From forced gradings to Q-Koszul algebras

Abstract

This paper has two parts. The main goal, carried out in Part I, is to survey some recent work by the authors in which "forced" grading constructions have played a significant role in the representation theory of semisimple algebraic groups $G$ in positive characteristic. The constructions begin with natural finite dimensional quotients of the distribution algebras Dist$(G)$, but then "force" gradings into the picture by passing to positively graded algebras constructed from ideal filtrations of these quotients. This process first guaranteed a place for itself by proving, for large primes, that all Weyl modules have $p$-Weyl filtrations. Later it led, under similar circumstances, to a new "good filtration" result for restricted Lie algebra Ext groups between restricted irreducible $G$-modules. In the process of proving these results, a new kind of graded algebra was invented, called a Q-Koszul algebra. Recent conjectures suggest these algebras arise in forced grading constructions as above, from quotients of Dist$(G)$, even for small primes and even in settings possibly involving singular weights. Related conjectures suggest a promising future for using Kazhdan-Lusztig theory to relate quantum and algebraic group cohomology and Ext groups in these same small prime and possibly singular weight settings. A part of one of these conjectures is proved in Part II of this paper. The proof is introduced by remarks of general interest on positively graded algebras and Morita equivalence, followed by a discussion of recent Koszulity results of Shan-Varagnalo-Vasserot, observing some extensions. Version 2 corrects some minor typos and inaccurate references. The paper will appear in PSPUM.