A criterion for discrete branching laws for Klein four symmetric pairs and its application to $E_{6(-14)}$

TL;DR

This paper establishes a necessary criterion for the discrete decomposability of unitarizable modules in Klein four symmetric pairs and applies it to classify such pairs for the group E6(-14), revealing when these modules decompose discretely.

Contribution

It introduces a necessary condition for discrete decomposability in Klein four symmetric pairs and provides a complete classification for E6(-14) with noncompact fixed points.

Findings

A necessary condition for discrete decomposability is established.

Complete classification of Klein four symmetric pairs for E6(-14) with certain decomposability properties.

Identification of pairs where modules are discretely decomposable for multiple involutions.

Abstract

Let $G$ be a noncompact connected simple Lie group, and $(G,G^\Gamma)$ a Klein four symmetric pair. In this paper, the author shows a necessary condition for the discrete decomposability of unitarizable simple $(\mathfrak{g},K)$-modules for Klein for symmetric pairs. Precisely, if certain conditions hold for $(G,G^\Gamma)$, there does not exist any unitarizable simple $(\mathfrak{g},K)$-module that is discretely decomposable as a $(\mathfrak{g}^\Gamma,K^\Gamma)$-module. As an application, for $G=\mathrm{E}_{6(-14)}$, the author obtains a complete classification of Klein four symmetric pairs $(G,G^\Gamma)$ with $G^\Gamma$ noncompact, such that there exists at least one nontrivial unitarizable simple $(\mathfrak{g},K)$-module that is discretely decomposable as a $(\mathfrak{g}^\Gamma,K^\Gamma)$-module and is also discretely decomposable as a $(\mathfrak{g}^\sigma,K^\sigma)$-module for some nonidentity element $\sigma\in\Gamma$.