Equivariant sheaves for classical groups acting on Grassmannians

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.

33 pages. v2: added results on tangent spaces and transverse slices