Lower semicontinuity of bounded property in the branching problem and sphericity of flag variety

Abstract

Vinberg--Kimel'fel'd [Funct. Anal. Appl., 1978] established that a quasi-projective normal $G$-variety $X$ is spherical if and only if $G$-modules on the spaces $\Gamma(X, \mathcal{L})$ of global sections of $G$-equivariant line bundles are multiplicity-free. This result was generalized by Kobayashi--Oshima [Adv. Math., 2013] and several researchers to (degenerate) principal series representations of reductive Lie groups. The purpose of this short article is to show that the boundedness of the multiplicities in the restrictions of cohomologically induced modules implies the sphericity of some partial flag variety. In our previous paper, we reduce the boundedness of the multiplicities to the finiteness of a ring-theoretic invariant $\mathrm{PIdeg}$. To show the main result, we discuss the lower semicontinuity of $\mathrm{PIdeg}$ on the space $\mathrm{Prim}(\mathcal{U}(\mathfrak{g}))$ of primitive ideals. We also treat the finiteness of the lengths of the restrictions of cohomologically induced modules.