An exact category approach to Hecke endomorphism algebras

Abstract

Let $G$ be a finite group of Lie type. In studying the cross-characteristic representation theory of $G$, the (specialized) Hecke algebra $H=\End_G(\ind_B^G1_B)$ has played a important role. In particular, when $G=G{L}_n({\mathbb{F}}_q)$ is a finite general linear group, this approach led to the Dipper-James theory of $q$-Schur algebras $A$. These algebras can be constructed over $\sZ:=\mathbb Z[t,t^{-1}]$ as the $q$-analog (with $q=t^2$) of an endomorphism algebra larger than $H$, involving parabolic subgroups. The algebra $A$ is quasi-hereditary over $\sZ$. An analogous algebra, still denoted $A$, can always be constructed in other types. However, these algebras have so far been less useful than in the $G{L}_n$ case, in part because they are not generally quasi-hereditary. Several years ago, reformulating a 1998 conjecture, the authors proposed (for all types) the existence of a $\sZ$-algebra $A^{+}$ having a stratified derived module category, with strata constructed via Kazhdan-Lusztig cell theory. The algebra $A$ is recovered as $A=e{A}^{+}e$ for an idempotent $e\in A^{+}$. A main goal of this monograph is to prove this conjecture completely. The proof involves several new homological techniques using exact categories. Following the proof, we show that $A^{+}$ does become quasi-hereditary after the inversion of the bad primes. Some first applications of the result -- e.g., to decomposition matrices -- are presented, together with several open problems.

144 pages