Bases of T-equivariant cohomology of Bott-Samelson varieties

TL;DR

This paper constructs combinatorial bases for the T-equivariant cohomology of Bott-Samelson varieties, enabling new calculations and insights into the structure of these cohomologies and their restrictions.

Contribution

It introduces explicit combinatorial bases for the T-equivariant cohomology of Bott-Samelson varieties and applies them to compute cohomology of certain embeddings, avoiding division by 2.

Findings

Constructed combinatorial bases for $H_T^ullet( ext{Bott-Samelson})$

Proved surjectivity of restriction maps in equivariant cohomology

Reestablished 2-torsion for parity sheaves in Braden's example

Abstract

We construct combinatorial bases of the $T$-equivariant ($T$ is the maximal torus) cohomology $H^\bullet_T(\Sigma,k)$ of the Bott-Samelson variety $\Sigma$ under some mild restrictions on the field of coefficients $k$. This bases allow us to prove the surjectivity of the restrictions $H^\bullet_T(\Sigma,k)\to H^\bullet_T(\pi^{-1}(x),k)$ and $H^\bullet_T(\Sigma,k)\to H^\bullet_T(\Sigma\setminus\pi^{-1}(x),k)$, where $\pi:\Sigma\to G/B$ is the canonical resolution. In fact, we also construct bases of the targets of these restrictions by picking up certain subsets of certain bases of $H^\bullet_T(\Sigma,k)$ and restricting them to $\pi^{-1}(x)$ or $\Sigma\setminus\pi^{-1}(x)$ respectively. As an application, we calculate the cohomology of the costalk-to-stalk embedding for the direct image $\pi_*{\underline k}_\Sigma$. This algorithm avoids division by 2, which allows us to reestablish 2-torsion for parity sheaves in Braden's example.