Unitary Representations of Lie Groups with Reflection Symmetry

TL;DR

This paper studies special unitary representations of Lie groups with reflection symmetry, focusing on selfsimilarity, order-covariance, and positivity, and classifies such representations for certain groups.

Contribution

It introduces a framework for unitary representations with reflection symmetry and classifies possible spaces satisfying key axioms for specific Lie groups.

Findings

Classifies representations with reflection symmetry for certain Lie groups.

Identifies incompatibility of positivity with other axioms in some groups.

Provides a natural context for selfsimilarity and order-covariance in semisimple groups.

Abstract

We consider the following class of unitary representations $\pi $ of some (real) Lie group $G$ which has a matched pair of symmetries described as follows: (i) Suppose $G$ has a period-2 automorphism $\tau $, and that the Hilbert space $\mathbf{H} (\pi)$ carries a unitary operator $J$ such that $J\pi =(\pi \circ \tau)J$ (i.e., selfsimilarity). (ii) An added symmetry is implied if $\mathbf{H} (\pi)$ further contains a closed subspace $\mathbf{K}_0 $ having a certain order-covariance property, and satisfying the $\mathbf{K}_0 $-restricted positivity: $< v \mid Jv > \ge 0$, $\forall v\in \mathbf{K}_0 $, where $< \cdot \mid \cdot >$ is the inner product in $\mathbf{H} (\pi)$. From (i)--(ii), we get an induced dual representation of an associated dual group $G^c$. All three properties, selfsimilarity, order-covariance, and positivity, are satisfied in a natural context when $G$ is semisimple and hermitean; but when $G$ is the $(ax+b)$-group, or the Heisenberg group, positivity is incompatible with the other two axioms for the infinite-dimensional irreducible representations. We describe a class of $G$, containing the latter two, which admits a classification of the possible spaces $\mathbf{K}_0 \subset \mathbf{H} (\pi)$ satisfying the axioms of selfsimilarity and order-covariance.