On smooth-group actions on reductive groups and spherical buildings

Abstract

Let $k$ be a field, and suppose that $\Gamma$ is a smooth $k$-group that acts on a connected, reductive $k$-group $\widetilde G$. Let $G$ denote the maximal smooth, connected subgroup of the group of $\Gamma$-fixed points in $\widetilde G$. Under fairly general conditions, we show that $G$ is a reductive $k$-group, and that the image of the functorial embedding $\mathscr{S}(G) \longrightarrow \mathscr{S}(\widetilde G)$ of spherical buildings is the set of ``$\Gamma$-fixed points in $\mathscr{S}(\widetilde G)$'', in a suitable sense. In particular, we do not need to assume that $\Gamma$ has order relatively prime to the characteristic of $k$ (nor even that $\Gamma$ is finite), nor that the action of $\Gamma$ preserves a Borel-torus pair in $\widetilde G$.