Cross-sections, quotients, and representation rings of semisimple algebraic groups

TL;DR

This paper characterizes when cross-sections of regular conjugacy classes exist in semisimple algebraic groups, relates this to algebraic invariants, and constructs rational sections, answering longstanding questions in the theory.

Contribution

It establishes a criterion for the existence of cross-sections in arbitrary semisimple groups and describes minimal generating sets for class functions and representation rings.

Findings

Cross-section existence is equivalent to the universal covering being bijective.

Minimal generating sets for class functions and representation rings are described.

Existence of rational sections is proven and related to W-equivariant maps.

Abstract

Let $G$ be a connected semisimple algebraic group over an algebraically closed field $k$. In 1965 Steinberg proved that if $G$ is simply connected, then in $G$ there exists a closed irreducible cross-section of the set of closures of regular conjugacy classes. We prove that in arbitrary $G$ such a cross-section exists if and only if the universal covering isogeny $\tau\colon \tG\to G$ is bijective; this answers Grothendieck's question cited in the epigraph. In particular, for ${\rm char}\,k=0$, the converse to Steinberg's theorem holds. The existence of a cross-section in $G$ implies, at least for ${\rm char}\,k=0$, that the algebra $k[G]^G$ of class functions on $G$ is generated by ${\rm rk}\,G$ elements. We describe, for arbitrary $G$, a minimal generating set of $k[G]^G$ and that of the representation ring of $G$ and answer two Grothendieck's questions on constructing generating sets of $k[G]^G$. We prove the existence of a rational (i.e., local) section of the quotient morphism for arbitrary $G$ and the existence of a rational cross-section in $G$ (for ${\rm char}\,k=0$, this has been proved earlier); this answers the other Grothendieck's question cited in the epigraph. We also prove that the existence of a rational section is equivalent to the existence of a rational $W$-equivariant map $T\dashrightarrow G/T$ where $T$ is a maximal torus of $G$ and $W$ the Weyl group.