Cocharacter-closure and spherical buildings

Abstract

Let $k$ be a field, let $G$ be a reductive $k$-group and $V$ an affine $k$-variety on which $G$ acts. In this note we continue our study of the notion of cocharacter-closed $G(k)$-orbits in $V$. In earlier work we used a rationality condition on the point stabilizer of a $G$-orbit to prove Galois ascent/descent and Levi ascent/descent results concerning cocharacter-closure for the corresponding $G(k)$-orbit in $V$. In the present paper we employ building-theoretic techniques to derive analogous results.

arXiv comment16 pages; v 2 17 pages, exposition improved; to appear in the Robert Steinberg Memorial Issue of the Pacific Journal of Mathematics