Cohomology of $\text{PSL}_2(q)$ (2002.04183v3)

Abstract: In 2011, Guralnick and Tiep proved that if $G$ was a Chevalley group with Borel subgroup $B$ and $V$ an irreducible $G$-module in cross characteristic with $V^B = 0$, then the the dimension of $H^1(G,V)$ is determined by the structure of the permutation module on the cosets of $B$. We generalise this theorem to higher cohomology and an arbitrary finite group, so that if $H \leq G$ such that $O_{r'}(H) = O^r(H)$ and $V^H = 0$ for $V$ a $G$-module in characteristic $r$ then $\dim H^1(G,V)$ is determined by the structure of the permutation module on cosets of $H$, and $H^n(G,V)$ by $\text{Ext}_G^{n-1}(V^*,M)$ for some $kG$-module $M$ dependent on $H$. We also determine $\text{Ext}_G^n(V,W)$ for all irreducible $kG$-modules $V$, $W$ for $G \in {\text{PSL}_2(q), \text{PGL}_2(q), \text{SL}_2(q)}$ in cross characteristic.