Cohomology of GKM-sheaves

TL;DR

This paper explores the cohomology of GKM-sheaves associated with finite $T$-CW complexes, establishing conditions for their global sections to match equivariant cohomology and interpreting higher cohomology geometrically.

Contribution

It generalizes previous results by showing that the global sections of GKM-sheaves correspond to equivariant cohomology if and only if the cohomology is reflexive, and provides a geometric interpretation of higher cohomology.

Findings

Global sections of GKM-sheaves equal equivariant cohomology when cohomology is reflexive

Higher cohomology groups have a geometric interpretation

Established a necessary and sufficient condition for isomorphism with equivariant cohomology

Abstract

Let $T$ be a compact torus and $X$ be a a finite $T$-CW complex (e.g. a compact $T$-manifold). In earlier work, the second author introduced a functor which assigns to $X$ a so called GKM-sheaf $\mathcal{F}_X$ whose ring of global sections $H^0(\mathcal{F}_X)$ is isomorphic to the equivariant cohomology $H^*_T(X)$ whenever $X$ is equivariantly formal (meaning that $H^*_T(X)$ is a free module over $H^*(BT))$. In the current paper we prove more generally that $H^0(\mathcal{F}_X) \cong H^*_T(X)$ if and only if $H_T^*(X)$ is reflexive, and find a geometric interpretation of the higher cohomology $H^n(\mathcal{F}_X)$ for $n \geq 1$.