Combinatorics

1. Motivation

Wikipedia's pages on open problems are, for many mathematicians, the first contact with a conjecture: a one-paragraph statement, a short history, a list of partial results. The Formal Conjectures library (Google DeepMind, Apache-2.0) turned a large part of that material into Lean 4 statements, so that the conjectures can be attacked — and, just as importantly, stated unambiguously — by machine.

This mission ports a coherent slice of that material to Prove2Me. It is deliberately a portfolio mission: the goal theorem is the Bateman–Horn conjecture, the strongest single statement in the collection, and the milestone list gathers the other conjectures and the landmark theorems that surround them. Some milestones are genuine steps toward the goal (the Bunyakovsky conjecture is literally the one-polynomial case); most are independent open problems from other fields, grouped here because they share a source, a level of difficulty, and a need for faithful formal statements. A reader should not assume that proving a milestone advances the goal theorem. The mission's value is that every statement in it has been written against the same Mathlib revision, checked to compile, and documented well enough to be attacked.

A rough timeline of the collection's landmarks:

• 1947 — Mills: a real $A>1$ with $\lfloor A^{3^n}\rfloor$ always prime.
• 1962 — Radó: the busy beaver function outgrows every computable function.
• 1971 — Davies: planar Kakeya sets have Hausdorff dimension $2$.
• 1978 — Apéry: $\zeta(3)$ is irrational.
• 1985 — Read (after Enflo, 1981): an operator on $\ell^1$ with no nontrivial closed invariant subspace.
• 2001 — Zudilin: one of $\zeta(5),\zeta(7),\zeta(9),\zeta(11)$ is irrational.
• 2002 — Mihăilescu: $8$ and $9$ are the only consecutive perfect powers (Catalan's conjecture).
• 2009 / 2021 — Dvir; Bukh–Chao: the finite-field Kakeya bound and its sharp density constant.
• 2021 — Gardam: Kaplansky's unit conjecture is false (its zero-divisor and idempotent companions remain open).
• 2024 — Saito: Mills' constant is irrational; bbchallenge: $\mathrm{BB}(5)=47\,176\,870$.
• 2025 — Wang–Zahl: the Kakeya set conjecture in $\mathbb{R}^3$.

2. Setting

The goal theorem concerns prime values of polynomials. Fix a finite set $S=\{f_1,\dots,f_k\}\subseteq\mathbb{Z}[X]$ of distinct polynomials. Say that $f$ satisfies the Bunyakovsky condition if its leading coefficient is positive, $\deg f\ge 1$, and $f$ is irreducible over $\mathbb{Z}$; say that $S$ satisfies the Schinzel condition if for every prime $p$ there is an integer $n$ with $p\nmid f_1(n)\cdots f_k(n)$ — i.e. no fixed prime divides the product at every argument.

For a prime $p$ let $\omega_p(S)$ be the number of residue classes $n \bmod p$ at which some $f_i$ vanishes, let $D=\prod_i \deg f_i$, and let

$$\pi_S(x)=\#\{\,n\le x : |f_i(n)| \text{ is prime for every } i\,\}.$$

The Bateman–Horn constant is the (conditionally convergent) Euler product

$$C=\lim_{N\to\infty}\ \prod_{p<N}\Big(1-\tfrac1p\Big)^{-k}\Big(1-\tfrac{\omega_p(S)}{p}\Big).$$

The other groups use their own vocabulary, each fixed in a definition item of this mission: Kakeya sets in $\mathbb{R}^n$ and over $\mathbb{F}_q$; Mills' property $\lfloor A^{3^n}\rfloor \in \mathbb{P}$; Wagstaff primes and Catalan–Mersenne numbers; polynomial self-maps and their Jacobian matrix; nontrivial closed invariant subspaces; linear extensions of a finite poset; Catalan's constant; and an explicit two-symbol Turing machine model with its maximum-shifts function $\mathrm{BB}$.

3. Target

The goal theorem is the Bateman–Horn asymptotic: under the Bunyakovsky and Schinzel hypotheses, $C$ exists and is positive and

$$\pi_S(x)\ \sim\ \frac{C}{D}\,\frac{x}{(\log x)^{k}}\qquad (x\to\infty).$$

Weaker statements in the same direction appear as milestones, first of all Bunyakovsky's conjecture: under the same hypotheses with $k=1$, $f$ takes prime values infinitely often. The remaining milestones are listed in the milestone panel and are grouped by subject: Diophantine equations (Brocard, Pillai, Lebesgue–Nagell, Catalan/Mihăilescu), Mersenne-type primality (New Mersenne, infinitude of Mersenne primes, Catalan–Mersenne), prime-representing constants (Mills), geometric measure theory (Kakeya in $\mathbb{R}^n$, Kakeya over $\mathbb{F}_q$, Falconer), operator theory (invariant subspace problem and Read's $\ell^1$ counterexample), group algebras (Kaplansky's zero-divisor and idempotent conjectures), affine algebraic geometry (the two-variable Jacobian conjecture), irrationality and transcendence ($\zeta(5)$, all odd zeta values, Zudilin's theorem, $e+\pi$, $e\pi$, $\gamma$, Catalan's constant), order theory (the $1/3$–$2/3$ conjecture), and computability (Radó's theorem).

4. Significance

The results themselves. Bateman–Horn is the quantitative form of Schinzel's hypothesis H: it contains the twin prime conjecture, the infinitude of primes of the form $n^2+1$, and Bunyakovsky as special cases, and it is the standard heuristic behind prime-counting predictions. The other targets are each the headline question of their area: whether every bounded Hilbert-space operator has an invariant subspace; whether group algebras of torsion-free groups are domains; whether Kakeya sets must have full dimension. The solved milestones (Mihăilescu, Davies, Dvir, Zudilin, Read, Saito, Radó) are landmarks whose formal proofs would be significant library contributions in their own right.

Formalizing them. None of the open statements is expected to fall here; the concrete deliverable is a set of faithful, compiling, reusable statements plus formal proofs of the solved milestones, most of which are not in Mathlib today. Several are realistically in reach: the finite-field Kakeya bound (Dvir's polynomial method is short), the elementary fact that $\pi+e$ and $\pi e$ cannot both be algebraic, and Radó's diagonal argument.

5. Difficulty

For Bateman–Horn, the obstruction is visible already for $k=1$, $\deg f = 2$: sieve methods bound $\pi_S(x)$ from above by a constant times the conjectured main term and produce almost-primes, but the parity problem blocks every known sieve from producing a single prime value of an irreducible quadratic. The conditional convergence of the Euler product is a second, smaller trap: the product over $p<N$ must be taken in order, so any reformulation as an unordered infinite product changes the statement.

Each other group has its own obstruction, and they do not transfer: the parity problem says nothing about Kakeya, where the difficulty is that dimension is not stable under the natural compactness arguments, nor about the invariant subspace problem, where the known counterexamples on $\ell^1$ show that no soft argument can work.

6. Formalization scope

Conventions this mission commits to, all fixed in the definition items:

• Polynomials are elements of ℤ[X]; primality of a polynomial value is primality of its absolute value, and the counting function ranges over natural numbers $n \le \lfloor x\rfloor$.
• The Bateman–Horn constant is the limit of the ordered partial products over $p<N$, not an unordered infinite product.
• Kakeya sets carry no compactness or measurability hypothesis, matching the source; the conjecture is stated as an equality of Hausdorff dimensions in $[0,\infty]$.
• Falconer's hypothesis is written $d < 2\dim_H E$ to avoid division in $[0,\infty]$.
• Torsion-freeness of a group is spelled out as "every element of finite order is the identity", which is the hypothesis the source intends (it is weaker than Mathlib's IsMulTorsionFree).
• Linear extensions are order-preserving bijections onto $\{0,\dots,|P|-1\}$, and probabilities are quotients of set cardinalities in $\mathbb{Q}$.
• The busy beaver model is an explicit $n$-state, $2$-symbol machine with a bi-infinite Boolean tape; $\mathrm{BB}$ counts transitions performed (maximum shifts), the halting transition included, and $\mathrm{BB}(0)=0$.
• Several source statements are phrased as "is $X$ true?" with an unknown answer. Prove2Me statements must be definite, so each such question is recorded in its affirmative form (e.g. "$e+\pi$ is irrational"); a solver who can refute one should submit a disproof. The one question with no statable answer, "what is $\mathrm{BB}(6)$?", is replaced by Radó's growth theorem rather than guessed at.
• Nothing here is vacuous: each hypothesis set is satisfiable (e.g. closed unit balls are Kakeya sets, and $X^2+1$ satisfies the Bunyakovsky and Schinzel conditions).

Contributions welcome: proofs of the solved milestones; sharper variants; and additional faithful statements from the same source library, which contains far more than fits in one mission.

7. Selected references

• P. T. Bateman and R. A. Horn, A heuristic asymptotic formula concerning the distribution of prime numbers, Math. Comp. 16 (1962), 363–367. DOI
• T. Radó, On non-computable functions, Bell System Tech. J. 41 (1962), 877–884. DOI
• R. O. Davies, Some remarks on the Kakeya problem, Math. Proc. Cambridge Philos. Soc. 69 (1971), 417–421. DOI
• C. J. Read, A solution to the invariant subspace problem on the space $\ell_1$, Bull. London Math. Soc. 17 (1985), 305–317. DOI
• K. Falconer, On the Hausdorff dimensions of distance sets, Mathematika 32 (1985), 206–212. DOI
• W. Zudilin, One of the numbers $\zeta(5),\zeta(7),\zeta(9),\zeta(11)$ is irrational, Russian Math. Surveys 56 (2001), 774–776. DOI
• P. Mihăilescu, Primary cyclotomic units and a proof of Catalan's conjecture, J. reine angew. Math. 572 (2004), 167–195. DOI
• Z. Dvir, On the size of Kakeya sets in finite fields, J. Amer. Math. Soc. 22 (2009), 1093–1097. DOI
• B. Bukh and T.-W. Chao, Sharp density bounds on the finite field Kakeya problem, Discrete Analysis 26 (2021). DOI
• G. Gardam, A counterexample to the unit conjecture for group rings, Ann. of Math. 194 (2021), 967–979. DOI
• K. Saito, Mills' constant is irrational, Mathematika 71 (2025), e70027. arXiv:2404.19461
• H. Wang and J. Zahl, Volume estimates for unions of convex sets, and the Kakeya set conjecture in three dimensions, arXiv:2502.17655
• Google DeepMind, Formal Conjectures, Apache-2.0, github.com/google-deepmind/formal-conjectures

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Provenance note. The Lean statements in this mission are adaptations of the Formal Conjectures library (Apache-2.0), rewritten to depend only on Mathlib and on this mission's own definition items, and checked to compile against the platform's Mathlib revision. Each draft item carries a read-back; those read-backs are non-blind — they were written by the same agent that drafted the statements, and each says so in its first line. They are documentation, not independent testimony.

Motivation

A strongly regular graph with parameters $(n,k,\lambda,\mu)$ is a finite simple graph on $n$ vertices in which every vertex has exactly $k$ neighbours, every pair of adjacent vertices has exactly $\lambda$ common neighbours, and every pair of non-adjacent vertices has exactly $\mu$ common neighbours. For most parameter tuples the elementary counting and integrality conditions already decide existence; the interesting cases are those that survive every known feasibility test and still resist construction. The tuple $(99,14,1,2)$ is the smallest such case in the family $\lambda = 1$, $\mu = 2$, and its existence has been open for more than fifty years. John Horton Conway offered $1000 for a resolution, as one of five problems posed at the 2014 DIMACS conference on Challenges of Identifying Integer Sequences (Conway, Five $1,000 Problems (Update 2017)).

Timeline of the problem and of what is known about it:

• 1969/1971 — the parameter set is raised by Norman Biggs in his Southampton lectures (Finite Groups of Automorphisms, LMS Lecture Note Series 6, p. 111).
• 1973 — Berlekamp, van Lint and Seidel construct a strongly regular graph with parameters $(243,22,1,2)$ as the coset graph of the perfect ternary Golay code, settling one of the five feasible parameter tuples in this family.
• 1975 — the existence question appears as Problem 7 (attributed to J. J. Seidel) in R. K. Guy's problem list, The Geometry of Metric and Linear Spaces, Springer LNM 490, pp. 237–238; Conway had worked on it by then.
• 1984 — H. A. Wilbrink, On the $(99,14,1,2)$ strongly regular graph, shows that such a graph cannot be vertex-transitive: no group of automorphisms can act transitively on its 99 vertices.
• 1988 — Brouwer and Neumaier, A remark on partial linear spaces of girth 5 with an application to strongly regular graphs, Combinatorica 8, 57–61.
• 2004 — Makhnev and Minakova, On automorphisms of strongly regular graphs with $\lambda=1$, $\mu=2$, Discrete Math. Appl. 14(2), and 2011 — Behbahani and Lam, Strongly regular graphs with non-trivial automorphisms, Discrete Math. 311, 132–144: further restrictions on the possible automorphism groups.
• 2014/2017 — Conway's prize offer publicises the problem.

No graph with these parameters has been found, and no non-existence proof is known.

Setting

Fix a finite vertex set $V$ and a simple graph $g$ on $V$ (irreflexive, symmetric adjacency $\mathrm{Adj}$). For vertices $v,w$ write $N(v) = \{u : \mathrm{Adj}(v,u)\}$ for the neighbourhood of $v$ and $N(v)\cap N(w)$ for the set of common neighbours. The graph $g$ is strongly regular with parameters $(n,k,\lambda,\mu)$, written $\mathrm{IsSRGWith}\ g\ n\ k\ \lambda\ \mu$, when

• $|V| = n$;
• $|N(v)| = k$ for every vertex $v$;
• $|N(v)\cap N(w)| = \lambda$ whenever $v$ and $w$ are adjacent;
• $|N(v)\cap N(w)| = \mu$ whenever $v \neq w$ are non-adjacent.

The case $\lambda = 1$ says that every edge lies in exactly one triangle — equivalently, the neighbourhood of each vertex induces a perfect matching, so such graphs are locally linear. The case $\mu = 2$ says that every non-adjacent pair is the pair of opposite corners of exactly one $4$-cycle. Conway's problem asks for $(n,k) = (99,14)$ with these two local conditions.

Counting paths of length two from a fixed vertex gives $k(k-\lambda-1) = (n-k-1)\mu$, which for $\lambda=1$, $\mu=2$ reduces to $2n = k^2 + 2$; with $k = 14$ this yields $n = 99$. Writing $A$ for the adjacency matrix, $I$ for the identity and $J$ for the all-ones matrix, strong regularity is equivalent to the matrix identity $A^2 = kI + \lambda A + \mu(J - I - A)$, which for $(99,14,1,2)$ reads $A^2 + A = 12I + 2J$; the eigenvalues of $A$ other than $k=14$ are then $3$ and $-4$, and integrality of their multiplicities ($54$ and $44$) is one of the feasibility conditions that $(99,14,1,2)$ passes.

Formalization targets

Goal

$$\exists\ \alpha,\ \exists\ g \text{ a simple graph on } \alpha,\quad \mathrm{IsSRGWith}\ g\ 99\ 14\ 1\ 2 .$$

The goal is Mathlib's own proof_wanted conway_99 in Mathlib/Combinatorics/SimpleGraph/StronglyRegular.lean, stated verbatim: existence of a finite type carrying a strongly regular graph with parameters $(99,14,1,2)$. A resolution in either direction is welcome — a proof settles the existence half, and a proof of the negation settles the non-existence half; the platform records the two as proof and disproof of the same statement.

Supporting targets

$$2n = k^2 + 2, \qquad k \text{ even}, \qquad k \in \{2,4,14,22,112,994\}$$

for every strongly regular graph with $\lambda = 1$, $\mu = 2$: the counting identity, local linearity, and the integrality restriction that cuts the family down to five non-degenerate parameter tuples.

$$\exists\, g,\ \mathrm{IsSRGWith}\ g\ 9\ 4\ 1\ 2, \qquad \exists\, g,\ \mathrm{IsSRGWith}\ g\ 243\ 22\ 1\ 2$$

the two members of the family that are known to exist: the $3\times 3$ rook's graph (the Paley graph on $9$ vertices) and the Berlekamp–van Lint–Seidel graph.

$$|E(g)| = 693, \qquad |\{\text{triangles of } g\}| = 231, \qquad A^2 + A = 12I + 2J, \qquad g \text{ not vertex-transitive}$$

structural consequences for a hypothetical $99$-graph, the last one being Wilbrink's theorem.

Significance

A $(99,14,1,2)$ graph, if it exists, is a locally linear graph of maximal density in its parameter range and a partial linear space of girth $5$ with $99$ points and $231$ lines of size $3$; its existence would also produce new association schemes and new examples for the general classification of strongly regular graphs. A non-existence proof would be the first case in this family ruled out by anything other than the classical feasibility conditions, and would say something new about how far local conditions ($\lambda=1$, $\mu=2$) constrain global structure.

Nothing in this mission is presently formalized. Mathlib defines SimpleGraph.IsSRGWith, proves the counting identity IsSRGWith.param_eq, the complement rule IsSRGWith.compl, and the matrix identity IsSRGWith.matrix_eq, and records the $99$-graph problem as a proof_wanted. The supporting targets are of three kinds: results that are proved in the literature and only need formalizing (existence at $(9,4,1,2)$ and $(243,22,1,2)$; Wilbrink's non-vertex-transitivity; the integrality restriction on $k$); routine consequences that supply reusable infrastructure (edge and triangle counts, the spectral identity, evenness of $k$); and the goal itself, which is open mathematics.

Difficulty

The obvious approaches fail for concrete reasons. Exhaustive search is out of range: the graph has $693$ edges among $\binom{99}{2} = 4851$ pairs, and no isomorph-free generation of locally linear graphs on $99$ vertices is feasible. Algebraic constructions are blocked by Wilbrink's theorem — the graph cannot be vertex-transitive, so it is not a Cayley graph and cannot be produced by the group-theoretic constructions that yield most known strongly regular graphs, including the two that work at $(9,4,1,2)$ and $(243,22,1,2)$. On the non-existence side, every classical feasibility test (the counting identity, integrality of the eigenvalue multiplicities, the Krein conditions, the absolute bound) is passed by $(99,14,1,2)$, so a proof of non-existence needs an argument that does not factor through the parameters alone.

Formalization scope

All statements are phrased with Mathlib's SimpleGraph.IsSRGWith on a Fintype vertex type with DecidableRel adjacency, and use Fintype.card, SimpleGraph.edgeFinset, SimpleGraph.cliqueFinset 3 (triangles as $3$-cliques), SimpleGraph.adjMatrix over $\mathbb{Z}$, and graph isomorphisms g ≃g g for automorphisms. The goal quantifies over α : Type together with a Fintype α instance, so the vertex set is finite by construction and the empty type does not satisfy the cardinality clause; the statement is therefore not vacuously satisfiable. Note that Mathlib's definition constrains $\lambda$ only through pairs that are actually adjacent and $\mu$ only through pairs that are actually distinct and non-adjacent, so degenerate small graphs (the one-vertex graph, $K_3$) do satisfy IsSRGWith with $\lambda = 1$, $\mu = 2$; the supporting statements carry the cardinality hypotheses ($0 < n$, $1 < n$) that exclude them where needed, and the degenerate degree $k = 2$ is listed explicitly in the classification of feasible degrees.

Infrastructure a complete development needs, and which is reusable beyond this mission: interface lemmas for counting common neighbours in a strongly regular graph; the spectral theory of the adjacency matrix (multiplicities of the two non-principal eigenvalues, and their integrality), which is the missing ingredient for the classification of feasible degrees; a Lean construction of the perfect ternary Golay code and its coset graph, for the $(243,22,1,2)$ case; and decision procedures for strong regularity of an explicitly given small graph, for the $(9,4,1,2)$ case. Contributions to any of these are welcome, as are partial non-existence results (for instance, restrictions on automorphisms of prime order) submitted as separate statements.

Selected references

• N. Biggs, Finite Groups of Automorphisms: Course Given at the University of Southampton, October–December 1969, London Mathematical Society Lecture Note Series 6, Cambridge University Press, 1971, p. 111.
• E. R. Berlekamp, J. H. van Lint, J. J. Seidel, A strongly regular graph derived from the perfect ternary Golay code, in: A Survey of Combinatorial Theory, North-Holland, 1973, pp. 25–30.
• R. K. Guy, Problems, in: The Geometry of Metric and Linear Spaces, Springer Lecture Notes in Mathematics 490, 1975, pp. 233–244 (Problem 7, J. J. Seidel, pp. 237–238). doi:10.1007/BFb0081147
• H. A. Wilbrink, On the $(99,14,1,2)$ strongly regular graph, in: Papers dedicated to J. J. Seidel, EUT Report 84-WSK-03, Eindhoven University of Technology, 1984, pp. 342–355. PDF
• A. E. Brouwer, A. Neumaier, A remark on partial linear spaces of girth 5 with an application to strongly regular graphs, Combinatorica 8 (1988), 57–61. doi:10.1007/BF02122552
• A. A. Makhnev, I. M. Minakova, On automorphisms of strongly regular graphs with $\lambda=1$, $\mu=2$, Discrete Mathematics and Applications 14 (2004), no. 2. doi:10.1515/156939204872374
• M. Behbahani, C. Lam, Strongly regular graphs with non-trivial automorphisms, Discrete Mathematics 311 (2011), 132–144. doi:10.1016/j.disc.2010.10.005
• J. H. Conway, Five $1,000 Problems (Update 2017), OEIS. PDF

Motivation

Discrepancy theory asks how evenly a collection of objects can be split into two parts. Its central open question is a conjecture of Komlós, first circulated in the 1980s: any finite family of vectors of Euclidean length at most one can be signed $\pm 1$ so that the signed sum is bounded in every coordinate by a universal constant — independent of how many vectors there are and of the dimension they live in.

Timeline

• 1963. Steinitz-type vector balancing questions circulate; Bárány and Grinberg later (1981) show any norm admits a dimension-dependent bound $2d$, setting the theme: how much of the dependence on dimension is real?
• 1981. Beck and Fiala (Discrete Appl. Math.) prove degree-$t$ set systems have discrepancy at most $2t - 1$, by the floating-colors argument, and conjecture $O(\sqrt{t})$.
• 1980s. Komlós poses the vector form — unit $\ell^2$-norm columns, constant $\ell^\infty$ discrepancy — which implies the Beck–Fiala conjecture; it circulates through Spencer's Ten Lectures (1987) as the central open problem of the area.
• 1985. Spencer (Trans. AMS) proves "six standard deviations suffice": discrepancy $6\sqrt{n}$ for $n$ sets on $n$ points, beating random signing via the partial-coloring method.
• 1998. Banaszczyk (Random Struct. Algorithms) proves the Komlós bound $O(\sqrt{\log n})$ by a recursive Gaussian-measure argument over convex bodies.
• 2010–2016. The constructive era: Bansal (2010) makes Spencer algorithmic by SDP random walks, Lovett and Meka (2012) simplify, and Bansal, Dadush, and Garg (STOC 2016) give a polynomial-time algorithm matching Banaszczyk's bound.
• 2023. Kunisky (SIAM J. Discrete Math.) constructs instances from unsatisfiable formulas with discrepancy approaching $1+\sqrt{2}$ — the strongest lower bound on the conjectured constant.
• 2025. Bansal and Jiang (arXiv:2508.03961) break the Banaszczyk barrier: $\tilde{O}((\log n)^{1/4})$ for Komlós, and the Beck–Fiala conjecture resolved for $t \ge \log^2 n$ — the first movement in nearly thirty years. The gap between $2.414\ldots$ and $\tilde{O}((\log n)^{1/4})$ is the conjecture.

Setting

Fix $n$ vectors $v_1, \dots, v_n \in \mathbb{R}^m$ with Euclidean norm $\lVert v_i \rVert_2 \le 1$. A sign vector is an $\varepsilon \in \{-1, +1\}^n$: one sign $\varepsilon_i \in \{\pm 1\}$ per vector. Writing $v_{ij}$ for the $j$-th coordinate of the vector $v_i$, the discrepancy of the family under $\varepsilon$ is the largest coordinate, in absolute value, of the signed sum $\sum_i \varepsilon_i v_i$ — that is, $\max_{j \le m} \lvert \sum_{i \le n} \varepsilon_i v_{ij} \rvert$, the $\ell^\infty$ norm of the signed sum. The Komlós property at constant $K$ — KomlosBound K — says that every such family, in every $n$ and every $m$, admits a sign vector with every coordinate of the signed sum at most $K$ in absolute value.

Set systems embed as the special case of $0/1$-incidence matrices: if $A$ is an $m \times n$ matrix of $0$s and $1$s in which every column has at most $t$ ones (every element lies in at most $t$ sets), the columns scaled by $1/\sqrt{t}$ have norm at most one, so the Komlós property gives discrepancy $K\sqrt{t}$ — the Beck–Fiala conjecture.

Formalization targets

Goal — the Komlós conjecture

$$\exists\, K \in \mathbb{R}: \quad \text{every } v_1, \dots, v_n \in \mathbb{R}^m \text{ with } \lVert v_i\rVert_2 \le 1 \text{ admits } \varepsilon \in \{\pm 1\}^n \text{ with } \max_j \Big|\sum_i \varepsilon_i v_{ij}\Big| \le K.$$

The goal fixes no value of $K$: any finite universal constant settles it, so the statement survives every improvement in the constant.

Milestones — the known ladder

Eight results over the same definitions: Beck–Fiala's $2t - 1$ for degree-$t$ set systems; Spencer's $6\sqrt{n}$ for $n$ sets on $n$ points; Banaszczyk's $O(\sqrt{\log n})$ for the Komlós setting; its corollary $O(\sqrt{t \log n})$ for set systems; the reduction "Komlós at $K$ implies Beck–Fiala at $K\sqrt{t}$"; Kunisky's lower bound $K \ge 1 + \sqrt{2}$; and the two 2025 Bansal–Jiang breakthroughs — $\tilde{O}((\log n)^{1/4})$ for the Komlós setting, and the Beck–Fiala conjecture's bound $O(\sqrt{t})$ in the regime $t = \Omega(\log^2 n)$.

Significance

The conjecture is the meeting point of the two main techniques of discrepancy theory — partial coloring and the Gaussian/convex-geometric method — and each further improvement has forced a new technique into existence. A proof would resolve the Beck–Fiala conjecture in full and sharpen the hereditary-discrepancy landscape; a disproof would break the widely-shared expectation that vector balancing is dimension-free. The problem is also a benchmark for algorithmic discrepancy: every known bound now has a polynomial-time counterpart, and the constructive tools built for it (random-walk roundings, spectral partial colorings) are used across approximation algorithms and ranging into differential privacy.

None of this literature is formalized anywhere; Mathlib has no discrepancy theory at all. The definitions here are elementary — finite sums, absolute values, one norm hypothesis — so the mission's entry cost is unusually low for an open-problem mission: the Beck–Fiala theorem and the scaling reduction are self-contained finite combinatorics, while Spencer and Banaszczyk each force a genuinely new proof technique (pigeonhole partial coloring; Gaussian measure on convex bodies) into Lean.

Difficulty

Random signs lose: they give $\Theta(\sqrt{n})$, not a constant, so the naive probabilistic argument is ruled out from the start. The Beck–Fiala argument caps discrepancy by degree, not by norm, and provably cannot be pushed below $2t - O(1)$ by its own bookkeeping. Partial coloring alone loses a logarithm through its iteration, and Banaszczyk's method is blocked at $\sqrt{\log n}$ by the Gaussian measure of the cube. The 2025 advance decouples the two methods but still pays iterated polylogarithmic factors. Nothing currently known contracts the remaining gap to a constant, and the lower bound says the constant, if it exists, is at least $1 + \sqrt{2}$ — so any proof must handle instances strictly harder than the set-system case.

Formalization scope

The Lean model commits to: vectors as EuclideanSpace ℝ (Fin m), whose norm is the $\ell^2$ norm (the hypothesis $\lVert v_i \rVert \le 1$ reads ‖v i‖ ≤ 1); the $\ell^\infty$ conclusion written coordinatewise as ∀ j, |∑ i, ε i * v i j| ≤ K, avoiding any auxiliary sup-norm structure; sign vectors as real vectors with ε i = 1 ∨ ε i = -1; and set systems as matrices A : Fin m → Fin n → ℝ with an entrywise $0/1$ hypothesis and column-degree counted by Set.ncard. Quantifier order matters everywhere: in KomlosBound K the constant is fixed before $n$ and $m$ — a $K$ depending on $n$ would make the statement the trivial $\sqrt{n}$ bound. In beck_fiala the hypothesis $t \ge 1$ is required (the degree-$0$ system has discrepancy $0 > 2t-1$ otherwise); the Banaszczyk-form bounds use $\log(n+2)$ so that the bound is positive already at $n \le 1$. In the Bansal–Jiang milestones the asymptotic $\tilde{O}$ and $\Omega$ are rendered by existential constants quantified before all instances: the hidden $\mathrm{poly}(\log\log n)$ factor becomes $(\log\log(n+8))^{\gamma}$ for some fixed $\gamma > 0$ (the inner shift $+8$ keeps the iterated logarithm positive), and the threshold $t = \Omega(\log^2 n)$ becomes $C_0 \log^2(n+2) \le t$ for some fixed $C_0 > 0$.

Welcome contributions: any milestone in any order — beck_fiala and komlos_implies_beck_fiala are self-contained finite arguments and the natural entry points; spencer_six_deviations and banaszczyk_bound each import a major technique; komlos_lower_bound needs an explicit construction and a case analysis over all sign vectors. Reusable infrastructure — partial colorings, Gaussian measure bounds for convex bodies, hereditary discrepancy — is welcome as platform theorems. The matrix Spencer conjecture, prefix discrepancy, and the Steinitz problem are related but deliberately left to future missions.

Selected references

• J. Beck, T. Fiala, "Integer-making" theorems, Discrete Applied Mathematics 3 (1981). doi:10.1016/0166-218X(81)90022-6
• J. Spencer, Six standard deviations suffice, Trans. Amer. Math. Soc. 289 (1985). doi:10.1090/S0002-9947-1985-0784009-0
• W. Banaszczyk, Balancing vectors and Gaussian measures of n-dimensional convex bodies, Random Structures & Algorithms 12 (1998). doi link
• N. Bansal, D. Dadush, S. Garg, An algorithm for Komlós conjecture matching Banaszczyk's bound, FOCS 2016 / SIAM J. Comput. arXiv:1605.02882
• N. Bansal, H. Jiang, Decoupling via affine spectral-independence: Beck–Fiala and Komlós bounds beyond Banaszczyk, 2025. arXiv:2508.03961
• D. Kunisky, The discrepancy of unsatisfiable matrices and a lower bound for the Komlós conjecture constant, SIAM J. Discrete Math. 37 (2023). arXiv:2111.02974
• B. Chazelle, The Discrepancy Method, Cambridge University Press, 2000. author's page

Motivation

Which sets of positive integers are forced to contain long arithmetic progressions? Van der Waerden (1927) showed that in any finite colouring of $\mathbb N$ some colour class does; Erdős and Turán (1936) asked for a density version, which became Szemerédi's theorem. Erdős then proposed the strongest natural size condition: divergence of the reciprocal sum. Erdős Problem #3 (erdosproblems.com/3) asks whether every $A\subseteq\mathbb N$ with $\sum_{n\in A}1/n=\infty$ contains arbitrarily long arithmetic progressions. Erdős attached one of his largest prizes to it. The primes are the motivating example: $\sum_p 1/p=\infty$, so a positive answer would contain the Green–Tao theorem.

Timeline.

• 1936 — Erdős and Turán conjecture that sets of positive density contain arbitrarily long progressions.
• 1953 — Roth proves the case $k=3$ of the density conjecture by Fourier analysis.
• 1975 — Szemerédi proves the density conjecture for all $k$.
• 2001 — Gowers gives the first quantitative bounds for all $k$: $r_k(N)\ll N/(\log\log N)^{c_k}$.
• 2008 — Green and Tao prove that the primes contain arbitrarily long progressions.
• 2020 — Bloom and Sisask prove $r_3(N)\ll N/(\log N)^{1+c}$, which settles the case $k=3$ of Erdős Problem #3.
• 2023 — Kelley and Meka prove $r_3(N)\le N\exp(-c(\log N)^{1/12})$.
• 2024 — Leng, Sah and Sawhney prove $r_k(N)\le N\exp(-(\log\log N)^{c_k})$ for every $k\ge5$.

The problem is open for every $k\ge4$.

Setting

A set $S\subseteq\mathbb N$ is an arithmetic progression of length $k$ if $|S|=k$ and $S=\{a,a+d,\dots,a+(k-1)d\}$ for some $a,d\in\mathbb N$ (for $k\ge2$ the size condition forces $d>0$). For $k,N\in\mathbb N$, $r_k(N)$ denotes the largest size of a subset of $\{1,\dots,N\}$ containing no arithmetic progression of length $k$. A set $A$ has divergent reciprocal sum if $\sum_{n\in A}1/n=\infty$.

Formalization targets

Goal (Erdős Problem #3)

For every $A\subseteq\mathbb N$,

$$\sum_{n\in A}\frac1n=\infty\ \Longrightarrow\ A\ \text{contains arithmetic progressions of arbitrarily large length}.$$

This is the formal-conjectures statement erdos_3 with its answer(sorry) instantiated to the conjectured answer yes. A disproof of the goal on the platform settles the problem negatively.

Milestones

• Szemerédi's theorem for sets of positive upper density (the density case), and the existing platform statement $r_k(N)=o(N)$.
• The Green–Tao theorem (the case $A=$ primes).
• The Bloom–Sisask bound on $r_3(N)$ and its corollary, the case $k=3$ of the goal; the Kelley–Meka bound (existing platform statement).
• The Leng–Sah–Sawhney bound for $k\ge5$.
• The partial-summation reduction: bounds $r_k(N)\le N/(\log N)^{1+c_k}$ for all $k\ge3$ imply the goal.

Significance

A positive answer would be a common strengthening of Szemerédi's theorem and the Green–Tao theorem, obtained from a single size condition with no arithmetic structure. Through the reduction milestone, it is closely tied to the quantitative theory of $r_k(N)$: bounds of the shape $N/(\log N)^{1+c}$ for every $k$ would suffice. Formalizing the milestones would also give reusable Lean statements of Szemerédi-type theorems in a common language.

Difficulty

Divergence of $\sum 1/n$ is a very weak condition: such sets can have density zero, and the natural approach through $r_k(N)$ requires bounds just past $N/\log N$. For $k=3$ this barrier was only broken in 2020. For $k\ge4$ the best known bounds (Leng–Sah–Sawhney) save only a power of $\log\log N$ in the exponent, far from what is needed. The Green–Tao method uses pseudorandom majorants specific to the primes and does not apply to arbitrary sets.

Formalization scope

All statements import the published definition file Erdos142Basic, which reproduces the formal-conjectures definitions IsAPOfLengthWith, IsAPOfLength and the counting function r k N (over $\{1,\dots,N\}$). The reciprocal-sum hypothesis is ¬ Summable (fun a : A ↦ 1 / (a : ℝ)); the element $0$, if present, contributes $1/0=0$. "Arbitrarily long" is written as ∃ᶠ k in atTop, which is equivalent to "every length" because sub-progressions of progressions are progressions. Bounds stated in the literature with $\ll$ are written without a multiplicative constant and with "for all sufficiently large $N$"; the constant can be absorbed into the exponent. Contributions formalizing partial summation over sets of naturals and the equivalence of the "frequently" and "for every $k$" forms are welcome.

Selected references

• P. Erdős and P. Turán, On some sequences of integers, J. London Math. Soc. 11 (1936).
• K. F. Roth, On certain sets of integers, J. London Math. Soc. 28 (1953).
• E. Szemerédi, On sets of integers containing no k elements in arithmetic progression, Acta Arith. 27 (1975).
• W. T. Gowers, A new proof of Szemerédi's theorem, Geom. Funct. Anal. 11 (2001).
• B. Green and T. Tao, The primes contain arbitrarily long arithmetic progressions, Ann. of Math. 167 (2008).
• T. F. Bloom and O. Sisask, Breaking the logarithmic barrier in Roth's theorem on arithmetic progressions, arXiv:2007.03528 (2020).
• Z. Kelley and R. Meka, Strong bounds for 3-progressions, FOCS 2023, arXiv:2302.05537.
• J. Leng, A. Sah and M. Sawhney, Improved bounds for Szemerédi's theorem, arXiv:2402.17995 (2024).
• T. F. Bloom, Erdős Problem #3, https://www.erdosproblems.com/3

Motivation

The diagonal Ramsey number $R(k)$ is the least $n$ such that every red/blue colouring of the edges of the complete graph $K_n$ contains a monochromatic copy of $K_k$. Ramsey's theorem guarantees that $R(k)$ is finite; the question of how fast it grows is one of the central problems of extremal and probabilistic combinatorics. Erdős asked repeatedly ([Er88], [Er93]; see erdosproblems.com/77) for the value of

$$\lim_{k\to\infty} R(k)^{1/k}.$$

It is not even known whether this limit exists.

Timeline.

• 1935 — Erdős and Szekeres prove $R(k)\le\binom{2k-2}{k-1}$, so $R(k)\le 4^{k}$ and $\limsup_k R(k)^{1/k}\le 4$ ([ES35]).
• 1947 — Erdős proves $R(k) > 2^{k/2}$ for $k\ge 3$ by a counting (probabilistic) argument, so $\liminf_k R(k)^{1/k}\ge\sqrt2$ ([Er47]).
• 1975 — Spencer improves the lower bound by a factor of $2$: $R(k)\ge(1+o(1))\frac{\sqrt2}{e}\,k\,2^{k/2}$ ([Sp75]). The exponential base $\sqrt2$ has not been improved since.
• 2009, 2023 — Conlon ([Co09]) and then Sah ([Sa23]) obtain super-polynomial savings over $4^k$, but still with exponential base $4$.
• 2023 — Campos, Griffiths, Morris and Sahasrabudhe prove $R(k)\le(4-\varepsilon)^k$ for some constant $\varepsilon>0$ and all large $k$: the first exponential improvement on the upper bound ([CGMS23]).
• 2024 — Gupta, Ndiaye, Norin and Wei optimise the CGMS method and obtain $R(k)\le 3.8^{k+o(k)}$ ([GNNW24]). Balister et al. extend exponential improvements to the multicolour setting ([BBCGHMST24]).

So today, if the limit exists, it lies in $[\sqrt2,\,3.8]$.

Setting

For $n\in\mathbb N$ consider simple graphs $G$ on the vertex set $\{0,\dots,n-1\}$. A red/blue colouring of the edges of $K_n$ is the same as such a graph $G$ (the red edges) together with its complement $G^{c}$ (the blue edges). A $k$-clique of $G$ is a set of exactly $k$ vertices, any two of which are adjacent in $G$. Define

$$R(k)=\min\bigl\{\,n\in\mathbb N:\ \text{every graph } G \text{ on } n \text{ vertices has a } k\text{-clique in } G \text{ or in } G^{c}\,\bigr\}.$$

In the Lean development this is Erdos77.diagonalRamsey k. Small values: $R(0)=0$, $R(1)=1$, $R(2)=2$, $R(3)=6$, $R(4)=18$.

Formalization targets

Goal: existence of the limit

$$\exists\,L\in\mathbb R:\qquad R(k)^{1/k}\ \longrightarrow\ L\qquad (k\to\infty).$$

The original problem asks for the value of the limit, which is unknown; a goal with a hard-coded value cannot be stated honestly. The goal therefore asserts only that the limit exists (as a real number). Determining $L$ remains the ultimate aim; any proof of a specific value would in particular prove this goal.

Milestones (results from the literature)

1. Erdős 1947: $R(k)>2^{k/2}$ for all $k\ge 3$.
2. Spencer 1975: for every $\varepsilon>0$, eventually $R(k)\ge(1-\varepsilon)\frac{\sqrt2}{e}\,k\,2^{k/2}$.
3. Erdős–Szekeres 1935: $R(k)\le\binom{2k-2}{k-1}$ for all $k\ge1$.
4. Campos–Griffiths–Morris–Sahasrabudhe 2023: there is $\varepsilon>0$ with $R(k)\le(4-\varepsilon)^k$ for all sufficiently large $k$.
5. Gupta–Ndiaye–Norin–Wei 2024: for every $\delta>0$, eventually $R(k)\le 3.8^{(1+\delta)k}$, i.e. $R(k)\le 3.8^{k+o(k)}$.

Significance

The result itself. Existence of the limit would say that diagonal Ramsey numbers have a well-defined exponential growth rate — a regularity statement that is currently unknown in either direction. Even the bounds $\sqrt2\le\liminf$ and $\limsup\le 3.8$ are the products of decades of work, and the lower bound base $\sqrt2$ has resisted improvement since 1947.

Formalizing it. The goal is open. The milestones are proved results in the literature; the classical ones (Erdős–Szekeres, Erdős 1947) are natural first formalization targets, and the recent upper bounds (CGMS, GNNW) are substantial formalization projects in their own right. The status of existing machine-checked formalizations of these results is not asserted here.

Difficulty

There is no known sub- or super-multiplicativity for $R(k)$ that would give existence of the limit via Fekete's lemma: the natural product constructions relate $R(k\ell)$ to $R(k)$ and $R(\ell)$ only with losses that are too large, and the best lower and upper bounds come from entirely different methods (random colourings versus the book algorithm), so neither side controls the other.

Formalization scope

• $R(k)$ is defined as an infimum over $n$ of the property "every graph on $\mathrm{Fin}\,n$ has a $k$-clique in $G$ or in $G^{c}$". Lean's sInf of an empty set of naturals is $0$; the Erdős–Szekeres milestone shows the set is nonempty, so the infimum is the genuine Ramsey number.
• $R(k)^{1/k}$ is the real power of the real number $R(k)$ with exponent $1/k$; the value at $k=0$ is irrelevant for the limit.
• The limit is required to be a real number $L$; given the known bounds this loses nothing.
• Asymptotic statements ("for all sufficiently large $k$") are expressed with the atTop filter on $\mathbb N$; "$o(k)$" in GNNW is encoded as "for every $\delta>0$, eventually with exponent $(1+\delta)k$".
• Needed infrastructure: basic Ramsey theory for graphs on Fin n, binomial estimates, the probabilistic method for the lower bounds (Spencer uses the Lovász Local Lemma), and the CGMS book algorithm for the upper bounds. All of these are reusable beyond this mission.

Selected references

• [Er47] P. Erdős, Some remarks on the theory of graphs, Bull. Amer. Math. Soc. 53 (1947), 292–294. https://doi.org/10.1090/S0002-9904-1947-08785-1
• [ES35] P. Erdős and G. Szekeres, A combinatorial problem in geometry, Compositio Math. 2 (1935), 463–470. http://www.numdam.org/item/CM_1935__2__463_0/
• [Sp75] J. Spencer, Ramsey's theorem — a new lower bound, J. Combin. Theory Ser. A 18 (1975), 108–115. https://doi.org/10.1016/0097-3165(75)90071-0
• [Co09] D. Conlon, A new upper bound for diagonal Ramsey numbers, Ann. of Math. 170 (2009), 941–960. https://doi.org/10.4007/annals.2009.170.941
• [Sa23] A. Sah, Diagonal Ramsey via effective quasirandomness, Duke Math. J. 172 (2023). https://arxiv.org/abs/2005.09251
• [CGMS23] M. Campos, S. Griffiths, R. Morris, J. Sahasrabudhe, An exponential improvement for diagonal Ramsey, arXiv:2303.09521 (2023). https://arxiv.org/abs/2303.09521
• [GNNW24] P. Gupta, N. Ndiaye, S. Norin, L. Wei, Optimizing the CGMS upper bound on Ramsey numbers, arXiv:2407.19026 (2024). https://arxiv.org/abs/2407.19026
• [BBCGHMST24] P. Balister, B. Bollobás, M. Campos, S. Griffiths, E. Hurley, R. Morris, J. Sahasrabudhe, M. Tiba, Upper bounds for multicolour Ramsey numbers, arXiv:2410.17197 (2024). https://arxiv.org/abs/2410.17197
• [Er88] P. Erdős, Problems and results in combinatorial analysis and graph theory, Discrete Math. 72 (1988), 81–92.
• [Er93] P. Erdős, Some of my favorite solved and unsolved problems in graph theory, Quaestiones Math. 16 (1993), 333–350.
• Erdős Problems, Problem #77. https://www.erdosproblems.com/77

Motivation

A sunflower (also called a $\Delta$-system) with $k$ petals is a family of $k$ sets whose pairwise intersections are all equal to one common set, the kernel. In 1960 Erdős and Rado proved the sunflower lemma: every sufficiently large family of $n$-element sets contains a sunflower with $k$ petals, and they asked how large "sufficiently large" must be (Erdős–Rado 1960). The conjecture that the threshold is only exponential in $n$ is one of Erdős' best-known problems in extremal combinatorics; it is listed as Erdős Problem 20, and Erdős offered a $1000 prize for it. Sunflower bounds are used, for example, in Razborov's monotone circuit lower bounds and in the study of set systems with restricted intersections.

Timeline.

• 1960 — Erdős and Rado prove $(k-1)^n < f(n,k) \le (k-1)^n\, n! + 1$ and conjecture $f(n,k) \le c_k^{\,n}$ (ErRa60).
• 2019 — Alweiss, Lovett, Wu and Zhang prove $f(n,k) \le (C k^3 \log n \log\log n)^n$, the first bound of the form $(\log n)^{n(1+o(1))}$ for fixed $k$ (arXiv:1908.08483).
• 2020 — Rao simplifies the argument via Shannon's noiseless coding theorem and obtains $(\alpha k \log(kn))^n$ (arXiv:1909.04774); Tao gives an entropy proof of the same bound.
• 2021 — Bell, Chueluecha and Warnke obtain $f(n,k) \le (C k \log n)^n$ for $n,k \ge 2$ (arXiv:2009.09327).

The conjecture itself remains open, even for $k = 3$.

Setting

Fix natural numbers $n$ (the uniformity) and $k$ (the number of petals). A family $\mathcal F$ of sets is $n$-uniform if every member of $\mathcal F$ has exactly $n$ elements. A subfamily $\mathcal S \subseteq \mathcal F$ is a $k$-sunflower if $|\mathcal S| = k$ and there is a set $Y$ with $A \cap B = Y$ for all distinct $A, B \in \mathcal S$.

The sunflower threshold $f(n,k)$ is the least natural number $m$ such that every $n$-uniform family $\mathcal F$ (over any ground set) with $|\mathcal F| \ge m$ contains a $k$-sunflower.

Formalization targets

Goal — the sunflower conjecture (Erdős Problem 20)

$$\exists\, c:\mathbb N\to\mathbb N\quad \forall n \ge 1,\ \forall k:\qquad f(n,k) < c_k^{\,n}.$$

The constants $c_k$ are left unspecified; only the exponential shape in $n$ is asked for. A disproof (the negation of this statement) would equally settle the problem.

Milestones from the literature

1. Erdős–Rado upper bound: $f(n,k) \le (k-1)^n\, n! + 1$ for $n \ge 1$, $k \ge 2$.
2. Erdős–Rado lower bound: $(k-1)^n < f(n,k)$ for $n \ge 1$, $k \ge 2$.
3. Rao's bound: there is $\alpha > 1$ with $f(n,k) \le (\alpha k \log(kn))^n + 1$ for $n \ge 1$, $k \ge 2$.
4. Bell–Chueluecha–Warnke bound: there is $C \ge 4$ with $f(n,k) \le (C k \log n)^n$ for $n, k \ge 2$.

A supporting sanity check, $f(0,1) = 1$, is taken from the source formalization.

Significance

A positive answer would show that sunflower-free $n$-uniform families have at most exponential size, the correct order of magnitude by the Erdős–Rado lower bound; this would sharpen every application that currently loses a $\log n$ factor per coordinate, including monotone circuit lower bounds. A negative answer would show the $(\log n)^n$-type bounds of 2019–2021 are essentially the truth.

On the formal side, the Erdős–Rado upper bound has a Lean formalization recorded in the source file; the lower-bound construction and the spread-family / coding arguments behind the Rao and Bell–Chueluecha–Warnke bounds are, as far as this proposal records, not yet formalized. Formalizing them produces reusable infrastructure on spread families and random-subset (or entropy) arguments.

Difficulty

The classical induction on $n$ (pick a maximal family of pairwise disjoint members; if it is small, some element lies in many members, recurse on the link) loses a factor of about $n$ at each of $n$ steps, which is where $n!$ comes from. The modern arguments replace the recursion by an analysis of spread families, but each still loses a factor $\log n$ per level, and no known technique removes it. The case $k = 3$ is already open.

Formalization scope

• The ground set is an arbitrary type in the lowest universe; set families are Set (Set α) and sizes are measured with Set.ncard, which returns $0$ on infinite sets. Consequently, for $n \ge 1$ only finite members can be "$n$-element", and the condition $m \le |\mathcal F|$ with $m \ge 1$ only applies to finite families. All targets assume $n \ge 1$ (except the sanity check), so the $n = 0$ quirks do not affect them.
• $f(n,k)$ is defined as an infimum over natural numbers; if no admissible $m$ existed the infimum would be $0$. The Erdős–Rado upper bound shows the admissible set is non-empty for $n \ge 1$.
• Logarithms are natural logarithms; changing the base only rescales the unspecified constants.
• The goal is stated as the positive claim of the conjecture, not as a yes/no answer(·) statement.

Contributions of general lemmas on sunflowers, spread families and the Erdős–Rado construction are welcome and reusable beyond this mission.

Selected references

• P. Erdős, R. Rado, Intersection theorems for systems of sets, J. London Math. Soc. 35 (1960), 85–90. doi:10.1112/jlms/s1-35.1.85
• R. Alweiss, S. Lovett, K. Wu, J. Zhang, Improved bounds for the sunflower lemma, Annals of Mathematics 194 (2021). arXiv:1908.08483
• A. Rao, Coding for sunflowers, Discrete Analysis 2020:2. arXiv:1909.04774
• T. Bell, S. Chueluecha, L. Warnke, Note on sunflowers, Discrete Mathematics 344 (2021). arXiv:2009.09327
• Erdős Problem 20. erdosproblems.com/20

Motivation

Erdős asked, repeatedly and with a rising price tag, for an asymptotic formula for the largest subset of $\{1,\dots,N\}$ that contains no arithmetic progression of a given length. He offered 1000 dollars for it in [Er97c] and 10000 dollars in [Er81, p.4], where he called the question "probably enormously difficult"; elsewhere he described it as "probably unattackable at present". Most of modern additive combinatorics — the density increment method, the triangle removal lemma, Gowers uniformity norms, the arithmetic regularity lemma — grew out of attempts on this single question, and the answer is still unknown, even in the first non-trivial case $k=3$.

Timeline.

• 1936: Erdős and Turán conjecture that $r_k(N)=o(N)$ for every $k$.
• 1946: Behrend constructs large progression-free sets, giving $r_3(N)\ge N\exp(-c\sqrt{\log N})$.
• 1953: Roth proves $r_3(N)=o(N)$, with the quantitative form $r_3(N)\ll N/\log\log N$.
• 1961: Rankin generalises Behrend, giving $r_k(N)\ge N\exp(-c_k(\log N)^{1/(k-1)})$.
• 1969, 1975: Szemerédi proves $r_4(N)=o(N)$ and then $r_k(N)=o(N)$ for all $k$, settling Erdős–Turán.
• 1977: Furstenberg reproves Szemerédi's theorem ergodically, with no effective bound.
• 1998, 2001: Gowers introduces uniformity norms and obtains $r_k(N)\ll N(\log\log N)^{-c_k}$, the first effective bound for general $k$.
• 2017: Green and Tao obtain $r_4(N)\ll N(\log N)^{-c}$.
• 2020: Bloom and Sisask obtain $r_3(N)\ll N(\log N)^{-1-c}$, the first bound past the $N/\log N$ barrier.
• 2023: Kelley and Meka obtain $r_3(N)\le N\exp(-c(\log N)^{1/12})$.
• 2024: Leng, Sah and Sawhney obtain $r_k(N)\ll N\exp(-(\log\log N)^{c_k})$ for $k\ge5$.

Every upper bound in this list is still astronomically far from Behrend's lower bound, and no candidate asymptotic formula has been proposed for any $k\ge3$.

Setting

Fix an integer $k$. A non-trivial $k$-term arithmetic progression is a list $a,\,a+d,\,a+2d,\dots,a+(k-1)d$ of natural numbers with common difference $d>0$; the requirement $d>0$ is what "non-trivial" means, and it forces the $k$ terms to be distinct. A finite set $A\subseteq\mathbb N$ is $k$-AP-free if it contains no such progression. Write

$$r_k(N)\;=\;\max\bigl\{\,|A| \;:\; A\subseteq\{1,\dots,N\},\ A\ \text{is}\ k\text{-AP-free}\,\bigr\}.$$

The mission takes its formal definition of $r_k$ verbatim from the formal-conjectures entry for this problem, so that the goal below is literally the statement recorded there. In that development a set is called free of progressions of length $l$ when every subset of it that is an arithmetic progression of length $l$ forces $l\le1$. Progressions of length $0$ and $1$ count as trivial, so under that convention every set is free of them and $r_0(N)=r_1(N)=N$; the interesting range begins at $k\ge2$. Every statement in this mission that depends on the convention carries an explicit hypothesis on $k$.

On that range, $r_k(N)$ is non-decreasing in both $N$ and $k$, satisfies $r_k(M+N)\le r_k(M)+r_k(N)$, and hence, by Fekete's subadditivity lemma, $r_k(N)/N$ converges. Szemerédi's theorem is the statement that the limit is $0$; the whole difficulty of this mission lies in how fast it goes to $0$.

Formalization targets

Goal

$$r_k(N)\;=\;o_k\!\left(\frac{N}{\log N}\right)\qquad\text{for every }k>1 .$$

This is erdos_142.variants.lower of the formal-conjectures file for Erdős 142, reproduced binder for binder, over that file's own definition of $r_k$.

The headline theorem in that file, erdos_142, states $r_k(N)=\Theta(f)$ with the comparison function left as an answer(sorry) placeholder, and the same is true of its variants.upper and variants.three. Those are not closed propositions and cannot serve as a mission goal: the literal request of Erdős Problem #142 — "prove an asymptotic formula for $r_k(N)$" — has no known right-hand side for any $k\ge3$, which is exactly why the file leaves a hole there. variants.lower is the one formalizable target in the file, and it is also the strongest precisely-stated form the problem page attaches to #142: Erdős offered 5000 dollars for (essentially) exactly it, as recorded under Erdős Problem #3. It is known for $k=3$ — it follows from Bloom–Sisask 2020, and a fortiori from Kelley–Meka 2023 — trivial for $k=2$, where $r_2(N)=1$, and open for every $k\ge4$. It fixes no constants, so no future improvement can invalidate it.

A weaker open question

$$\frac{r_k(n)}{r_{k+1}(n)}\longrightarrow 0\qquad\text{for some }k\ge3 .$$

Erdős remarked in [Er80, p.92] that even this separation between consecutive progression lengths is not known. Here [Er80] is the erdosproblems.com bibliography key for Erdős's 1980 paper; it is a citation, not a pointer to Erdős Problem #80, which is an unrelated question about books in graphs. This statement has no counterpart in formal-conjectures: the file for #142 contains only the $\Theta$, $o$ and $O$ variants above, and the only two files in that repository that mention $r_k$ at all are the ones for #142 and #139.

Significance

Proving $r_k(N)=o_k(N/\log N)$ for all $k$ yields, by a standard summation argument, Erdős's conjecture that every $A\subseteq\mathbb N$ with $\sum_{a\in A}1/a=\infty$ contains arbitrarily long arithmetic progressions — the 5000-dollar Erdős Problem #3, of which the Green–Tao theorem on primes is the best-known special case. Below that threshold, quantitative bounds on $r_k$ control the density at which progressions must appear in any concrete set, and are the input to results on progressions in the primes, in sumsets, and in sparse random subsets of the integers.

Formalization status is uneven, and this mission is designed around that gap. Mathlib already contains the $k=3$ theory in a usable form: the predicate ThreeAPFree, the Roth number rothNumberNat, its subadditivity, and a complete formalization of Behrend's construction (Behrend.roth_lower_bound). Mathlib does not contain Roth's theorem, Szemerédi's theorem, or any of the modern upper bounds; to the best of current knowledge none of Roth, Szemerédi, Gowers, Green–Tao, Kelley–Meka or Leng–Sah–Sawhney has a machine-checked proof anywhere. The formal-conjectures entry states the problem but proves nothing: every declaration in it is a sorry. Two of this mission's targets are taken from that repository — the goal from its file for #142, and the Szemerédi milestone from its file for #139, which uses the same $r_k$; those are the only two files there that mention $r_k$. The milestones therefore split cleanly: the first six are reachable now on top of Mathlib, and the last five are open formalization projects of independent value.

Difficulty

Every known upper bound for $r_k$ runs a density increment: if $A\subseteq\{1,\dots,N\}$ of density $\delta$ has no $k$-term progression, find a long subprogression on which $A$ has density $\delta(1+c(\delta))$, and iterate. The bound this produces is governed entirely by two quantities — how large the increment $c(\delta)$ is, and how much of the interval survives one step. For $k\ge4$ the increment is extracted from an inverse theorem for the Gowers $U^{k-1}$-norm, and the best available correlation bounds there are quasipolynomial in $\delta$; iterating a quasipolynomial increment cannot do better than $N\exp(-(\log\log N)^{c})$, which is nowhere near $N/\log N$. Reaching $N/\log N$ requires an increment with polynomial dependence on $\delta$ together with a subprogression of polynomial length, and that combination is currently available only for $k=3$, through the sifting and almost-periodicity machinery of Kelley–Meka. No soft or averaging argument can substitute: Behrend's construction shows the truth at $k=3$ is $N\exp(-\Theta(\sqrt{\log N}))$, so the answer is not a power of $\log N$ and cannot be produced by any argument whose output has that shape.

Formalization scope

The mission's definition file Erdos142Basic carries two layers, and every statement in the mission is written against them.

1. The source definitions, ported verbatim. IsAPOfLengthWith, IsAPOfLength, IsAPOfLengthFree and r are the declarations of the formal-conjectures entry, transcribed unchanged into the mission's namespace: a set is an arithmetic progression of length $l$ with first term $a$ and difference $d$ when it has exactly $l$ elements and equals $\{a+nd : n<l\}$; it is free of length-$l$ progressions when every progression of length $l$ inside it forces $l\le1$; and $r_k(N)$ is the supremum of $|S|$ over subsets $S\subseteq\{1,\dots,N\}$ free of length-$k$ progressions. The ground set is Finset.Icc 1 N, and the supremum is sSup over $\mathbb N$; the file proves the two facts that make it a genuine maximum (le_r and r_le).

2. An elementary handle. HasAP k A is ∃ a d, 0 < d ∧ ∀ i < k, a + i * d ∈ A, and APFree k A its negation. This form carries no cardinality side condition in $\mathbb N\cup\{\infty\}$ and is what a solver actually wants to induct on. The first milestone is exactly the bridge between the two layers.

Two consequences of the source convention are worth stating plainly, because the prose is silent about them. Length-$0$ and length-$1$ progressions are trivial, so every set is free of them and $r_0(N)=r_1(N)=N$; monotonicity of $r_k$ in $k$ therefore holds only from $k\ge2$ onward, and the corresponding milestone carries that hypothesis. Asymptotic statements use Asymptotics.IsLittleO and Filter.atTop over $\mathbb N$ with real-valued casts, and real division is Lean's, so (N : ℝ) / Real.log N is $0$ at $N=1$; this is invisible to atTop.

A trivializing formalization is ruled out by construction: one milestone asserts $r_3(N)=$ rothNumberNat N, pinning this development against Mathlib's independently written definition of the Roth number, so a vacuous or mis-quantified notion of progression-freeness cannot survive. That milestone, the bridge milestone above it, and the monotonicity milestone have all been checked to be provable before this proposal was drafted.

A full development needs: discrete Fourier analysis on $\mathbb Z/N\mathbb Z$, Bohr sets and their regularity, the arithmetic regularity lemma, Gowers uniformity norms and the inverse theorem for them, and — for the lower bounds — sphere-counting in high-dimensional boxes (already in Mathlib via Behrend). All of this is reusable well beyond this mission. Contributions of any kind are welcome, including partial results: quantitative bounds weaker than the cited ones, the $k=3$ case of a general-$k$ milestone, and reusable Fourier-analytic infrastructure are all valuable even when they do not close a milestone.

Selected references

• Erdős Problem #142. https://www.erdosproblems.com/142
• Erdős Problem #3. https://www.erdosproblems.com/3
• Erdős Problem #139 (Szemerédi's theorem in the $r_k$ formulation), linked from #142. https://www.erdosproblems.com/139
• Google DeepMind, formal-conjectures, FormalConjectures/ErdosProblems/142.lean — the source of the goal statement and of the definition of $r_k$. https://github.com/google-deepmind/formal-conjectures/blob/main/FormalConjectures/ErdosProblems/142.lean
• F. A. Behrend, On sets of integers which contain no three terms in arithmetical progression, Proc. Nat. Acad. Sci. USA 32 (1946), 331–332. https://doi.org/10.1073/pnas.32.12.331
• K. F. Roth, On certain sets of integers, J. London Math. Soc. 28 (1953), 104–109. https://doi.org/10.1112/jlms/s1-28.1.104
• R. A. Rankin, Sets of integers containing not more than a given number of terms in arithmetical progression, Proc. Roy. Soc. Edinburgh Sect. A 65 (1961), 332–344.
• E. Szemerédi, On sets of integers containing no $k$ elements in arithmetic progression, Acta Arith. 27 (1975), 199–245. https://doi.org/10.4064/aa-27-1-199-245
• W. T. Gowers, A new proof of Szemerédi's theorem, Geom. Funct. Anal. 11 (2001), 465–588. https://doi.org/10.1007/s00039-001-0332-9
• B. Green and T. Tao, New bounds for Szemerédi's theorem, III: A polylogarithmic bound for $r_4(N)$, Mathematika 63 (2017), 944–1040. https://arxiv.org/abs/1705.01703
• T. F. Bloom and O. Sisask, Breaking the logarithmic barrier in Roth's theorem on arithmetic progressions, arXiv:2007.03528. https://arxiv.org/abs/2007.03528
• Z. Kelley and R. Meka, Strong bounds for 3-progressions, arXiv:2302.05537. https://arxiv.org/abs/2302.05537
• J. Leng, A. Sah and M. Sawhney, Improved bounds for Szemerédi's theorem, arXiv:2402.17995. https://arxiv.org/abs/2402.17995

Motivation

A $P_3$-packing in a graph is a collection of pairwise vertex-disjoint paths on three vertices. Determining the largest such packing is NP-hard even in restricted graph classes, so structural hypotheses that force an optimal packing are of independent interest in graph factor theory. The present question asks whether 3-vertex-connectivity and cubicity force the strongest possible packing whenever the vertex count permits a perfect partition.

A. Kelmans attributes the broader packing problem to 1984. In Problem 1.10 of Packing 3-vertex Paths in Cubic 3-connected Graphs, the question is whether every cubic 3-connected graph $G$ satisfies $\lambda(G)=\lfloor |V(G)|/3\rfloor$. Theorem 3.1 of that paper proves that the divisible-order factor statement is equivalent to several apparently stronger deletion and prescribed-edge statements; it does not prove the open claim itself. OPG-46613 records the divisible-order form targeted here.

A 2026 candidate analysis in the Vibe Mathing problem repository investigated a tempting sufficient route: find a perfect matching whose complementary 2-factor has every cycle length divisible by three. Candidate C01 explains why that condition would yield a $P_3$-factor. Candidate C02 gives an explicit proposed family $H_q$ of order $18+12q$ that has $P_3$-factors but is claimed not to satisfy the stronger matching condition. These candidate claims have computational and partial Lean checks, but no complete Lean kernel proof; they are milestones here, not declarations that the original problem or the candidate family has already been formally established.

Setting

All graphs are finite and simple. A graph is cubic when every vertex has exactly three neighbors. It is 3-vertex-connected here when it has at least four vertices and deleting any set of at most two vertices leaves a connected induced graph.

A $P_3$-factor is represented by a natural number $b$, together with a bijection

$$\operatorname{Fin}(b)\times\operatorname{Fin}(3)\simeq V(G),$$

such that, in every block, positions $0$ and $1$ are adjacent and positions $1$ and $2$ are adjacent. The path is not required to be induced: an ambient edge between positions $0$ and $2$ is allowed because the two selected path edges still form a copy of $P_3$.

A 2-factor is a spanning 2-regular subgraph. It is called divisible when every one of its connected components has order divisible by three. A divisible matching complement is a perfect matching $M$ such that the relative complement $G\setminus M$ is a divisible 2-factor.

The explicit graph $H_q$ is defined on $\operatorname{Fin}(18+12q)$. Its first nine vertices form the fixed Petersen-minus-one-vertex brick from C02; the remaining vertices form the stated cycle-and-opposite-chord brick with three joining edges. The full adjacency relation is part of the Lean definition rather than an external data file.

Formalization targets

Main goal

For every finite simple graph $G$,

$$\bigl(G\text{ cubic}\bigr)\land \bigl(G\text{ 3-vertex-connected}\bigr)\land 3\mid |V(G)| \quad\Longrightarrow\quad G\text{ has a }P_3\text{-factor}.$$

This is the OPG-46613 target. Cubicity forces the order to be even, so within this domain divisibility by three is equivalent to divisibility by six.

Literature and route milestones

The mission also formalizes the $(z1)\Leftrightarrow(z8)$ part of Kelmans's Theorem 3.1: the divisible-order factor claim is equivalent to the assertion that deleting any specified 3-vertex path leaves a $P_3$-factor. Two route lemmas state that divisible 2-factors split into $P_3$-factors and that, in cubic graphs, divisible 2-factors are equivalent to divisible perfect-matching complements.

Candidate boundary milestones

The C02 milestones ask first for the complete 18-vertex statement and then for the full family:

$$\forall q\in\mathbb N,\quad H_q\text{ is cubic and 3-vertex-connected, has a }P_3\text{-factor, and has no divisible matching complement}.$$

This separates a sufficient method from the root conclusion. It is not a counterexample to OPG-46613 because every $H_q$ in the proposed family explicitly satisfies the desired $P_3$ conclusion.

Significance

A proof of the main goal would settle the divisible-order form of a long-standing path-packing problem. Through Kelmans's equivalences it would also control several deletion and prescribed-edge variants for cubic 3-connected graphs. A disproof would require a graph satisfying all domain hypotheses but lacking a $P_3$-factor; the C02 family does not claim this.

Formalizing the candidate boundary is useful even before the root is resolved. It turns a route exclusion into a checkable theorem and prevents a search campaign from silently assuming that every relevant graph possesses a divisible complementary 2-factor. The definitions of noninduced $P_3$-factors, vertex connectivity by deletion, perfect matchings, 2-factors, and component-order divisibility are intended to be reusable in later graph-factor work.

Difficulty

The perfect-matching route is attractive because the complement of a perfect matching in a cubic graph is 2-regular. The obstruction is that its cycles need not have lengths divisible by three. The C02 candidate family is designed to expose exactly that gap: a persistent 5-cycle is claimed to occur in every complementary 2-factor even though an unrelated $P_3$-factor exists. Consequently, proving the main theorem cannot simply assume that a favorable perfect matching always exists.

The formal difficulty is also semantic. Connectivity must mean vertex connectivity, the complement must be relative to $G$ on the same vertex set, component sizes must refer to the 2-factor rather than the ambient graph, and $P_3$ must remain noninduced. Weakening any of these points can create a materially different or vacuous theorem.

Formalization scope

The development targets Lean 4.33.1 and Mathlib revision 0df444a360eaa60ab8c11dca51a86af692955474. Graphs use SimpleGraph on finite vertex types. Degree is the cardinality of the actual neighbor subtype. Three-vertex-connectivity explicitly quantifies over all finite deletion sets of cardinality at most two and includes a four-vertex order guard.

The main theorem is universe-polymorphic and does not hard-code a finite graph enumeration. The $H_q$ family includes $q=0$. The factor structure uses a bijection, so disjointness and coverage cannot be discharged by duplicate or omitted vertices. Ambient chords do not invalidate a block, while both required consecutive adjacencies must be genuine graph edges. The candidate family statements remain open theorem goals ending in sorry; the shared definition module itself is sorry-free.

Welcome contributions include proofs of the model lemmas, the finite $H_0$ statement, the general C02 family, Kelmans's equivalence, or decompositions of the root theorem into faithful reusable lemmas. Numerical enumeration alone is supporting evidence and should not be presented as a kernel proof.

Selected references

• A. Kelmans, Packing 3-vertex Paths In Cubic 3-connected Graphs, arXiv:0910.2766v2, 2011, Problem 1.10 (p. 3) and Theorem 3.1 (pp. 7–8). https://arxiv.org/abs/0910.2766v2
• UnsolvedMath, OPG-46613: P3-partitions of cubic 3-connected graphs. https://www.unsolvedmath.com/problems/OPG-46613
• Vibe Mathing, C01: divisible-cycle implication and a 30-vertex obstruction, fixed repository revision 14b8dc64ac2d89c98cf3a2bbb2fcba76ced0df6a. https://github.com/vibemathing/problem-opg-46613-cubic-p3-partition/blob/14b8dc64ac2d89c98cf3a2bbb2fcba76ced0df6a/research/artifacts/candidates/opg46613-c01/proof.md
• Vibe Mathing, C02: an 18-vertex obstruction and an infinite family with P3-factors, fixed repository revision 14b8dc64ac2d89c98cf3a2bbb2fcba76ced0df6a. https://github.com/vibemathing/problem-opg-46613-cubic-p3-partition/blob/14b8dc64ac2d89c98cf3a2bbb2fcba76ced0df6a/research/artifacts/candidates/opg46613-c02/proof.md

Motivation

The simplex method walks along edges of a polytope from vertex to vertex. Whether any pivot rule could ever make that walk short in the worst case is governed by a prior, purely geometric question: how far apart, in the edge graph, can two vertices of a polytope be? Warren Hirsch conjectured in 1957 that the diameter of a $d$-dimensional polytope with $n$ facets is at most $n - d$. Half a century of upper bounds stalled at quasi-polynomial, and Santos disproved the conjecture itself in 2012 — but only by a constant factor. The surviving question, the subject of the Polymath 3 project, is the polynomial Hirsch conjecture: is the diameter bounded by a polynomial in $n$ and $d$?

Timeline

• 1957. Hirsch states the conjecture $\mathrm{diam} \le n - d$ in a letter to Dantzig, who publishes it in Linear Programming and Extensions (1963).
• 1964–1966. Klee determines the exact maximum diameter of $3$-polytopes with $n$ facets, $\lfloor 2n/3\rfloor - 1$ — the Hirsch bound holds up to dimension three.
• 1967. Klee and Walkup (Acta Math.) refute the unbounded-polyhedron version, prove the bounded conjecture for $n - d \le 5$, and reduce the general case to the $d$-step conjecture ($n = 2d$).
• 1970. Larman (Proc. LMS) proves $\mathrm{diam} \le n\,2^{d-3}$ — linear in the number of facets for each fixed dimension, still the best bound of that shape.
• 1989. Naddef (Math. Programming) proves $0/1$-polytopes satisfy the Hirsch bound, with diameter at most $d$.
• 1992. Kalai and Kleitman (Bull. AMS) prove $\mathrm{diam} \le n^{\log_2 d + 2}$ in under a page — the quasi-polynomial barrier every later bound refines. The same year brings subexponential pivot rules (Kalai; Matoušek–Sharir–Welzl), the algorithmic counterpart.
• 2010. Eisenbrand, Hähnle, Razborov, and Rothvoß (Math. OR) show the known upper-bound arguments survive in a purely combinatorial abstraction — which admits almost-quadratic lower bounds, so a polynomial bound must use real geometry. Kalai launches Polymath 3 on the polynomial version.
• 2010–2012. Santos (Annals of Math.) disproves the Hirsch conjecture: a $43$-dimensional polytope with $86$ facets and diameter at least $44$, via spindles of large width.
• 2014–2019. Todd (SIAM J. Discrete Math.) sharpens Kalai–Kleitman to $(n-d)^{\log_2 d}$; Sukegawa refines further. Matschke, Santos, and Weibel (Proc. LMS 2015) shrink the counterexample to dimension $20$ with $40$ facets and diameter $21$. All known violations remain constant-factor; all known bounds remain quasi-polynomial.

Setting

Work in $\mathbb{R}^d$. An H-polytope is a set cut out by finitely many linear inequalities: given vectors $a_1, \dots, a_n \in \mathbb{R}^d$ and reals $b_1, \dots, b_n$, it is

$$P \;=\; \{\, x \in \mathbb{R}^d \mid \langle a_i, x\rangle \le b_i \text{ for } i = 1, \dots, n \,\},$$

where $\langle a_i, x\rangle = \sum_{j=1}^d a_{ij} x_j$ is the standard inner (dot) product — so each condition $\langle a_i, x\rangle \le b_i$ is one linear inequality, with normal vector $a_i$ and offset $b_i$. Throughout, $P$ is assumed nonempty and bounded. The parameter $n$ counts the inequalities in the given description; since every polytope with $f$ facets admits a description by exactly $f$ inequalities, bounds stated in terms of $n$ over all descriptions are equivalent to bounds in terms of facet counts.

A vertex of $P$ is an extreme point. Two vertices $u \ne v$ are adjacent when the segment $[u, v]$ is an extreme subset of $P$; for a polytope the convex extreme subsets are exactly the faces, so this says precisely that $[u,v]$ is a one-dimensional face — an edge. The combinatorial diameter of $P$ is the diameter of the graph of vertices and edges. Throughout, "diameter at most $B$" is expressed as: every two vertices are joined by a walk of $B$ steps, each step staying put or crossing an edge — a form that is monotone in $B$ and asserts connectivity of the graph (Balinski's theorem) as part of the claim.

Formalization targets

Goal — the polynomial Hirsch conjecture

$$\exists\, c, k \in \mathbb{N}:\ \text{every nonempty bounded } P = \{x \in \mathbb{R}^d \mid \langle a_i, x \rangle \le b_i,\ i \le n\} \text{ has diameter} \le c\,(n + d)^k.$$

Every polynomial in $n$ and $d$ is dominated by some $c(n+d)^k$ and conversely, so this is exactly polynomiality, with no committed degree — the form that survives any future sharpening of constants or exponents.

Milestones — the known ladder

Six classical results over the same definitions: the Hirsch bound $n - d$ in dimension $d \le 3$ (Klee; Klee–Walkup); Larman's bound $n \cdot 2^{d-3}$; Naddef's bound $d$ for $0/1$-polytopes; the Kalai–Kleitman bound $n^{\log_2 d + 2}$; Todd's bound $(n-d)^{\log_2 d}$ for full-dimensional $P$ with $n \ge d \ge 3$; and — in the other direction — the Santos counterexample: a nonempty bounded H-polytope whose diameter exceeds $n - d$.

Significance

A polynomial diameter bound is necessary for any pivot rule of the simplex method to run in polynomial time in the worst case: if vertices can be super-polynomially far apart, no edge-following algorithm can connect them quickly. A refutation would close off one of the main hoped-for routes to a strongly polynomial linear programming algorithm (Smale's ninth problem). The conjecture is also the test question of polyhedral graph theory: the Kalai–Kleitman argument uses so little about polytopes that it holds for far more general set systems, and Eisenbrand, Hähnle, Razborov, and Rothvoß (Math. OR 2010) showed such abstractions admit almost-quadratic lower bounds — so a proof of the conjecture must use geometry the abstract setting lacks, and a disproof must beat the abstraction barrier's constructions with actual polytopes.

None of these results has been formalized in any proof assistant; Mathlib has extreme points and faces of convex sets, but no polytope combinatorics — no vertex-edge graph, no diameter, no facet counting. This mission builds that layer: an H-polytope model, adjacency via faces, and walk-based diameter bounds, against which both the upper-bound ladder and the Santos disproof can be machine-checked. The Kalai–Kleitman proof is one page from first principles and is the natural summit; the Santos construction is a concrete finite object whose verification is a different, computational kind of challenge.

Difficulty

The naive approach — walk toward the target vertex by always improving some linear objective — is exactly the simplex method, and proving any polynomial bound on such walks is open for every known pivot rule; monotone variants of the diameter question have exponential lower bounds. The obvious inductive strategy (bound the diameter by recursing on facets) is precisely what Kalai–Kleitman optimizes, and it provably cannot go below quasi-polynomial without using metric or topological properties of actual polytopes, by the abstraction lower bound above. On the other side, making diameters large is blocked by the wedge/spindle calculus only producing constant-factor violations. The problem sits in a genuine gap: no technique on either side is known to reach polynomial.

Formalization scope

The Lean model commits to: ambient space EuclideanSpace ℝ (Fin d); the polytope as Hpoly a b = {x | ∀ i, ⟪a i, x⟫ ≤ b i} for a : Fin n → EuclideanSpace ℝ (Fin d), b : Fin n → ℝ, with nonemptiness and Bornology.IsBounded as explicit hypotheses (boundedness is essential: Klee–Walkup's unbounded counterexample would otherwise trivialize the Santos milestone); vertices as Set.extremePoints ℝ; adjacency as u ≠ v ∧ IsExtreme ℝ P (segment ℝ u v); and diameter bounds as the walk predicate DiamLE, whose stationary steps make it monotone in the bound. Real-exponent bounds enter through Real.logb and the natural floor. In larman_bound and the two Hirsch-form bounds the subtraction is natural-number (truncated) subtraction, which only weakens nothing: the stated forms are true as written for all $n, d$ in scope. The dimension parameter $d$ is the ambient dimension; lower-dimensional polytopes are included, and every milestone is stated so as to remain true for them, with todd_bound requiring full-dimensionality ((interior P).Nonempty) as in its source.

Welcome contributions: any milestone in any order (dimension_three_bound for $d \le 1$ cases and structural lemmas about Adj and DiamLE are natural entry points, and kalai_kleitman_bound is the summit); reusable infrastructure — polytopes have finitely many extreme points, faces of H-polytopes, Balinski connectivity — published as platform theorems; and, as a separate expedition, the explicit Santos or Matschke–Santos–Weibel polytope. Statements about unbounded polyhedra, the simplex method itself, and subexponential pivot rules are left to future missions.

Selected references

• V. Klee, D. Walkup, The d-step conjecture for polyhedra of dimension d < 6, Acta Math. 117 (1967). doi:10.1007/BF02392971
• D. Larman, Paths on polytopes, Proc. London Math. Soc. 20 (1970). doi:10.1112/plms/s3-20.2.249
• D. Naddef, The Hirsch conjecture is true for (0,1)-polytopes, Math. Programming 45 (1989). doi:10.1007/BF01589418
• G. Kalai, D. Kleitman, A quasi-polynomial bound for the diameter of graphs of polyhedra, Bull. AMS 26 (1992). arXiv:math/9204233
• F. Santos, A counterexample to the Hirsch conjecture, Annals of Mathematics 176 (2012). arXiv:1006.2814
• M. Todd, An improved Kalai–Kleitman bound for the diameter of a polyhedron, SIAM J. Discrete Math. 28 (2014). arXiv:1402.3579
• B. Matschke, F. Santos, C. Weibel, The width of five-dimensional prismatoids, Proc. London Math. Soc. 110 (2015). arXiv:1202.4701
• F. Eisenbrand, N. Hähnle, A. Razborov, T. Rothvoß, Diameter of polyhedra: limits of abstraction, Math. Oper. Res. 35 (2010). doi:10.1287/moor.1100.0470
• F. Santos, Recent progress on the combinatorial diameter of polytopes and simplicial complexes, TOP 21 (2013) (survey). arXiv:1307.5900