Representations and cohomology for Frobenius-Lusztig kernels

TL;DR

This paper investigates the structure, representations, and cohomology of Frobenius-Lusztig kernels, a class of finite-dimensional Hopf subalgebras of quantum groups at roots of unity, extending classical algebraic group concepts.

Contribution

It classifies irreducible modules and proves finite-generation of cohomology rings for Frobenius-Lusztig kernels and related subalgebras, generalizing known results.

Findings

Classified irreducible modules for Frobenius-Lusztig kernels.

Proved cohomology rings are finitely-generated for certain types.

Extended finite-generation results to nilpotent and Borel subalgebras.

Abstract

Let $U_\zeta$ be the quantum group (Lusztig form) associated to the simple Lie algebra $\mathfrak{g}$, with parameter $\zeta$ specialized to an $\ell$-th root of unity in a field of characteristic $p>0$. In this paper we study certain finite-dimensional normal Hopf subalgebras $U_\zeta(G_r)$ of $U_\zeta$, called Frobenius-Lusztig kernels, which generalize the Frobenius kernels $G_r$ of an algebraic group $G$. When $r=0$, the algebras studied here reduce to the small quantum group introduced by Lusztig. We classify the irreducible $U_\zeta(G_r)$-modules and discuss their characters. We then study the cohomology rings for the Frobenius-Lusztig kernels and for certain nilpotent and Borel subalgebras corresponding to unipotent and Borel subgroups of $G$. We prove that the cohomology ring for the first Frobenius-Lusztig kernel is finitely-generated when $\g$ has type $A$ or $D$, and that the cohomology rings for the nilpotent and Borel subalgebras are finitely-generated in general.