Spherical subgroups in simple algebraic groups

Abstract

Let $G$ be a simple algebraic group. A closed subgroup $H$ of $G$ is called spherical provided it has a dense orbit on the flag variety $G/B$ of $G$. Reductive spherical subgroups of simple Lie groups were classified by Krämer in 1979. In 1997, Brundan showed that each example from Krämer's list also gives rise to a spherical subgroup in the corresponding simple algebraic group in any positive characteristic. Nevertheless, there is no classification of all such instances in positive characteristic to date. The goal of this paper is to complete this classification. It turns out that there is only one additional instance (up to isogeny) in characteristic $2$ which has no counterpart in Krämer's classification. As one of our key tools, we prove a general deformation result for subgroup schemes allowing us to deduce the sphericality of subgroups in positive characteristic from this property for subgroups in characteristic $0$.