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arxiv: 2607.01355 · v1 · pith:SXLBQFCLnew · submitted 2026-07-01 · 🧮 math.AG · math.RT

Hecke operators on symplectic surfaces and chi-independence

Pith reviewed 2026-07-03 18:35 UTC · model grok-4.3

classification 🧮 math.AG math.RT
keywords BPS cohomologychi-independenceHecke operatorscohomological Hall algebrasymplectic surfacesmoduli spaces of sheavestautological classesMukai vectors
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The pith

BPS cohomology of one-dimensional sheaves on quasi-projective symplectic surfaces is chi-independent relative to the Chow variety.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper proves Toda's chi-independence conjecture for the BPS cohomology of moduli spaces of one-dimensional sheaves on quasi-projective symplectic surfaces. It constructs this result using Hecke operators that act on BPS cohomology by modifying sheaves with zero-dimensional quotients, derived from a bialgebra structure on the cohomological Hall algebra. A sympathetic reader would care because the result resolves an open conjecture and simultaneously identifies the associated BPS Lie algebra with the subspace of tautological classes, extending a known generation theorem. The argument relies on reducing a factorization coproduct from three-dimensional algebras to obtain the necessary bialgebra structure.

Core claim

We prove Toda's chi-independence conjecture for the BPS cohomology of moduli spaces of one-dimensional sheaves on quasi-projective symplectic surfaces, relative to the Chow variety. We also identify the BPS Lie algebra associated with one-dimensional Mukai vectors with the subspace of tautological classes, giving an extension of Markman's tautological generation theorem from primitive to arbitrary Mukai vectors. The main structure input is a bialgebra structure on the cohomological Hall algebra of coherent sheaves on a quasi-projective symplectic variety S. The coproduct is obtained, by dimensional reduction, from a factorization coproduct for 3d cohomological Hall algebras, and gives rise t

What carries the argument

Hecke operators on BPS cohomology that modify one-dimensional sheaves by zero-dimensional quotients, obtained from the bialgebra structure on the cohomological Hall algebra via dimensional reduction from a 3d factorization coproduct.

If this is right

  • BPS cohomology of the moduli spaces is independent of the Euler characteristic relative to the Chow variety.
  • The BPS Lie algebra for one-dimensional Mukai vectors coincides exactly with the subspace of tautological classes.
  • Markman's tautological generation theorem extends from primitive to arbitrary Mukai vectors.
  • A global BPS Lie algebra is attached to the stack of all coherent sheaves on the symplectic variety S.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same bialgebra and Hecke construction may yield explicit formulas for BPS invariants in low-dimensional examples.
  • The approach could extend to compute similar invariants for moduli spaces with different stability conditions.
  • The identification with tautological classes suggests these classes control the full ring structure in the BPS setting.

Load-bearing premise

The bialgebra structure on the cohomological Hall algebra arises by dimensional reduction, and the affinized BPS cohomology of the semistable locus equals the primitive part of the coproduct on the full moduli stack.

What would settle it

Compute the graded dimensions of the BPS cohomology for two different Euler characteristics on the same component of the Chow variety for a fixed symplectic surface and one-dimensional Mukai vector; a mismatch would falsify chi-independence.

read the original abstract

We prove Toda's chi-independence conjecture for the BPS cohomology of moduli spaces of one-dimensional sheaves on quasi-projective symplectic surfaces, relative to the Chow variety. We also identify the BPS Lie algebra associated with one-dimensional Mukai vectors with the subspace of tautological classes, giving an extension of Markman's tautological generation theorem from primitive to arbitrary Mukai vectors. The main structure input is a bialgebra structure on the cohomological Hall algebra of coherent sheaves on a quasi-projective symplectic variety S. The coproduct is obtained, by dimensional reduction, from a factorization coproduct for 3d cohomological Hall algebras, and gives rise to a global BPS Lie algebra attached to the stack of coherent sheaves on S. The link between this structure and the applications to chi-independence and tautological generation is provided by Hecke operators on BPS cohomology, which modify one-dimensional sheaves by zero-dimensional quotients. To make this construction work, we prove that there is an identification between the affinized BPS cohomology of the semistable locus and the primitive part of the coproduct on the entire moduli stack

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 2 minor

Summary. The manuscript proves Toda's χ-independence conjecture for the BPS cohomology of moduli spaces of one-dimensional sheaves on quasi-projective symplectic surfaces, relative to the Chow variety. It also identifies the BPS Lie algebra associated with one-dimensional Mukai vectors with the subspace of tautological classes, extending Markman's tautological generation theorem. The central technical input is a bialgebra structure on the cohomological Hall algebra of coherent sheaves on a quasi-projective symplectic variety S, obtained by dimensional reduction from a factorization coproduct for 3d cohomological Hall algebras; this yields a global BPS Lie algebra. The link to the applications is provided by Hecke operators on BPS cohomology together with the identification of the affinized BPS cohomology of the semistable locus with the primitive part of the coproduct on the full moduli stack.

Significance. If the derivations hold, the work constitutes a substantial advance on chi-independence and tautological generation for BPS cohomology in the setting of symplectic surfaces. The construction of the bialgebra via dimensional reduction supplies a concrete structural input that is then applied via Hecke operators, and the identification with tautological classes extends an existing theorem from primitive to arbitrary Mukai vectors. These results strengthen the dictionary between cohomological Hall algebras, BPS Lie algebras, and geometric invariants on moduli stacks.

minor comments (2)
  1. §2.3: the definition of the affinized BPS cohomology is introduced after its first use in the statement of the key identification; moving the definition forward would improve readability.
  2. Notation for the Mukai vector v and the Chow variety is used consistently but would benefit from a single consolidated table of symbols in the preliminaries.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive summary and recommendation of minor revision. No major comments were listed in the report, so we have no specific points requiring response or revision at this stage. We are prepared to incorporate any minor suggestions that may arise.

Circularity Check

0 steps flagged

Derivation is self-contained with independent structure inputs

full rationale

The paper constructs a bialgebra on the CoHA via dimensional reduction from a 3d factorization coproduct, states an identification of affinized BPS cohomology on the semistable locus with the primitive part of the coproduct as a theorem with supporting arguments, and applies Hecke operators to reach chi-independence and tautological generation. These steps are presented with explicit definitions and reductions internal to the manuscript; no load-bearing step reduces by construction to a fitted parameter, self-definition, or unverified self-citation chain. The derivation chain remains independent of its target conclusions.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Based solely on the abstract, no specific free parameters, axioms, or invented entities are identifiable. The work relies on established structures like cohomological Hall algebras and BPS cohomology.

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Reference graph

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