T. Kobayashi, Admissible restrictions of irreducible representations of reductive Lie groups: Symplectic geometry and discrete decomposability, Pure and Applied Mathematics Quarterly 17 (2021), no. 4, 1321-1343, (special issue: in memory of Prof. Bertram Kostant). doi: 10.4310/PAMQ.2021.v17.n4.a5. arXiv: 1907.12964..

Let $G$ be a real reductive Lie group, $L$ a compact subgroup, and $\pi$ an irreducible admissible representation of $G$. In this article we prove a necessary and sufficient condition for the finiteness of the multiplicities of $L$-types occurring in $\pi$ based on symplectic techniques. This leads us to a simple proof of the criterion for discrete decomposability of the restriction of unitary representations with respect to noncompact subgroups (the author, Ann. Math. 1998), and also provides a proof of a reverse statement which was announced in [Proc. ICM 2002, Thm. D]. A number of examples are presented in connection with Kostant's convexity theorem and also with non-Riemannian locally symmetric spaces.

[ DOI |  arXiv | preprint version(pdf) ]

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