Motivation
Integrable spin chains with a Lie superalgebra symmetry gl(M∣N) are lattice regularizations of two-dimensional field theories with target-space supersymmetry, such as the gl(M∣N) Gross–Neveu models and sigma models on superspaces, which appear in the study of disordered systems, polymers and the AdS/CFT correspondence. Their spectra are described by the nested Bethe ansatz: each eigenvalue of the transfer matrix corresponds to a family of complex numbers, the Bethe roots, solving a system of algebraic equations, the Bethe ansatz equations (BAE). The nested algebraic Bethe ansatz for general closed gl(M∣N) chains was carried out by Belliard and Ragoucy (arXiv:0804.2822).
C. Candu, Continuum Limit of gl(M|N) Spin Chains (arXiv:1012.0050), studies the antiferromagnetic chain (V⊗V∗)⊗L of alternating fundamental and dual representations. Its §4 shows that a class of solutions of the gl(M∣N) BAE is fully characterized by solutions of the gl(M−1∣N−1) BAE, which embeds the gl(M−1∣N−1) spectrum into the gl(M∣N) spectrum. This is the algebraic counterpart of cohomological reduction (Candu–Creutzig–Mitev–Schomerus, arXiv:1001.1344), in which the gl(M−1∣N−1) chain is the cohomology of an odd nilpotent element acting on the gl(M∣N) chain. The paper uses this relation to compare all gl(n+N∣N) chains with n fixed, and concludes that their continuum limits share the gl(n) Gross–Neveu-like particle content.
This mission formalizes that restriction and lift of Bethe ansatz equations.
Setting
Let d=M+N and r=d−1. A grading assigns to each position i∈{1,…,d} a parity ∣i∣∈{0,1} and a sign σi=(−1)∣i∣; M counts the even and N the odd positions. The loop weight is n=∑i=1dσi=M−N. The simple roots are αk=ϵk−ϵk+1, k=1,…,r; αk is odd when σkσk+1=−1.
For a chain of length L the source terms are
Λ1(u)=(u+σ1)L(u+2n)L,Λi(u)=uL(u+2n)L (2≤i≤r),Λd(u)=uL(u+2n−σd)L.
For each type k=1,…,r let {uj(k)}j=1ν(k) be a finite set of pairwise distinct complex numbers and Qk(u)=∏j(u−uj(k)), with Q0=Qd=1. The BAE (2.9) state that for every k and every root u=uj(k),
Λk+1(u)Λk(u)=−σkσk+1Qk−1(u+σk)Qk(u−σk)Qk+1(u)Qk−1(u)Qk(u+σk+1)Qk+1(u−σk+1).
With xj(k)=iuj(k)+2i∑l≤kσl (2.11), the energy (2.18) of a solution is
E=−j∑(xj(1))2+1/4σ1−j∑(xj(r))2+1/4σd.
The paper works throughout under n=M−N>0.
Fix k with A1 (αk odd) and A2 (αk has no source term, k=1,r). R1 identifies the roots of types k−1 and k+1. The reduced system deletes grading positions k,k+1 and root types k,k+1: it is the BAE of a gl(M−1∣N−1) chain with simple roots {α1,…,αk−2,ϵk−1−ϵk+2,αk+2,…,αr}.
Formalization targets
Goal: restriction and lift (§4.1–4.2)
Under A1, A2, R1 and non-degeneracy of the roots,
BAEgl(M∣N)({u(l)}l=1r)⟺BAEgl(M−1∣N−1)({u(l)}l=k,k+1) ∧ (2.9)k−1 at the roots of type k−1,
and Egl(M∣N)=Egl(M−1∣N−1). The forward direction is the restriction R1–R2; the backward direction is the reduction of the lift to "a solution … to either eq. (4.2) or eq. (4.4)".
Milestones
- Under A1–A2 the equations of types k−1, k, k+1 take the printed forms (4.2), (4.3), (4.4) (p. 17).
- Under R1, (4.3) holds automatically (p. 18).
- R2: (4.2) and (4.4) at a common root give (4.6), in which the type-k roots no longer appear (p. 18).
- The restricted equations are exactly the gl(M−1∣N−1) BAE for the reduced simple root system, with the same loop weight n (p. 18).
- Given a reduced solution and R1, (4.2) and (4.4) are equivalent (p. 19).
Significance
The restriction gives an explicit map from gl(M−1∣N−1) Bethe solutions to gl(M∣N) Bethe solutions that preserves the energy. Iterated, it relates every gl(n+N∣N) chain to the gl(n) chain, which is how the paper identifies the vacuum and the massive excitations of the gl(n+N∣N) chains in the continuum limit (§5–§6). It is also a concrete, equation-level check of cohomological reduction.
The result is proved in the paper by direct algebra. The result is not formalized anywhere on Prove2Me, which has no Bethe ansatz equations of any kind. This mission produces a precise statement of the nested BAE with their grading-dependent signs, shifts and boundary conventions, and a verified version of the restriction, including the bookkeeping that the paper leaves implicit: the reindexing of types, the change of the source terms of the reduced chain, and the invariance of the energy.
Difficulty
Each step is elementary algebra; the difficulty is bookkeeping. The sign −σkσk+1, the shifts ±σk, ±σk+1 and the boundary cases Q0=Qd=1 differ between the original and the reduced system: after deletion, the neighbour of type k+2 is type k−1, and the last source term Λd and the partial sums ∑l≤kσl in (2.11) are those of a different grading. A naive argument that matches equations by position, or that treats the reduced system with the original source terms, misses these shifts. The backward direction needs non-degeneracy, because recovering (4.4) from (4.6) and (4.2) requires dividing by the two sides of (4.2).
Formalization scope
Everything is in the namespace SuperBAE.Restriction and works over C. Positions 1,…,d and types 1,…,d−1 are natural numbers, and a grading is a Boolean function (true = odd), with σi∈{±1}. Root sets are Finset ℂ, which encodes the paper's requirement that no two roots of the same type coincide. Qk equals 1 outside 1≤k≤d−1. Only the homogeneous chain is modelled. The BAE are in division form, so every theorem carries a regularity hypothesis: at each root u of type k, the values Λk(u), Λk+1(u), Qk−1(u+σk), Qk(u−σk) and Qk+1(u) are nonzero. This is the non-degeneracy the division form presupposes; it is imposed on the original system only. The standing assumption n>0 is kept as N<M, although the algebra of §4 does not use it. A1 is the Boolean condition ∣k∣=∣k+1∣, and A2 is 2≤k, k+2≤d.
The reduced system is the same BAE predicate applied to the deleted grading on d−2 positions and the reindexed roots, with its own source terms and its own loop weight. A formalization that writes the reduced equations by hand, or passes n to the reduced chain as a parameter, is trivializing and is ruled out. Without the regularity hypothesis, Lean's convention x/0=0 would make degenerate configurations satisfy (2.9) vacuously; regularity excludes this, and a verification file checks that the hypotheses of the goal are jointly satisfiable and that regularity is not vacuous.
Out of scope: the Yang–Baxter and Yangian structure, the walled Brauer algebra, the weight constraints (4.1) and (4.8), existence and uniqueness of lifted type-k roots, inhomogeneous chains, the boundary case k=1,r of §4.3, and the thermodynamic limit. Proofs of the milestones are welcome, as are lemmas that put the finite-product manipulations of Q-polynomials into a reusable form.
Selected references
- C. Candu, Continuum Limit of gl(M|N) Spin Chains, arXiv preprint (v1, 2010), JHEP 2011. https://arxiv.org/abs/1012.0050
- S. Belliard and E. Ragoucy, Nested Bethe ansatz for 'all' closed spin chains, J. Phys. A 41, 295202 (2008). https://arxiv.org/abs/0804.2822
- C. Candu, T. Creutzig, V. Mitev and V. Schomerus, Cohomological Reduction of Sigma Models, JHEP 05, 047 (2010). https://arxiv.org/abs/1001.1344
- F. H. L. Eßler, H. Frahm and H. Saleur, Continuum Limit of the Integrable sl(2|1) 3–3̄ Superspin Chain, Nucl. Phys. B 712, 513 (2005). https://arxiv.org/abs/cond-mat/0501197
- E. Ogievetsky and P. Wiegmann, Factorized S-Matrix and the Bethe Ansatz for Simple Lie Groups, Phys. Lett. B 168, 360 (1986).