Affine Embeddings of Homogeneous Spaces

Let G be a reductive algebraic group and H a closed subgroup of G. An affine embedding of the homogeneous space G/H is an affine G-variety with an open G-orbit isomorphic to G/H . The homogeneous space G/H admits an affine embedding if and only if G/H is a quasi-affine algebraic variety. We start with some basic properties of affine embeddings and consider the cases, where the theory is well-developed: toric varieties, normal SL(2)-embeddings, S-varieties, and algebraic monoids. We discuss connections between the theory of affine embeddings and Hilbert’s 14th problem via a theorem of Grosshans. We characterize affine homogeneous spaces G/H such that any affine embedding of G/H contains a finite number of G-orbits. The maximal value of modality over all affine embeddings of a given affine homogeneous space G/H is computed and the group of equivariant automorphisms of an embedding is studied. As applications of the theory, we describe invariant algebras on homogeneous spaces of a compact Lie group and G-algebras with finitely generated invariant subalgebras. AMS 2000 Math. Subject Classification: Primary 13A50, 14M17, 14R20; Secondary 14L30, 14M25, 14R05, 22C05, 32M12, 46J10