Semigroups in 3-graded Lie groups and endomorphisms of standard subspaces

Abstract

Let V be a standard subspace in the complex Hilbert space H and U : G \to U(H) be a unitary representation of a finite dimensional Lie group. We assume the existence of an element h in the Lie algebra of G such that U(exp th) is the modular group of V and that the modular involution J_V normalizes U(G). We want to determine the semigroup $S_V=\{g\in G:U(g)V\subseteq V\}.$ In previous work we have seen that its infinitesimal generators span a Lie algebra on which ad h defines a 3-grading, and here we completely determine the semigroup S_V under the assumption that ad h defines a 3-grading. Concretely, we show that the ad h-eigenspaces for the eigenvalue $\pm 1$ contain closed convex cones $C_{\pm }$, such that $S_V=exp(C_{+})G_{V}exp(C_{-})$, where $G_V$ is the stabilizer of V in G. To obtain this result we compare several subsemigroups of G specified by the grading and the positive cone $C_U$ of U. In particular, we show that the orbit U(G)V, endowed with the inclusion order, is an ordered symmetric space covering the adjoint orbit $Ad(G)h$, endowed with the partial order defined by~$C_U$.

29 pages, some typos corrected. This paper will appear in Kyoto J. Math

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Cited by in corpus (1)

• Covariant homogeneous nets of standard subspaces