Equivariant sheaves for classical groups acting on Grassmannians

TL;DR

This paper develops a stratification of Grassmannians under classical group actions, studying associated sheaves and their cohomology, with implications for Springer theory in representation theory.

Contribution

It introduces a new stratification of Grassmannians related to orthogonal and symplectic groups and analyzes the topology and sheaf theory on these stratifications.

Findings

Existence of enough parity sheaves.

Parity-vanishing property of hypercohomology.

Applications to Springer theory for classical groups.

Abstract

Let $V$ be a finite-dimensional complex vector space. Assume that $V$ is a direct sum of subspaces each of which is equipped with a nondegenerate symmetric or skew-symmetric bilinear form. In this paper, we introduce a stratification of the Grassmannian $\mathrm{Gr}_k(V)$ related to the action of the appropriate product of orthogonal and symplectic groups, and we study the topology of this stratification. The main results involve sheaves with coefficients in a field of characteristic other than $2$. We prove that there are "enough" parity sheaves, and that the hypercohomology of each parity sheaf also satisfies a parity-vanishing property. This situation arises in the following context: let $x$ be a nilpotent element in the Lie algebra of either $G = \mathrm{Sp}_N(\mathbb{C})$ or $G = \mathrm{SO}_N(\mathbb{C})$, and let $V = \ker x \subset \mathbb{C}^N$. Our stratification of $\mathrm{Gr}_k(V)$ is preserved by the centralizer $G^x$, and we expect our results to have applications in Springer theory for classical groups.