Cohomology of $\mathbf{GL}_d(\mathbb{F})$ in non-defining characteristic via the quantum schur algebra

Abstract

Let $G = \mathbf{GL}_d(\mathbb{F})$ be the general linear group over a field of cardinal $q$, and let $\mathbb{k}$ be a field of positive characteristic which does not divide $q(q-1)$. Building on the works of Cline, Parshall, and Scott, we show how to compute Ext-groups between $\mathbb{k}G$-modules using the quantum Schur algebra. The main novelty is our ability to compute these Ext-groups in higher degree than what was done before. More precisely, let $\ell$ be the order of $q$ in $\mathbb{k}$. In previous work, this method enabled the computation of the cohomology groups $H^*(\mathbf{GL}_d,M)$ in degree $*\leq \ell-1$. We show that for a lot of modules $M$, we can compute these cohomology groups in higher degree, with an example where we can compute until degree $3(\ell-1)$. We also show some new result on Ext-groups between modules over the quantum Schur algebra along the way.