Enumerative invariants and wall-crossing formulae in abelian categories

Abstract: Enumerative invariants in Algebraic Geometry 'count' $\tau$-(semi)stable objects $E$ with fixed topological invariants $[E]=a$ in some geometric problem, using a virtual class $[{\cal M}a^{\rm ss}(\tau)]{\rm virt}$ in homology, for the moduli spaces ${\cal M}a^{\rm st}(\tau)\subseteq{\cal M}_a^{\rm ss}(\tau)$ of $\tau$-(semi)stable objects. We get numbers by taking integrals $\int{[{\cal M}a^{\rm ss}(\tau)]{\rm virt}}\Upsilon$ for cohomology classes $\Upsilon$. Let $\cal A$ be a $\mathbb C$-linear abelian category in Algebraic Geometry. There are two moduli stacks of objects in $\cal A$: the usual moduli stack $\cal M$, and the 'projective linear' moduli stack $\cal M^{\rm pl}$. We give $H_({\cal M})$ the structure of a vertex algebra, and $H_({\cal M}^{\rm pl})$ a Lie algebra. Virtual classes $[{\cal M}a^{\rm ss}(\tau)]{\rm virt}$ lie in $H_({\cal M}^{\rm pl})$. We develop a universal theory of enumerative invariants in such $\mathcal A$. Virtual classes $[{\cal M}a^{\rm ss}(\tau)]{\rm virt}$ are only defined when ${\cal M}a^{\rm st}(\tau)={\cal M}_a^{\rm ss}(\tau)$. We define invariants $[{\cal M}_a^{\rm ss}(\tau)]{\rm inv}$ in $H_({\cal M}^{\rm pl})$ for all $a$, with $[{\cal M}a^{\rm ss}(\tau)]{\rm inv}=[{\cal M}a^{\rm ss}(\tau)]{\rm virt}$ when ${\cal M}a^{\rm st}(\tau)={\cal M}_a^{\rm ss}(\tau)$. If $\tau,\tau'$ are stability conditions on $\cal A$, we prove a wall-crossing formula writing $[{\cal M}_a^{\rm ss}(\tau')]{\rm inv}$ in terms of the $[{\cal M}b^{\rm ss}(\tau)]{\rm inv}$, using the Lie bracket on $H_*({\cal M}^{\rm pl})$. We apply our results for $\cal A$ the representations of a quiver or quiver with relations, or coh$(X)$ for $X$ a curve, surface or Fano 3-fold, or a category of 'pairs' in coh$(X)$ for $X$ a curve or surface. This proves conjectures in Gross-Joyce-Tanaka arXiv:2005.05637.