$G$-complete reducibility and semisimple modules

Abstract

Let $G$ be a connected reductive algebraic group defined over an algebraically closed field %$k$ of characteristic $p>0$. Our first aim in this note is to give concise and uniform proofs for two fundamental and deep results in the context of Serre's notion of $G$-complete reducibility, at the cost of less favourable bounds. Here are some special cases of these results: Suppose that the index $(H:H^{\circ })$ is prime to $p$ and that $p>2\dim V-2$ for some faithful $G$-module $V$. Then the following hold: (i) $V$ is a semisimple $H$-module if and only if $H$ is $G$-completely reducible; (ii) $H^{\circ }$ is reductive if and only if $H$ is $G$-completely reducible. We also discuss two new related results: (i) if $p\ge \dim V$ for some $G$-module $V$ and $H$ is a $G$-completely reducible subgroup of $G$, then $V$ is a semisimple $H$-module -- this generalizes Jantzen's semisimplicity theorem (which is the case $H=G$); (ii) if $H$ acts semisimply on $V\otimes V^{\ast }$ for some faithful $G$-module $V$, then $H$ is $G$-completely reducible.

9 pages