Equivariant automorphism group and real forms of complexity-one varieties

Abstract

Let $G$ be a connected reductive real algebraic group. We prove that every real $G$-variety of complexity one admits only finitely many pairwise non-isomorphic $(\mathbb{R},G)$-forms. Our approach relies on representability and structural results for equivariant automorphism groups. More generally, over a perfect field, the equivariant automorphism group of an almost homogeneous variety under a smooth group scheme of finite type is represented by a smooth group scheme of finite type, and is linear whenever the acting group is linear. In characteristic zero, the equivariant automorphism group of every complexity-one variety under a connected reductive group is represented by a smooth group scheme locally of finite type. In the case that is not almost homogeneous, we further describe the subgroup acting trivially on the rational quotient as an extension of an étale group scheme, locally isomorphic to ${\mathbb{Z}}^m$, by a group of multiplicative type.

arXiv comment22 pages. Compared with the previous version, the results have been significantly strengthened, and several proofs have also been simplified