Strong cohomological integrality for symmetric stacks

Abstract: We prove a strong form of the cohomological integrality theorem, decomposing the cohomology of smooth symmetric stacks as the cohomological Hall induction of the intersection cohomology of the good moduli spaces of stacks of graded points. This generalizes the previous result by the second author together with Bu--Davison--Ib\'a~nez Nu~nez--P\u{a}durariu to non-orthogonal stacks, and confirms a conjecture of the first author that the algebraic BPS cohomology of the quotient stack of a symmetric representation matches the intersection cohomology group whenever it is nonzero. As a consequence, we obtain a version of the cohomological integrality theorem for general 0-shifted symplectic stacks with good moduli spaces, as well as for the character stacks of general compact oriented $3$-manifolds with reductive gauge groups. As an application, we prove Halpern-Leistner's conjecture on the purity of the Borel--Moore homology of $0$-shifted symplectic stacks admitting proper good moduli spaces. Our proof combines a cohomological bound for the algebraic BPS cohomology, due to the first author and based on Efimov's lemma, with a vanishing-cycle argument due to the second author in collaboration with Bu--Davison--Ib\'a~nez Nu~nez--P\u{a}durariu.