alpha)e - \mu(\alpha)e\|_2 \le |1-\alpha|\,\|XSe - \mu e\|_2 + \alpha^2\|\Delta X\Delta S e\|_2 \le |1-\alpha|\theta\mu + \alpha^2\frac{\theta^2 + n(1-\sigma)^2}{2^{3/2}(1-\theta)}\mu∥X(α)S(α)e−μ(α)e∥2​≤∣1−α∣∥XSe−μe∥2​+α2∥ΔXΔSe∥2​≤∣1−α∣θμ+α223/2(1−θ)θ2+n(1−σ)2​μ.

Further results on the same objects

  1. Lemma 5.7 (predictor step): from N2(0.25)\mathcal N_2(0.25)N2​(0.25) with σ=0\sigma = 0σ=0, every α∈[0,αˉ]\alpha \in [0, \bar\alpha]α∈[0,αˉ] stays in N2(0.5)\mathcal N_2(0.5)N2​(0.5), where αˉ=min⁡(1/2,(μ/(8∥ΔXΔSe∥2))1/2)\bar\alpha = \min(1/2, (\mu/(8\|\Delta X\Delta Se\|_2))^{1/2})αˉ=min(1/2,(μ/(8∥ΔXΔSe∥2​))1/2); the largest admissible step is at least αˉ\bar\alphaαˉ and reduces μ\muμ to at most (1−αˉ)μ(1-\bar\alpha)\mu(1−αˉ)μ.
  2. Lemma 5.8 (corrector step): from N2(0.5)\mathcal N_2(0.5)N2​(0.5) with σ=1\sigma = 1σ=1, the full step lands in N2(0.25)\mathcal N_2(0.25)N2​(0.25) and μ(1)=μ\mu(1) = \muμ(1)=μ.
  3. Lemma 5.10: on N−∞(γ)\mathcal N_{-\infty}(\gamma)N−∞​(γ), ∥ΔXΔSe∥2≤2−3/2(1+1/γ)nμ\|\Delta X\Delta S e\|_2 \le 2^{-3/2}(1 + 1/\gamma)n\mu∥ΔXΔSe∥2​≤2−3/2(1+1/γ)nμ.
  4. Theorem 5.11 (long-step method): the iterates of Algorithm LPF satisfy μk+1≤(1−δ/n)μk\mu_{k+1} \le (1 - \delta/n)\mu_kμk+1​≤(1−δ/n)μk​ with δ=23/2γ1−γ1+γmin⁡{σmin⁡(1−σmin⁡),σmax⁡(1−σmax⁡)}\delta = 2^{3/2}\gamma\frac{1-\gamma}{1+\gamma}\min\{\sigma_{\min}(1-\sigma_{\min}), \sigma_{\max}(1-\sigma_{\max})\}δ=23/2γ1+γ1−γ​min{σmin​(1−σmin​),σmax​(1−σmax​)}.

Significance

Theorem 5.6 is the step that turns the linear decrease of μ\muμ (Lemma 5.1) into a convergence proof: once the full step is known to stay in N2(θ)\mathcal N_2(\theta)N2​(θ), the duality measure falls by the factor 1−0.4/n1 - 0.4/\sqrt n1−0.4/n​ at every iteration, and the generic complexity theorem of Chapter 3 gives the O(nlog⁡(1/ϵ))O(\sqrt n\log(1/\epsilon))O(n​log(1/ϵ)) iteration bound, the standard bound for primal-dual interior-point methods. Lemmas 5.7 and 5.8 give the same bound for the adaptive predictor-corrector method, and Lemma 5.10 with Theorem 5.11 the O(nlog⁡(1/ϵ))O(n\log(1/\epsilon))O(nlog(1/ϵ)) bound for the long-step method, which is the variant closest to practical codes.

On the platform, the Mizuno–Todd–Ye predictor-corrector analysis has been formalized in a different parametrization (pairs (x,s)(x, s)(x,s) with an existential dual multiplier, and a different step bound); the results of this chapter in Wright's triple form, the general Lemma 5.3, the short-step invariant and the long-step reduction are not yet formalized. A machine-checked version yields reusable infrastructure: the step equations, the two neighborhoods and the pairwise-product bounds are used by every later chapter of the book (infeasible methods, superlinear convergence, finite termination).

Difficulty

The step equations are linear, but neighborhood membership is not: the products xi(α)si(α)x_i(\alpha)s_i(\alpha)xi​(α)si​(α) are quadratic in α\alphaα, and the quadratic coefficient ΔxiΔsi\Delta x_i\Delta s_iΔxi​Δsi​ is not controlled by the equations individually — only the sum ∑iΔxiΔsi\sum_i \Delta x_i\Delta s_i∑i​Δxi​Δsi​ vanishes. A bound on ∥ΔXΔSe∥2\|\Delta X\Delta Se\|_2∥ΔXΔSe∥2​ in terms of μ\muμ alone, uniformly over the neighborhood, is the crux; a naive bound through the norms of Δx\Delta xΔx and Δs\Delta sΔs separately involves the conditioning of AAA and is not uniform. The second subtlety is strict positivity: the proximity inequality at every α\alphaα does not by itself rule out a coordinate of x(α)x(\alpha)x(α) or s(α)s(\alpha)s(α) changing sign, and the conclusion of Theorem 5.6 requires x(α)>0x(\alpha) > 0x(α)>0, s(α)>0s(\alpha) > 0s(α)>0 for the whole segment.

Formalization scope

Vectors are Fin n → ℝ, matrices Matrix (Fin m) (Fin n) ℝ, points are triples (x, lam, s). The norm is the explicit Euclidean norm ∑ivi2\sqrt{\sum_i v_i^2}∑i​vi2​​ (Mathlib's default norm on Fin n → ℝ is the sup norm). N2(θ)\mathcal N_2(\theta)N2​(θ) and N−∞(γ)\mathcal N_{-\infty}(\gamma)N−∞​(γ) are subsets of F∘\mathcal F^\circF∘, so membership includes both equality constraints and strict positivity. The direction is defined by its linear equations, never by a closed-form solution, so no rank assumption and no nonsingularity are presupposed. Where the page leaves the ranges implicit, the standing ones are hypotheses: θ∈(0,1)\theta \in (0,1)θ∈(0,1) for N2\mathcal N_2N2​, γ∈(0,1)\gamma \in (0,1)γ∈(0,1) for N−∞\mathcal N_{-\infty}N−∞​, σ∈[0,1]\sigma \in [0,1]σ∈[0,1] for the step equations. In Lemmas 5.1 and 5.5, α\alphaα ranges over all reals, as the page places no restriction on it.

Explicit constants: the book's "there is a constant δ\deltaδ independent of nnn" in Theorem 5.11 is replaced by the δ\deltaδ its proof derives (p. 100); every other constant is printed in the statements. In αˉ\bar\alphaαˉ (5.19), the case ΔXΔSe=0\Delta X\Delta Se = 0ΔXΔSe=0 is read as αˉ=1/2\bar\alpha = 1/2αˉ=1/2; real division by zero, which would make αˉ=0\bar\alpha = 0αˉ=0 and Lemma 5.7 trivial on the interval [0,0][0, 0][0,0], is excluded by an explicit case split. "The largest α\alphaα" in Algorithms PC and LPF is a hypothesis (IsGreatest) on the chosen step, never a construction. A trivializing formalization — one that states only the proximity inequality ∥X(α)S(α)e−μ(α)e∥2≤θμ(α)\|X(\alpha)S(\alpha)e - \mu(\alpha)e\|_2 \le \theta\mu(\alpha)∥X(α)S(α)e−μ(α)e∥2​≤θμ(α) without strict feasibility — is ruled out by the definition of N2(θ)\mathcal N_2(\theta)N2​(θ).

Beyond the paper: one variant is posed beside the faithful goal. theorem_5_6_strong (strengthened, finding F1) is Theorem 5.6 for every real σ\sigmaσ: condition (5.15) already forces σ>0\sigma > 0σ>0 when n≥1n \ge 1n≥1, and the bound σ<1\sigma < 1σ<1 is not used by the argument.

Needed infrastructure: finite sums and the Euclidean norm on Fin n → ℝ, Matrix.mulVec and dot products, real powers. Proofs of Lemma 5.3 and of the positivity argument along the segment are reusable well beyond this mission; contributions of either are welcome.

Selected references

  • S. J. Wright, Primal-Dual Interior-Point Methods, SIAM, Philadelphia, 1997. https://doi.org/10.1137/1.9781611971453
  • S. Mizuno, M. J. Todd, Y. Ye, On adaptive-step primal-dual interior-point algorithms for linear programming, Mathematics of Operations Research 18 (1993), 964–981. https://doi.org/10.1287/moor.18.4.964
  • M. Kojima, S. Mizuno, A. Yoshise, A primal-dual interior point algorithm for linear programming, in N. Megiddo (ed.), Progress in Mathematical Programming, Springer, 1989, 29–47. https://doi.org/10.1007/978-1-4613-9617-8_2
  • R. D. C. Monteiro, I. Adler, Interior path following primal-dual algorithms. Part I: Linear programming, Mathematical Programming 44 (1989), 27–41. https://doi.org/10.1007/BF01587075
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Primal-Dual Interior-Point Methods II: The Kojima–Mizuno–Yoshise Step Lowers the Potential Φ_ρ by 0.15Textbook

Motivation

Interior-point methods solve linear programs by moving through the interior of the feasible region instead of along its edges. Two families dominate the theory: path-following methods, which keep the iterates near a central path, and potential-reduction methods, which instead measure progress by a single merit function and only ask that it fall by a fixed amount at every iteration. The second family grew out of Karmarkar's projective algorithm (Karmarkar 1984). The primal-dual potential used here was introduced by Tanabe and by Todd and Ye (Todd–Ye 1990), and the primal-dual step analysed in this mission is that of Kojima, Mizuno and Yoshise (Kojima–Mizuno–Yoshise 1991). It attains the O(n ∣log⁡ϵ∣)O(\sqrt n\,|\log\epsilon|)O(n​∣logϵ∣) iteration bound, the best known for any interior-point method.

The mission follows Chapter 4 of S. J. Wright, Primal-Dual Interior-Point Methods (SIAM 1997), the standard graduate text on the subject. Its argument is self-contained and fully quantitative: a fixed decrease of 0.150.150.15 per step, and an explicit iteration count.

Setting

Let A∈Rm×nA \in \mathbb R^{m\times n}A∈Rm×n, b∈Rmb \in \mathbb R^mb∈Rm and c∈Rnc \in \mathbb R^nc∈Rn, with no rank assumption on AAA. The primal problem is min⁡cTx\min c^TxmincTx subject to Ax=bAx = bAx=b, x≥0x \ge 0x≥0, and the dual is max⁡bTλ\max b^T\lambdamaxbTλ subject to ATλ+s=cA^T\lambda + s = cATλ+s=c, s≥0s \ge 0s≥0. The strictly feasible set is

F∘={(x,λ,s):Ax=b, ATλ+s=c, x>0, s>0},\mathcal F^\circ = \{(x,\lambda,s) : Ax = b,\ A^T\lambda + s = c,\ x > 0,\ s > 0\},F∘={(x,λ,s):Ax=b, ATλ+s=c, x>0, s>0},

inequalities being componentwise. The duality measure is μ=xTs/n\mu = x^Ts/nμ=xTs/n. Write e=(1,…,1)Te = (1,\dots,1)^Te=(1,…,1)T, X=diag⁡(x)X = \operatorname{diag}(x)X=diag(x) and S=diag⁡(s)S = \operatorname{diag}(s)S=diag(s).

For a parameter ρ>n\rho > nρ>n, the Tanabe–Todd–Ye potential is

Φρ(x,s)=ρlog⁡xTs−∑i=1nlog⁡xisi,\Phi_\rho(x,s) = \rho\log x^Ts - \sum_{i=1}^n \log x_is_i,Φρ​(x,s)=ρlogxTs−i=1∑n​logxi​si​,

evaluated where x,s>0x, s > 0x,s>0. Algorithm PR iterates as follows. At (x,λ,s)∈F∘(x,\lambda,s) \in \mathcal F^\circ(x,λ,s)∈F∘ it computes the search direction (Δx,Δλ,Δs)(\Delta x,\Delta\lambda,\Delta s)(Δx,Δλ,Δs), the solution of

AΔx=0,ATΔλ+Δs=0,SΔx+XΔs=−XSe+nρμe.A\Delta x = 0,\qquad A^T\Delta\lambda + \Delta s = 0,\qquad S\Delta x + X\Delta s = -XSe + \frac n\rho\mu e.AΔx=0,ATΔλ+Δs=0,SΔx+XΔs=−XSe+ρn​μe.

It then sets αmax⁡=sup⁡{α∈[0,1]:(x,s)+α(Δx,Δs)≥0}\alpha_{\max} = \sup\{\alpha\in[0,1] : (x,s) + \alpha(\Delta x,\Delta s) \ge 0\}αmax​=sup{α∈[0,1]:(x,s)+α(Δx,Δs)≥0} and takes a step length αk\alpha_kαk​ that minimizes α↦Φρ(x+αΔx,s+αΔs)\alpha\mapsto\Phi_\rho(x+\alpha\Delta x, s+\alpha\Delta s)α↦Φρ​(x+αΔx,s+αΔs) over (0,αmax⁡)(0,\alpha_{\max})(0,αmax​).

The analysis compares Φρ\Phi_\rhoΦρ​ along the direction with a quadratic. For τ∈(0,1)\tau\in(0,1)τ∈(0,1) define

g1=ρsTΔx+xTΔsxTs−eT(X−1Δx+S−1Δs),g2=11−τ(∥X−1Δx∥2+∥S−1Δs∥2),g_1 = \rho\frac{s^T\Delta x + x^T\Delta s}{x^Ts} - e^T(X^{-1}\Delta x + S^{-1}\Delta s),\qquad g_2 = \frac{1}{1-\tau}\big(\|X^{-1}\Delta x\|^2 + \|S^{-1}\Delta s\|^2\big),g1​=ρxTssTΔx+xTΔs​−eT(X−1Δx+S−1Δs),g2​=1−τ1​(∥X−1Δx∥2+∥S−1Δs∥2),

and the quadratic estimate q(α)=Φρ(x,s)+αg1+12α2g2q(\alpha) = \Phi_\rho(x,s) + \alpha g_1 + \frac12\alpha^2g_2q(α)=Φρ​(x,s)+αg1​+21​α2g2​. Let vi=(xisi)1/2v_i = (x_is_i)^{1/2}vi​=(xi​si​)1/2, vmin⁡=min⁡iviv_{\min} = \min_i v_ivmin​=mini​vi​ and r=−v+nρμV−1er = -v + \frac n\rho\mu V^{-1}er=−v+ρn​μV−1e with V=diag⁡(v)V = \operatorname{diag}(v)V=diag(v). Here ∥⋅∥\|\cdot\|∥⋅∥ is the Euclidean norm and ∥⋅∥∞\|\cdot\|_\infty∥⋅∥∞​ the max-norm.

Formalization targets

Goal: Theorem 4.6

For n≥1n \ge 1n≥1, (x,λ,s)∈F∘(x,\lambda,s)\in\mathcal F^\circ(x,λ,s)∈F∘, ρ≥n+n\rho \ge n+\sqrt nρ≥n+n​, the direction above, τ=0.5\tau = 0.5τ=0.5 and αˉ=vmin⁡/(2∥r∥)\bar\alpha = v_{\min}/(2\|r\|)αˉ=vmin​/(2∥r∥):

q(αˉ)−q(0)≤−0.15,q(\bar\alpha) - q(0) \le -0.15,q(αˉ)−q(0)≤−0.15,

and every minimizer αk∈(0,αmax⁡)\alpha_k \in (0,\alpha_{\max})αk​∈(0,αmax​) of the potential along the direction satisfies

Φρ(x+αkΔx,s+αkΔs)≤Φρ(x,s)−0.15.\Phi_\rho(x+\alpha_k\Delta x, s+\alpha_k\Delta s) \le \Phi_\rho(x,s) - 0.15 .Φρ​(x+αk​Δx,s+αk​Δs)≤Φρ​(x,s)−0.15.

Milestones

  1. Lemma 4.1. For β>−1\beta > -1β>−1, log⁡(1+β)≤β\log(1+\beta)\le\betalog(1+β)≤β, with equality exactly at β=0\beta = 0β=0. For ∥t∥∞≤τ<1\|t\|_\infty\le\tau<1∥t∥∞​≤τ<1, −∑ilog⁡(1+ti)≤−eTt+∥t∥2/(2(1−τ))-\sum_i\log(1+t_i) \le -e^Tt + \|t\|^2/(2(1-\tau))−∑i​log(1+ti​)≤−eTt+∥t∥2/(2(1−τ)).
  2. Eq. (4.16). Along any direction with AΔx=0A\Delta x = 0AΔx=0 and ATΔλ+Δs=0A^T\Delta\lambda + \Delta s = 0ATΔλ+Δs=0, Φρ(x+αΔx,s+αΔs)≤q(α)\Phi_\rho(x+\alpha\Delta x, s+\alpha\Delta s) \le q(\alpha)Φρ​(x+αΔx,s+αΔs)≤q(α) whenever 0<αmax⁡(∥X−1Δx∥∞,∥S−1Δs∥∞)≤τ0 < \alpha\max(\|X^{-1}\Delta x\|_\infty,\|S^{-1}\Delta s\|_\infty) \le \tau0<αmax(∥X−1Δx∥∞​,∥S−1Δs∥∞​)≤τ.
  3. Eqs. (4.26)–(4.27). Along the PR direction, g2≤∥r∥2/((1−τ)vmin⁡2)g_2 \le \|r\|^2/((1-\tau)v_{\min}^2)g2​≤∥r∥2/((1−τ)vmin2​) and g1=−ρnμ∥r∥2g_1 = -\frac{\rho}{n\mu}\|r\|^2g1​=−nμρ​∥r∥2.
  4. Lemma 4.5. For ρ>n+n\rho > n+\sqrt nρ>n+n​, ∥r∥≥32vmin⁡⋅nμρ\|r\| \ge \frac{\sqrt3}{2v_{\min}}\cdot\frac{n\mu}{\rho}∥r∥≥2vmin​3​​⋅ρnμ​.
  5. Lemma 4.2. For x,s>0x, s > 0x,s>0, Φn(x,s)≥nlog⁡n\Phi_n(x,s) \ge n\log nΦn​(x,s)≥nlogn, with equality iff XSe=μeXSe = \mu eXSe=μe.
  6. Lemma 4.3. If F∘≠∅\mathcal F^\circ \ne\emptysetF∘=∅, Φρ\Phi_\rhoΦρ​ is unbounded below on it; and μ≤exp⁡(Φρ(x,s)/(ρ−n))\mu \le \exp(\Phi_\rho(x,s)/(\rho-n))μ≤exp(Φρ​(x,s)/(ρ−n)) on F∘\mathcal F^\circF∘.
  7. Theorem 4.4. Any sequence in F∘\mathcal F^\circF∘ whose potential falls by δ>0\delta>0δ>0 per step has μk≤ϵ\mu_k\le\epsilonμk​≤ϵ for all k≥⌈Φρ(x0,s0)/δ+((ρ−n)/δ)∣log⁡ϵ∣⌉k \ge \lceil\Phi_\rho(x^0,s^0)/\delta + ((\rho-n)/\delta)|\log\epsilon|\rceilk≥⌈Φρ​(x0,s0)/δ+((ρ−n)/δ)∣logϵ∣⌉, for every ϵ∈(0,1)\epsilon\in(0,1)ϵ∈(0,1).
  8. Corollary 4.7. For ρ≥n+n\rho\ge n+\sqrt nρ≥n+n​, ϵ>0\epsilon > 0ϵ>0 and Φρ(x0,s0)≤κ(ρ−n)∣log⁡ϵ∣\Phi_\rho(x^0,s^0)\le\kappa(\rho-n)|\log\epsilon|Φρ​(x0,s0)≤κ(ρ−n)∣logϵ∣, the iterates of Algorithm PR are in F∘\mathcal F^\circF∘ and have μk≤ϵ\mu_k\le\epsilonμk​≤ϵ for all k≥⌈((κ+1)/0.15)(ρ−n)∣log⁡ϵ∣⌉k\ge\lceil((\kappa+1)/0.15)(\rho-n)|\log\epsilon|\rceilk≥⌈((κ+1)/0.15)(ρ−n)∣logϵ∣⌉.

The goal fixes τ=0.5\tau = 0.5τ=0.5 and the constant 0.150.150.15 because the book does. Its proof in fact gives (1−3)/4≈−0.183(1-\sqrt3)/4 \approx -0.183(1−3​)/4≈−0.183.

Significance

Theorem 4.6 is the per-iteration guarantee of the method. Combined with Theorem 4.4 it yields Corollary 4.7. At ρ=n+n\rho = n+\sqrt nρ=n+n​ this is an O(n ∣log⁡ϵ∣)O(\sqrt n\,|\log\epsilon|)O(n​∣logϵ∣) bound on the number of iterations needed to reduce the duality measure below ϵ\epsilonϵ. The same template underlies potential-reduction methods for linear complementarity and semidefinite programming: a fixed decrease of a potential, plus the inequality linking the potential to μ\muμ.

Formalizing the chapter yields a machine-checked complexity proof of a primal-dual interior-point method for linear programming, with every constant explicit. It also yields reusable pieces: the logarithmic estimates of Lemma 4.1, the bound of μ\muμ by the potential, and the generic counting argument of Theorem 4.4. The results are classical and proved on paper. No formal version of this chapter exists on the platform. The related published items cover a different potential-reduction method (Mizuno–Todd–Ye, in a wide neighbourhood) and a different counting bound (Bertsimas–Tsitsiklis).

Difficulty

The proof of Lemma 4.5 is the centre of the argument. It needs a lower bound on ∥r∥\|r\|∥r∥ uniform over all strictly feasible points, including badly centred ones. A naive bound through ∥r∥≥nμ(1−n/ρ)\|r\| \ge \sqrt{n\mu}(1-n/\rho)∥r∥≥nμ​(1−n/ρ) loses the factor 1/vmin⁡1/v_{\min}1/vmin​ that cancels against αˉ\bar\alphaαˉ.

The hypothesis ρ≥n+n\rho \ge n+\sqrt nρ≥n+n​ is essential, and Theorem 4.6 does not print it. It is stated for Lemma 4.5 and Corollary 4.7. For n<ρ<n+nn<\rho<n+\sqrt nn<ρ<n+n​ the conclusion fails: at n=4n=4n=4, ρ=4.1\rho=4.1ρ=4.1 the potential can fall by at most about 0.00250.00250.0025.

The step from the quadratic estimate to the potential has its own subtleties. The open interval (0,αmax⁡)(0,\alpha_{\max})(0,αmax​) need not contain a minimizer, and αˉ\bar\alphaαˉ can equal αmax⁡=1\alpha_{\max}=1αmax​=1 (at n=1n=1n=1, ρ=2\rho = 2ρ=2). The second conclusion of the goal is therefore stated for every minimizer, and a proof must not assume αˉ<αmax⁡\bar\alpha<\alpha_{\max}αˉ<αmax​.

Formalization scope

Vectors are Fin n → ℝ and matrices are Matrix (Fin m) (Fin n) ℝ. All definitions are in Definitions.Def_WrightPDIPM_Potential_Setting. These are Fo, mu, Phi, alphaMax, IsFeasibleDirection, IsPRDirection, g1, g2, q, vvec, vmin, rvec, alphaBar and IsPRStep. The search direction is defined by its linear equations, never by a closed-form inverse. The arg min of the line search is a hypothesis (IsMinOn on the open interval), never constructed.

Φρ\Phi_\rhoΦρ​ is the formula itself, and every statement evaluates it only at positive vectors. Standing hypotheses of the chapter are explicit binders: n≥1n\ge1n≥1, ρ>n\rho > nρ>n, F∘≠∅\mathcal F^\circ\ne\emptysetF∘=∅ for Lemma 4.3(i), and ρ≥n+n\rho\ge n+\sqrt nρ≥n+n​ for Theorem 4.6. Indices KKK are integer ceilings compared with kkk in Z\mathbb ZZ.

The book's O(·) in (4.32) is decoration: the explicit K=⌈((κ+1)/0.15)(ρ−n)∣log⁡ϵ∣⌉K = \lceil((\kappa+1)/0.15)(\rho-n)|\log\epsilon|\rceilK=⌈((κ+1)/0.15)(ρ−n)∣logϵ∣⌉ is what is stated. Theorem 4.6 has no O(·); its constants are τ=0.5\tau = 0.5τ=0.5 and δ=0.15\delta = 0.15δ=0.15.

A trivializing formalization is ruled out. The direction is the solution of the PR system, not an arbitrary feasible direction (for which the claim is false). The coefficients g1,g2g_1, g_2g1​,g2​ are computed from it, not free reals (for which it would be empty). αˉ\bar\alphaαˉ never takes Lean's junk value x/0=0x/0 = 0x/0=0, since r≠0r\ne0r=0 whenever ρ>n\rho>nρ>n.

Beyond the paper. Two variants are posed next to the faithful items.

  • lemma_4_5_strong (strengthened, finding F2) proves (4.28) for ρ≥n+n\rho\ge n+\sqrt nρ≥n+n​. Lemma 4.5 prints a strict inequality, but its proof covers the endpoint, which Theorem 4.6 and Corollary 4.7 use.
  • theorem_4_6_probe (sharpness, finding F1) is Theorem 4.6 under only ρ>n\rho>nρ>n. It is expected to be false, since a centred point with n=4n = 4n=4, ρ=4.1\rho = 4.1ρ=4.1 falls by at most 0.00250.00250.0025, and a disproof is welcome.

Contributions welcome: proofs of any milestone, a proof of the goal, and a disproof of the probe.

Selected references

  • S. J. Wright, Primal-Dual Interior-Point Methods, SIAM, Philadelphia, 1997. https://doi.org/10.1137/1.9781611971453
  • M. Kojima, S. Mizuno, A. Yoshise, An O(nL)O(\sqrt n L)O(n​L) iteration potential reduction algorithm for linear complementarity problems, Mathematical Programming 50 (1991) 331–342. https://doi.org/10.1007/BF01586927
  • M. J. Todd, Y. Ye, A centered projective algorithm for linear programming, Mathematics of Operations Research 15 (1990) 508–529. https://doi.org/10.1287/moor.15.3.508
  • N. Karmarkar, A new polynomial-time algorithm for linear programming, Combinatorica 4 (1984) 373–395. https://doi.org/10.1007/BF02579150
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The Nature of Statistical Learning Theory VII: Tikhonov Regularization of Ill-Posed ProblemsTextbook

Why ill-posed problems appear in learning

Estimating a probability density, a conditional probability or a conditional density from data can be written as solving a linear operator equation Af=FAf = FAf=F whose right-hand side FFF is a distribution function. The true FFF is unknown; only its empirical estimate FℓF_\ellFℓ​ is available, built from ℓ\ellℓ observations. The Glivenko–Cantelli theorem and the Kolmogorov–Smirnov law (Vapnik 2000, §7.3) guarantee that FℓF_\ellFℓ​ is close to FFF in the uniform metric. However, the operator AAA (for densities, integration) has an unbounded inverse. A small change in the right-hand side can produce an arbitrarily large change in the solution, so solving Af=FℓAf = F_\ellAf=Fℓ​ directly gives nonsense. Hadamard called such problems ill-posed.

Regularization handles this. Tikhonov (1963) proposed replacing the equation by the minimization of a penalized residual. Tikhonov and Arsenin (1977) proved that the regularized solutions converge to the true solution when the data error and the regularization parameter go to zero together in a controlled way. Vapnik and Stefanyuk (1978) extended this to random right-hand sides, the case of statistical estimation, and Stefanyuk (1986) to the case where the operator is estimated as well. Chapter 7 of The Nature of Statistical Learning Theory collects these results, without proofs, as the foundation of its SVM approach to density and conditional-probability estimation. This mission formalizes them.

Setting

Let (E1,ρE1)(E_1, \rho_{E_1})(E1​,ρE1​​) and (E2,ρE2)(E_2, \rho_{E_2})(E2​,ρE2​​) be metric spaces and A:E1→E2A : E_1 \to E_2A:E1​→E2​ a continuous one-to-one operator. The operator equation (7.12) is Af=FAf = FAf=F. Its solution fff exists and is unique, but A−1A^{-1}A−1 need not be continuous.

A regularization functional is a map Ω:E1→[0,∞]\Omega : E_1 \to [0, \infty]Ω:E1​→[0,∞] that is lower semicontinuous and whose sublevel sets {g:Ω(g)≤c}\{g : \Omega(g) \le c\}{g:Ω(g)≤c} are compact for every c>0c > 0c>0. Its domain is {Ω<∞}\{\Omega < \infty\}{Ω<∞}. In Lean this is NatureSLT.IllPosed.IsRegularizer.

Tikhonov's functional (Method T) for a parameter γ>0\gamma > 0γ>0 and a right-hand side F′F'F′ is

WT(g)=ρE22(Ag,F′)+γ Ω(g),W_T(g) = \rho^2_{E_2}(A g, F') + \gamma\, \Omega(g),WT​(g)=ρE2​2​(Ag,F′)+γΩ(g),

which is NatureSLT.IllPosed.tikhonovFunctional. A regularized solution is any minimizer of WTW_TWT​ over E1E_1E1​.

In the deterministic problem, for every δ>0\delta > 0δ>0 an approximation FδF_\deltaFδ​ with ρE2(F,Fδ)≤δ\rho_{E_2}(F, F_\delta) \le \deltaρE2​​(F,Fδ​)≤δ is given, and fδf_\deltafδ​ minimizes WTW_TWT​ with F′=FδF' = F_\deltaF′=Fδ​ and γ=γ(δ)\gamma = \gamma(\delta)γ=γ(δ). In the stochastic problem, FℓF_\ellFℓ​ are random elements of E2E_2E2​ converging to FFF in probability, and fℓf_\ellfℓ​ minimizes WTW_TWT​ with F′=FℓF' = F_\ellF′=Fℓ​ and γ=γℓ→0\gamma = \gamma_\ell \to 0γ=γℓ​→0. When the operator is also estimated by random continuous operators AℓA_\ellAℓ​, their distance to AAA is measured by (7.48),

∥Aℓ−A∥=sup⁡Ω(f)<∞ρE2(Aℓf,Af)Ω1/2(f),\|A_\ell - A\| = \sup_{\Omega(f) < \infty} \frac{\rho_{E_2}(A_\ell f, A f)}{\Omega^{1/2}(f)},∥Aℓ​−A∥=Ω(f)<∞sup​Ω1/2(f)ρE2​​(Aℓ​f,Af)​,

which is NatureSLT.IllPosed.operatorDistance.

Formalization targets

Goal: Theorem 7.1 (p. 238)

Let the exact solution fff lie in the domain of Ω\OmegaΩ, and suppose γ(δ)>0\gamma(\delta) > 0γ(δ)>0 satisfies (7.17):

γ(δ)→0 (δ→0),lim sup⁡δ→0+δ2γ(δ)≤r<∞.\gamma(\delta) \to 0 \ (\delta \to 0), \qquad \limsup_{\delta \to 0^+} \frac{\delta^2}{\gamma(\delta)} \le r < \infty .γ(δ)→0 (δ→0),δ→0+limsup​γ(δ)δ2​≤r<∞.

Then every family of minimizers fδf_\deltafδ​ of ρE22(Ag,Fδ)+γ(δ)Ω(g)\rho^2_{E_2}(Ag, F_\delta) + \gamma(\delta)\Omega(g)ρE2​2​(Ag,Fδ​)+γ(δ)Ω(g) satisfies fδ→ff_\delta \to ffδ​→f in E1E_1E1​ as δ→0+\delta \to 0^+δ→0+. This is the weakest statement in the chapter: it fixes no rate and assumes only metric structure.

Milestones

  1. Theorem 7.2 (p. 238). E1E_1E1​ is a real Hilbert space, AAA is linear, Ω(f)=∥f∥2\Omega(f) = \|f\|^2Ω(f)=∥f∥2, γ(δ)→0\gamma(\delta) \to 0γ(δ)→0 and δ2/γ(δ)→0\delta^2/\gamma(\delta) \to 0δ2/γ(δ)→0 (7.18). Then minimizers of (7.19) converge to fff in norm.
  2. Theorem 7.3 (p. 239). For any ε,μ>0\varepsilon, \mu > 0ε,μ>0 there is n(ε,μ)n(\varepsilon,\mu)n(ε,μ) such that for all ℓ>n(ε,μ)\ell > n(\varepsilon,\mu)ℓ>n(ε,μ)
P{ρE1(fℓ,f)>ε}≤P{ρE2(Fℓ,F)>γℓ μ}.(7.21)P\{\rho_{E_1}(f_\ell, f) > \varepsilon\} \le P\{\rho_{E_2}(F_\ell, F) > \sqrt{\gamma_\ell\,\mu}\}. \tag{7.21}P{ρE1​​(fℓ​,f)>ε}≤P{ρE2​​(Fℓ​,F)>γℓ​μ​}.(7.21)
  1. Theorem 7.4 (p. 239). E1E_1E1​ Hilbert, AAA linear, Ω(f)=∥f∥2\Omega(f) = \|f\|^2Ω(f)=∥f∥2. For any ε>0\varepsilon > 0ε>0 there is n(ε)n(\varepsilon)n(ε) such that for all ℓ>n(ε)\ell > n(\varepsilon)ℓ>n(ε)
P{∥fℓ−f∥2>ε}<2 P{ρE22(Fℓ,F)>ε2γℓ}.P\{\|f_\ell - f\|^2 > \varepsilon\} < 2\,P\{\rho^2_{E_2}(F_\ell, F) > \tfrac{\varepsilon}{2}\gamma_\ell\}.P{∥fℓ​−f∥2>ε}<2P{ρE2​2​(Fℓ​,F)>2ε​γℓ​}.
  1. Theorem 7.5 (p. 251). For any ε>0\varepsilon > 0ε>0 and constants C1,C2>0C_1, C_2 > 0C1​,C2​>0 there is γ0>0\gamma_0 > 0γ0​>0 such that for any γℓ≤γ0\gamma_\ell \le \gamma_0γℓ​≤γ0​
P{ρE1(fℓ,f)>ε}≤P{ρE2(Fℓ,F)>C1γℓ}+P{∥Aℓ−A∥>C2γℓ}.(7.49)P\{\rho_{E_1}(f_\ell, f) > \varepsilon\} \le P\{\rho_{E_2}(F_\ell, F) > C_1\sqrt{\gamma_\ell}\} + P\{\|A_\ell - A\| > C_2\sqrt{\gamma_\ell}\}. \tag{7.49}P{ρE1​​(fℓ​,f)>ε}≤P{ρE2​​(Fℓ​,F)>C1​γℓ​​}+P{∥Aℓ​−A∥>C2​γℓ​​}.(7.49)

The constants are the book's: the rrr of (7.17), the γℓμ\sqrt{\gamma_\ell\mu}γℓ​μ​ of (7.21), the factor 222 and the threshold ε2γℓ\tfrac{\varepsilon}{2}\gamma_\ell2ε​γℓ​ of Theorem 7.4, and the C1γℓC_1\sqrt{\gamma_\ell}C1​γℓ​​, C2γℓC_2\sqrt{\gamma_\ell}C2​γℓ​​ of (7.49).

Significance

Theorem 7.1 is what makes regularization a method rather than a heuristic: it names the admissible parameter choices, γ(δ)→0\gamma(\delta) \to 0γ(δ)→0 no faster than δ2\delta^2δ2, under which the regularized solution recovers the true one. Theorems 7.3 and 7.4 turn it into probability statements. Combined with any rate of convergence of the empirical distribution function, they give consistency of density estimation by Method T, which is the Corollary on p. 239. Theorem 7.5 does the same when the operator itself is estimated from data, as in conditional probability estimation (§7.9). Chapter 7's SVM estimators of densities and conditional probabilities rest on these statements.

The results are classical and proved in the literature cited. None of them is machine-checked. The platform has Tikhonov-regularized dynamics (TikhonovHDD) and finite-dimensional ridge identities (RobustLS.Tikhonov), but not the convergence theory of Tikhonov's method for operator equations. A formalization supplies a reusable notion of regularization functional on a metric space, the Tikhonov functional over [0,∞][0,\infty][0,∞], and the operator distance (7.48), for later work on inverse problems and nonparametric estimation.

Difficulty

The obvious approach, inverting AAA and passing to the limit, fails because A−1A^{-1}A−1 is not continuous: ρE2(Afδ,F)→0\rho_{E_2}(Af_\delta, F) \to 0ρE2​​(Afδ​,F)→0 alone does not force fδ→ff_\delta \to ffδ​→f. The deterministic theorems need control of the regularized solutions that the residual does not give, and in Theorem 7.2 that control must be obtained without compact sublevel sets, since the balls of an infinite-dimensional Hilbert space are not compact. In the stochastic theorems the difficulty is uniformity: a deterministic convergence statement for each outcome does not give the explicit comparison of probabilities that (7.21) and (7.49) assert. In (7.49) the bound must hold for every parameter below a threshold that depends only on ε\varepsilonε, C1C_1C1​, C2C_2C2​.

Two warnings from reading the chapter apply. Theorem 7.4 as printed is false: with exact data (Fℓ=FF_\ell = FFℓ​=F) both probabilities vanish for all large ℓ\ellℓ, and the strict inequality 0<00 < 00<0 fails (finding F1). Also, the three "uniqueness" properties of Ω\OmegaΩ listed on p. 235 are mutually inconsistent at f=0f = 0f=0, which is why they are not assumed (finding F3).

Formalization scope

All statements live in NatureSLT.IllPosed. Ω\OmegaΩ takes values in ℝ≥0∞. The book's "positive" is nonnegativity and its domain F\mathcal FF is {Ω≠∞}\{\Omega \ne \infty\}{Ω=∞}. "Semicontinuous" is lower semicontinuity. The exact solution is assumed to lie in the domain of Ω\OmegaΩ (the book assumes this tacitly); otherwise every point could minimize WTW_TWT​. Every theorem is stated for every family of minimizers, given as a hypothesis, never for a chosen sequence, and does not assert that minimizers exist. Statements of the form "there exists a sequence that converges" are a trivializing formalization and are ruled out. The limit in (7.17) is read as a limsup in [0,∞][0,\infty][0,∞], and δ→0\delta \to 0δ→0 is taken along δ>0\delta > 0δ>0. Hilbert spaces are real. Linearity of AAA (Theorems 7.2 and 7.4) makes E2E_2E2​ a real normed space. In the stochastic theorems the probability space (S,P)(S, P)(S,P) is arbitrary, and the random elements FℓF_\ellFℓ​, fℓf_\ellfℓ​ (and in 7.5 the random distance ∥Aℓ−A∥\|A_\ell - A\|∥Aℓ​−A∥) are Borel measurable by hypothesis, so no event gets probability zero by convention. In (7.48) a point with Ω(f)=0\Omega(f) = 0Ω(f)=0 contributes 000 if Aℓf=AfA_\ell f = AfAℓ​f=Af and ∞\infty∞ otherwise. In Theorem 7.5, γ0\gamma_0γ0​ is quantified before the random data, so it is uniform over all of them.

Beyond the paper. One variant item is posed: [Corrected] Theorem 7.4, the same statement with ≤\le≤ in place of <<<. Its evidence is the exact-data counterexample above, which has been checked in Lean. The faithful Theorem 7.4 stays a milestone.

Not included: the Glivenko–Cantelli theorem in the book's in-probability form (the platform's GlivenkoCantelli.glivenko_cantelli is the almost-sure form), the Kolmogorov–Smirnov limit laws (7.10)–(7.11), and Theorem 7.6, whose notion of consistency is not pinned down on the page. Contributions welcome: compactness arguments in metric spaces, weak compactness and the Radon–Riesz property in Hilbert spaces, and measurability lemmas for minimizers.

Selected references

  • V. N. Vapnik, The Nature of Statistical Learning Theory, 2nd ed., Springer, 2000, Chapter 7. https://doi.org/10.1007/978-1-4757-3264-1
  • A. N. Tikhonov, "On solving ill-posed problem and method of regularization", Doklady Akademii Nauk USSR 153 (1963), 501–504 (as cited in Vapnik 2000; no stable link).
  • A. N. Tikhonov and V. Y. Arsenin, Solution of Ill-Posed Problems, W. H. Winston, Washington, DC, 1977 (no stable link).
  • V. N. Vapnik and A. R. Stefanyuk, "Nonparametric methods for estimating probability densities", Automation and Remote Control (8), 1978 (as cited in Vapnik 2000; no stable link).
  • A. R. Stefanyuk, 1986, cited in Vapnik 2000, p. 251, for Theorem 7.5; the book's bibliography gives no entry for it.
  • V. N. Vapnik, Statistical Learning Theory, Wiley, 1998, which contains the proofs the 2000 book omits (no stable link).
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The Nature of Statistical Learning Theory VI: Kernels Generating Regularized Fourier ExpansionsTextbook

Motivation

A support vector machine never computes its feature map. It only evaluates a kernel K(x,y)K(x, y)K(x,y), the inner product of the feature vectors of xxx and yyy. When the features are the first NNN terms of an expansion, or infinitely many terms damped by regularizing weights, the learning algorithm is practical only if that inner product has a closed form. Chapter 6 of V. N. Vapnik, The Nature of Statistical Learning Theory (2nd ed., Springer 2000, doi:10.1007/978-1-4757-3264-1), §6.3–6.5, gives such closed forms for the kernels generating orthogonal-polynomial, spline and Fourier expansions. It also states Theorem 6.1, which lifts any one-dimensional kernel to nnn dimensions. The book prints these identities without proof and cites classical sources (Mikhlin 1964; Gradshtein and Ryzhik 1980). Several of them, as printed, are off by a constant factor, which a formal check settles.

Setting

Kernels in this mission are real functions of two real variables (or of two vectors of Rn\mathbb R^nRn) of the form K(x,y)=∑iri ψi(x) ψi(y)K(x, y) = \sum_i r_i\, \psi_i(x)\, \psi_i(y)K(x,y)=∑i​ri​ψi​(x)ψi​(y), a finite or convergent sum over basis functions ψi\psi_iψi​ with weights ri≥0r_i \ge 0ri​≥0.

  • Orthonormal polynomials. P1,P2,…P_1, P_2, \dotsP1​,P2​,… are real polynomials with deg⁡Pk=k−1\deg P_k = k - 1degPk​=k−1 that are orthonormal for a measure μ\muμ on R\mathbb RR with finite moments. Kn(x,y)=∑k=1nPk(x)Pk(y)K_n(x, y) = \sum_{k=1}^n P_k(x) P_k(y)Kn​(x,y)=∑k=1n​Pk​(x)Pk​(y) is the kernel of the first nnn of them.
  • Hermite. Pk(x)=(−1)kex2(d/dx)ke−x2P_k(x) = (-1)^k e^{x^2} (d/dx)^k e^{-x^2}Pk​(x)=(−1)kex2(d/dx)ke−x2, and Hk(x)=μkPk(x)e−x2H_k(x) = \mu_k P_k(x) e^{-x^2}Hk​(x)=μk​Pk​(x)e−x2 with normalization constants μk\mu_kμk​ (6.23). The regularized kernel is ∑i≥0qiHi(x)Hi(y)\sum_{i \ge 0} q^i H_i(x) H_i(y)∑i≥0​qiHi​(x)Hi​(y) with 0≤q<10 \le q < 10≤q<1.
  • Tensor products. Given coordinatewise basis functions bib_ibi​ on a set XXX, the nnn-dimensional basis is bi1…in(x)=bi1(x1)⋯bin(xn)b_{i_1 \dots i_n}(x) = b_{i_1}(x^1) \cdots b_{i_n}(x^n)bi1​…in​​(x)=bi1​​(x1)⋯bin​​(xn) (6.25).
  • Splines. The truncated power (x−t)+d(x - t)^d_+(x−t)+d​ is 000 for x≤tx \le tx≤t and (x−t)d(x - t)^d(x−t)d for x>tx > tx>t. The spline kernel with infinitely many nodes on (0,a)(0, a)(0,a) is ∫0a(xj−t)+d(xi−t)+d dt+∑r=0dxjrxir\int_0^a (x_j - t)^d_+ (x_i - t)^d_+\, dt + \sum_{r=0}^d x_j^r x_i^r∫0a​(xj​−t)+d​(xi​−t)+d​dt+∑r=0d​xjr​xir​ for xj,xi∈[0,a]x_j, x_i \in [0, a]xj​,xi​∈[0,a].
  • Fourier. KN(x,xi)=12+∑k=1N(sin⁡kxsin⁡kxi+cos⁡kxcos⁡kxi)K_N(x, x_i) = \tfrac12 + \sum_{k=1}^N (\sin kx \sin kx_i + \cos kx \cos kx_i)KN​(x,xi​)=21​+∑k=1N​(sinkxsinkxi​+coskxcoskxi​) is the kernel of the trigonometric polynomials of degree NNN. Its regularized versions damp the kkk-th harmonic by qkq^kqk (0<q<10 < q < 10<q<1) or by 1/(1+γ2k2)1/(1 + \gamma^2 k^2)1/(1+γ2k2) (γ>0\gamma > 0γ>0).

Formalization targets

Goal: Eq. (6.33)

For γ>0\gamma > 0γ>0 and real xi,xjx_i, x_jxi​,xj​ with 0≤∣xi−xj∣≤2π0 \le |x_i - x_j| \le 2\pi0≤∣xi​−xj​∣≤2π, the series converges and

12+∑k=1∞cos⁡kxicos⁡kxj+sin⁡kxisin⁡kxj1+γ2k2=π2γ cosh⁡π−∣xi−xj∣γsinh⁡πγ.\frac12 + \sum_{k=1}^{\infty} \frac{\cos kx_i \cos kx_j + \sin kx_i \sin kx_j}{1 + \gamma^2 k^2} = \frac{\pi}{2\gamma}\, \frac{\cosh\frac{\pi - |x_i - x_j|}{\gamma}}{\sinh\frac{\pi}{\gamma}} .21​+k=1∑∞​1+γ2k2coskxi​coskxj​+sinkxi​sinkxj​​=2γπ​sinhγπ​coshγπ−∣xi​−xj​∣​​.

The only numbered theorem of the section, Theorem 6.1, is close to an unfolding of the definitions. The goal is therefore the deepest identity of the section that is correct as printed.

Milestones

  1. (6.21), Christoffel–Darboux. There is a constant ana_nan​ such that Kn(x,y)=anPn+1(x)Pn(y)−Pn(x)Pn+1(y)x−yK_n(x, y) = a_n \frac{P_{n+1}(x)P_n(y) - P_n(x)P_{n+1}(y)}{x - y}Kn​(x,y)=an​x−yPn+1​(x)Pn​(y)−Pn​(x)Pn+1​(y)​ for x≠yx \ne yx=y and Kn(x,x)=an[Pn+1′(x)Pn(x)−Pn′(x)Pn+1(x)]K_n(x, x) = a_n[P'_{n+1}(x)P_n(x) - P'_n(x)P_{n+1}(x)]Kn​(x,x)=an​[Pn+1′​(x)Pn​(x)−Pn′​(x)Pn+1​(x)].
  2. (6.24), Hermite kernel. For some constants μk\mu_kμk​: ∑i≥0qiHi(x)Hi(y)=1π(1−q2)exp⁡{2xyq1+q−(x−y)2q21−q2}\sum_{i \ge 0} q^i H_i(x)H_i(y) = \frac{1}{\sqrt{\pi(1-q^2)}} \exp\{\frac{2xyq}{1+q} - \frac{(x-y)^2q^2}{1-q^2}\}∑i≥0​qiHi​(x)Hi​(y)=π(1−q2)​1​exp{1+q2xyq​−1−q2(x−y)2q2​} for all 0≤q<10 \le q < 10≤q<1 and real x,yx, yx,y.
  3. Theorem 6.1. ∑(i1,…,in)bi1…in(x) bi1…in(y)=∏k=1n∑ibi(xk)bi(yk)\sum_{(i_1, \dots, i_n)} b_{i_1 \dots i_n}(x)\, b_{i_1 \dots i_n}(y) = \prod_{k=1}^n \sum_i b_i(x^k) b_i(y^k)∑(i1​,…,in​)​bi1​…in​​(x)bi1​…in​​(y)=∏k=1n​∑i​bi​(xk)bi​(yk), for finite bases or bases whose one-dimensional kernel converges.
  4. (6.26). ∏i=1n1π(1−q2)exp⁡{⋯ }=1(1−q2)n/2exp⁡{2(x∗y)q1+q−∣x−y∣2q21−q2}\prod_{i=1}^n \frac{1}{\sqrt{\pi(1-q^2)}} \exp\{\cdots\} = \frac{1}{(1-q^2)^{n/2}} \exp\{\frac{2(x*y)q}{1+q} - \frac{|x-y|^2q^2}{1-q^2}\}∏i=1n​π(1−q2)​1​exp{⋯}=(1−q2)n/21​exp{1+q2(x∗y)q​−1−q2∣x−y∣2q2​}.
  5. (6.30), splines. For d≥0d \ge 0d≥0, the spline kernel equals ∑r=0dCdr2d−r+1(xj∧xi)2d−r+1∣xj−xi∣r+∑r=0dxjrxir\sum_{r=0}^d \frac{C_d^r}{2d - r + 1} (x_j \wedge x_i)^{2d-r+1} |x_j - x_i|^r + \sum_{r=0}^d x_j^r x_i^r∑r=0d​2d−r+1Cdr​​(xj​∧xi​)2d−r+1∣xj​−xi​∣r+∑r=0d​xjr​xir​. For d=1d = 1d=1 this is 1+xjxi+12∣xj−xi∣(xj∧xi)2+(xj∧xi)3/31 + x_jx_i + \frac12|x_j - x_i|(x_j \wedge x_i)^2 + (x_j \wedge x_i)^3/31+xj​xi​+21​∣xj​−xi​∣(xj​∧xi​)2+(xj​∧xi​)3/3.
  6. Dirichlet kernel (p. 197). KN(x,xi)=12+∑k=1Ncos⁡k(x−xi)=sin⁡(2N+1)(x−xi)2/sin⁡x−xi2K_N(x, x_i) = \frac12 + \sum_{k=1}^N \cos k(x - x_i) = \sin\frac{(2N+1)(x - x_i)}{2} \big/ \sin\frac{x - x_i}{2}KN​(x,xi​)=21​+∑k=1N​cosk(x−xi​)=sin2(2N+1)(x−xi​)​/sin2x−xi​​ for x−xi∉2πZx - x_i \notin 2\pi\mathbb Zx−xi​∈/2πZ.
  7. (6.32), Poisson kernel. For 0<q<10 < q < 10<q<1: 12+∑k≥1qkcos⁡k(xi−xj)=1−q22(1−2qcos⁡(xi−xj)+q2)\frac12 + \sum_{k \ge 1} q^k \cos k(x_i - x_j) = \frac{1 - q^2}{2(1 - 2q\cos(x_i - x_j) + q^2)}21​+∑k≥1​qkcosk(xi​−xj​)=2(1−2qcos(xi​−xj​)+q2)1−q2​.

Milestones 2, 4 and 6 are stated exactly as printed and are believed false (see Formalization scope). A disproof of each is a complete answer.

Significance

The result itself. Each closed form replaces an infinite or high-dimensional inner product by a constant number of elementary operations. That is what makes the regularized Fourier, Hermite and spline kernels usable in an SVM. Theorem 6.1 is the mechanism by which every multidimensional kernel of Chapter 6 is obtained from a one-dimensional one. The constants matter in practice. A wrong prefactor rescales the kernel matrix, and so the effective regularization of the SVM solution.

Formalizing it. The identities themselves are classical (Christoffel 1858, Darboux 1878, Mehler 1866, and the Fourier series of cosh⁡\coshcosh on [0,2π][0, 2\pi][0,2π]). As far as a search of the platform shows, none is stated on Prove2Me in these forms. SpikedWishart.Separated.eq_299 (Open) is one Christoffel–Darboux instance for a specific Hermite family. AronszajnRK.Product.direct_product_kernel concerns reproducing-kernel Hilbert spaces, not finite or summable feature sums. The mission produces machine-checked versions of the book's catalogue, together with a formal verdict on the three displays that are off by a constant.

Difficulty

The goal and (6.32) are infinite-series identities with no ready-made counterpart in the library, and the goal holds only on ∣xi−xj∣≤2π|x_i - x_j| \le 2\pi∣xi​−xj​∣≤2π. Outside that range the left side is 2π2\pi2π-periodic and the right side is not, so no argument that ignores the range can succeed. The Christoffel–Darboux formula needs a theory of orthogonal polynomials for a general measure, which Mathlib does not yet provide. (6.24) involves the iterated derivatives of e−x2e^{-x^2}e−x2 and a doubly infinite parameter family. The spline identity requires evaluating a piecewise-polynomial integral exactly. Theorem 6.1 for infinite bases requires summability over a product index set ιn\iota^nιn. The three false displays (2, 4, 6) hide their error in a constant factor. Two of them still reduce to an identity a solver may expect to hold.

Formalization scope

Everything is over R\mathbb RR. Series are stated with HasSum, or as Summable together with an equation for tsum. A divergent series therefore never reads as 000. Sums over k≥1k \ge 1k≥1 are written over k∈Nk \in \mathbb Nk∈N with k+1k + 1k+1 in place of kkk. The Christoffel–Darboux polynomials are indexed as in the book (P1P_1P1​ is the constant), they are orthonormal up to Pn+1P_{n+1}Pn+1​, and the measure has all moments finite. The Hermite PkP_kPk​ is the physicists' family, defined through iteratedDeriv. It is not Mathlib's probabilists' Polynomial.hermite. The normalization constants of (6.24) are existentially quantified, since the book does not give them. The range of qqq in (6.24) and (6.26) is 0≤q<10 \le q < 10≤q<1: the book's 0≤q≤10 \le q \le 10≤q≤1 without the singular point q=1q = 1q=1. The Dirichlet display is stated where sin⁡x−xi2≠0\sin\frac{x - x_i}{2} \ne 0sin2x−xi​​=0. The goal adds γ>0\gamma > 0γ>0, which the page leaves unstated. Spline points lie in [0,a][0, a][0,a] with a>0a > 0a>0. In Theorem 6.1, the multidimensional kernel is the sum over multi-indices of products of basis functions, a statement to be proved and not the product of kernels by definition. A formalization that defined the nnn-dimensional kernel as ∏kK(xk,yk)\prod_k K(x^k, y^k)∏k​K(xk,yk) would make the theorem trivial, and is ruled out.

Beyond the paper. Three corrected variants sit beside the faithful statements. Each changes one clause only. dirichlet_kernel_corrected (finding F1) uses the denominator 2sin⁡x−xi22\sin\frac{x - x_i}{2}2sin2x−xi​​, since at N=0N = 0N=0 and x−xi=πx - x_i = \pix−xi​=π the printed display reads 12=1\frac12 = 121​=1. hermite_kernel_corrected (F2) takes Hk=μkPkH_k = \mu_k P_kHk​=μk​Pk​ without the Gaussian factor. At q=0q = 0q=0 the printed (6.24) forces μ02e−x2−y2=π−1/2\mu_0^2 e^{-x^2-y^2} = \pi^{-1/2}μ02​e−x2−y2=π−1/2 for all x,yx, yx,y, and the corrected form is Mehler's formula. hermite_kernel_multidim_corrected (F3) uses the prefactor (π(1−q2))−n/2(\pi(1 - q^2))^{-n/2}(π(1−q2))−n/2, since at n=1n = 1n=1, x=y=0x = y = 0x=y=0, q=0q = 0q=0 the printed (6.26) reads π−1/2=1\pi^{-1/2} = 1π−1/2=1. One further observation is recorded without a variant: (6.33) also holds for γ<0\gamma < 0γ<0, because both sides are even in γ\gammaγ.

Reusable beyond this mission are the two definitions (the physicists' Hermite functions and the truncated power) and any proof of the summable form of Theorem 6.1. Proofs, disproofs of the three false milestones, and general lemmas on Fourier series of cosh⁡\coshcosh and qkcos⁡kθq^k\cos k\thetaqkcoskθ are all welcome.

Selected references

  • V. N. Vapnik, The Nature of Statistical Learning Theory, 2nd ed., Springer, 2000. doi:10.1007/978-1-4757-3264-1
  • I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series, and Products, Academic Press, 1980 (7th ed. 2007). doi:10.1016/C2009-0-22516-5
  • G. Szegő, Orthogonal Polynomials, AMS Colloquium Publications 23, 1939 (Christoffel–Darboux formula, §3.2). doi:10.1090/coll/023
  • F. G. Mehler, Ueber die Entwicklung einer Function von beliebig vielen Variablen nach Laplaceschen Functionen höherer Ordnung, J. reine angew. Math. 66 (1866), 161–176. doi:10.1515/crll.1866.66.161
  • S. G. Mikhlin, Variational Methods in Mathematical Physics, Pergamon, 1964.
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Primal-Dual Interior-Point Methods I: The Primal-Dual Log Barrier Has a Unique Minimizer, So the Central Path ExistsTextbook

Motivation

Primal-dual interior-point methods are the standard polynomial-time algorithms for linear programming and the basis of most commercial LP solvers. They do not move along the boundary of the feasible polyhedron, as the simplex method does. Instead they follow a curve through its interior, the central path, toward the solution set. Every convergence and complexity result about these methods assumes that this curve exists. Stephen J. Wright's monograph Primal-Dual Interior-Point Methods (SIAM, 1997, doi:10.1137/1.9781611971453) proves existence in its Chapter 2 from the standard LP background: existence of solutions, boundedness of solution sets, the Goldman–Tucker partition, and a primal-dual logarithmic barrier.

This mission is the first in a series that formalizes the book chapter by chapter. Later missions (potential reduction, path following, infeasible methods, finite termination, the homogeneous self-dual form) take the objects defined here as given.

Setting

Let A∈Rm×nA \in \mathbb{R}^{m\times n}A∈Rm×n, b∈Rmb \in \mathbb{R}^mb∈Rm and c∈Rnc \in \mathbb{R}^nc∈Rn. Nothing is assumed about the rank of AAA. The primal and dual linear programs in standard form are

(2.1)min⁡x cTx  s.t. Ax=b, x≥0,(2.2)max⁡λ,s bTλ  s.t. ATλ+s=c, s≥0,\text{(2.1)}\quad \min_x\ c^{\mathsf T}x \ \ \text{s.t.}\ Ax = b,\ x \ge 0, \qquad\qquad \text{(2.2)}\quad \max_{\lambda,s}\ b^{\mathsf T}\lambda \ \ \text{s.t.}\ A^{\mathsf T}\lambda + s = c,\ s \ge 0,(2.1)xmin​ cTx  s.t. Ax=b, x≥0,(2.2)λ,smax​ bTλ  s.t. ATλ+s=c, s≥0,

with all vector inequalities componentwise. The primal-dual feasible set F\mathcal FF consists of the triples (x,λ,s)(x,\lambda,s)(x,λ,s) with Ax=bAx = bAx=b, ATλ+s=cA^{\mathsf T}\lambda + s = cATλ+s=c and (x,s)≥0(x,s) \ge 0(x,s)≥0. The strictly feasible set F∘\mathcal F^\circF∘ is the same set with (x,s)>0(x,s) > 0(x,s)>0. The solution sets are ΩP\Omega_PΩP​, the minimizers of (2.1); ΩD\Omega_DΩD​, the maximizers (λ∗,s∗)(\lambda^*, s^*)(λ∗,s∗) of (2.2); and Ω=ΩP×ΩD\Omega = \Omega_P \times \Omega_DΩ=ΩP​×ΩD​. The index sets B\mathcal BB and N\mathcal NN collect the coordinates jjj for which some x∗∈ΩPx^* \in \Omega_Px∗∈ΩP​ has xj∗≠0x^*_j \neq 0xj∗​=0, and some (λ∗,s∗)∈ΩD(\lambda^*, s^*) \in \Omega_D(λ∗,s∗)∈ΩD​ has sj∗≠0s^*_j \ne 0sj∗​=0, respectively.

For τ>0\tau > 0τ>0, the central path conditions (2.25) ask for (x,λ,s)(x,\lambda,s)(x,λ,s) with

ATλ+s=c,Ax=b,xjsj=τ  (j=1,…,n),(x,s)>0.A^{\mathsf T}\lambda + s = c, \qquad Ax = b, \qquad x_j s_j = \tau \ \ (j = 1,\dots,n), \qquad (x,s) > 0 .ATλ+s=c,Ax=b,xj​sj​=τ  (j=1,…,n),(x,s)>0.

The reduced strictly feasible set H∘\mathcal H^\circH∘ is the set of pairs (x,s)(x,s)(x,s) such that (x,λ,s)∈F∘(x,\lambda,s) \in \mathcal F^\circ(x,λ,s)∈F∘ for some λ∈Rm\lambda \in \mathbb{R}^mλ∈Rm. The primal-dual log barrier on it is

fτ(x,s)=1τ xTs−∑j=1nlog⁡(xjsj).f_\tau(x,s) = \frac1\tau\, x^{\mathsf T}s - \sum_{j=1}^n \log(x_j s_j).fτ​(x,s)=τ1​xTs−j=1∑n​log(xj​sj​).

In Lean all of these objects live in the single definition module WrightPDIPM.CentralPath.LP, under the names F, Fo, OmegaP, OmegaD, Omega, indexB, indexN, Hcirc, ftau and CentralPathConditions.

Formalization targets

Goal: Theorem 2.8 (p. 39)

If F∘≠∅\mathcal F^\circ \ne \emptysetF∘=∅ and τ>0\tau > 0τ>0, then two things hold. First, fτf_\taufτ​ has exactly one local minimizer relative to H∘\mathcal H^\circH∘:

∃! (x,s)∈H∘ such that (x,s) is a local minimizer of fτ on H∘.\exists!\,(x,s) \in \mathcal H^\circ \ \text{such that}\ (x,s) \text{ is a local minimizer of } f_\tau \text{ on } \mathcal H^\circ .∃!(x,s)∈H∘ such that (x,s) is a local minimizer of fτ​ on H∘.

Second, the central path conditions (2.25) have a solution (x,λ,s)(x,\lambda,s)(x,λ,s).

Milestones

  1. Theorem 2.1 (p. 25). If F≠∅\mathcal F \neq \emptysetF=∅, then Ω≠∅\Omega \ne \emptysetΩ=∅. If either problem has an optimal solution, so does the other, and the optimal values agree.
  2. Theorem 2.3 (p. 26). Suppose F≠∅\mathcal F \neq \emptysetF=∅. If the dual is strictly feasible, then ΩP\Omega_PΩP​ is nonempty and bounded. If the primal is strictly feasible, then {s∗:(λ∗,s∗)∈ΩD}\{s^* : (\lambda^*, s^*) \in \Omega_D\}{s∗:(λ∗,s∗)∈ΩD​} is nonempty and bounded.
  3. Theorem 2.4 (Goldman–Tucker, p. 28). B∪N={1,…,n}\mathcal B \cup \mathcal N = \{1,\dots,n\}B∪N={1,…,n}; equivalently, some x∗∈ΩPx^* \in \Omega_Px∗∈ΩP​ and (λ∗,s∗)∈ΩD(\lambda^*, s^*) \in \Omega_D(λ∗,s∗)∈ΩD​ satisfy x∗+s∗>0x^* + s^* > 0x∗+s∗>0. It is stated here under F≠∅\mathcal F \ne \emptysetF=∅.
  4. Lemma 2.5 (p. 30). If F∘≠∅\mathcal F^\circ \ne \emptysetF∘=∅, then for every K≥0K \ge 0K≥0 the set {(x,s):(x,λ,s)∈F for some λ, xTs≤K}\{(x,s) : (x,\lambda,s) \in \mathcal F \text{ for some } \lambda,\ x^{\mathsf T}s \le K\}{(x,s):(x,λ,s)∈F for some λ, xTs≤K} is bounded.

Significance

The result itself. Theorem 2.8 shows that the central path {(xτ,λτ,sτ):τ>0}\{(x_\tau, \lambda_\tau, s_\tau) : \tau > 0\}{(xτ​,λτ​,sτ​):τ>0} exists as soon as the problem has a strictly feasible primal-dual point. Every path-following and potential-reduction method in the book either tracks this curve or measures its iterates against neighbourhoods of it, so the theorem is the foundation the book's convergence and complexity results stand on. Theorem 2.4 provides the strictly complementary partition B∪N\mathcal B \cup \mathcal NB∪N, which the local and superlinear convergence analysis and the finite-termination results use. Theorems 2.1 and 2.3 are the duality and boundedness facts beneath both.

Formalizing it. These are classical results with complete textbook proofs. The work left is machine-checking them in the book's own form: standard-form LP, no rank assumption, the primal-dual barrier rather than the primal barrier, and minimality relative to H∘\mathcal H^\circH∘. Related statements are already on the platform: Farkas's lemma and LP strong duality in Bertsimas–Tsitsiklis form, a central-path theorem in Matoušek–Gärtner form under the assumption rank⁡A=m\operatorname{rank} A = mrankA=m with the primal barrier, and Goldman–Tucker for the inequality-form LP. None of them is any of the statements above, but their proofs may be reusable.

Difficulty

The natural first step is to minimize fτf_\taufτ​ by compactness. This does not work directly, because H∘\mathcal H^\circH∘ is neither closed nor bounded and fτf_\taufτ​ is not coercive on all of R2n\mathbb{R}^{2n}R2n. The quadratic term xTsx^{\mathsf T}sxTs is indefinite in general, and only its restriction to the affine set of feasible pairs is linear. To obtain a minimizer one must show that the level sets of fτf_\taufτ​ inside H∘\mathcal H^\circH∘ are compact. This needs a boundedness statement (Lemma 2.5) that uses a strictly feasible point in an essential way, together with lower bounds that keep xjsjx_j s_jxj​sj​ away from 000. A second difficulty is that H∘\mathcal H^\circH∘ has empty interior in R2n\mathbb{R}^{2n}R2n whenever A≠0A \ne 0A=0. Passing from the minimizer to the conditions (2.25) is therefore a constrained first-order argument with multipliers, and it must work without AAA having full row rank, so the multiplier λ\lambdaλ is not unique. For Goldman–Tucker, the obstacle is the step from "every index is positive at some optimum" to a single optimum that is positive on every index of B\mathcal BB and of N\mathcal NN at once.

Formalization scope

Vectors are Fin n → ℝ and matrices are Matrix (Fin m) (Fin n) ℝ. Inequalities are componentwise. Indices are 0-based in Lean and {1,…,n}\{1,\dots,n\}{1,…,n} is Set.univ. Primal-dual points are triples (x, λ, s). "Bounded" means Bornology.IsBounded. ΩP\Omega_PΩP​ and ΩD\Omega_DΩD​ are defined as sets of optimal points (feasible, and at least as good as every feasible point), so no infimum or supremum is taken and an unattained optimum gives the empty set. No rank assumption on AAA is made anywhere. No result in this chapter has an O(⋅)O(\cdot)O(⋅) or an unspecified constant, so no constants are instantiated.

"Local minimizer in H∘\mathcal H^\circH∘" means IsLocalMinOn (ftau τ) (Hcirc A b c), that is, minimality among nearby points of H∘\mathcal H^\circH∘. Local minimality in all of R2n\mathbb{R}^{2n}R2n would make the claim false. Uniqueness is asserted for the minimizer (x,s)(x,s)(x,s) only, never for λ\lambdaλ. fτf_\taufτ​ uses Real.log and is only ever evaluated on H∘\mathcal H^\circH∘, where every xjsj>0x_j s_j > 0xj​sj​>0.

The hypothesis F∘≠∅\mathcal F^\circ \neq \emptysetF∘=∅ is part of the goal and cannot be weakened. If F∘=∅\mathcal F^\circ = \emptysetF∘=∅, then H∘=∅\mathcal H^\circ = \emptysetH∘=∅ and the first conclusion becomes false. A formalization that drops it, or one that replaces "unique local minimizer on H∘\mathcal H^\circH∘" with "a global minimizer exists", is not this theorem.

The book prints Theorem 2.4 without a hypothesis. The faithful item carries the section's standing case F≠∅\mathcal F \ne \emptysetF=∅, which by Theorem 2.1 gives Ω≠∅\Omega \neq \emptysetΩ=∅.

Beyond the paper. One sharpness probe, theorem_2_4_probe, states Theorem 2.4 exactly as printed, with no hypothesis. It is expected to be false. For m=n=1m = n = 1m=n=1, A=(1)A = (1)A=(1), b=−1b = -1b=−1, c=0c = 0c=0 the primal is infeasible and the dual objective is unbounded, so B∪N=∅\mathcal B \cup \mathcal N = \emptysetB∪N=∅. A disproof is the sought outcome.

Infrastructure that is reusable beyond this mission includes the standard-form LP objects, the solution sets and the B\mathcal BB/N\mathcal NN partition, all of which later chapters use; KKT conditions for linearly constrained convex minimization; and strict convexity of ∑j−log⁡tj\sum_j -\log t_j∑j​−logtj​. Contributions of general lemmas toward any of these are welcome.

Selected references

  • S. J. Wright, Primal-Dual Interior-Point Methods, SIAM, Philadelphia, 1997, Chapter 2. doi:10.1137/1.9781611971453
  • A. J. Goldman and A. W. Tucker, "Theory of linear programming," in Linear Inequalities and Related Systems, Annals of Mathematics Studies 38, Princeton University Press, 1956, pp. 53–97.
  • N. Megiddo, "Pathways to the optimal set in linear programming," in Progress in Mathematical Programming, Springer, 1989, pp. 131–158.
  • D. Bertsimas and J. N. Tsitsiklis, Introduction to Linear Optimization, Athena Scientific, 1997.
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