On the classification of $C^*$-algebras of twisted isometries with finite dimensional wandering spaces

TL;DR

This paper classifies certain twisted isometry C*-algebras with finite-dimensional wandering spaces, providing representation theory, K-theory calculations, and conditions for universality under specific spectral assumptions.

Contribution

It offers a complete classification of these twisted isometry C*-algebras, including irreducible representations, K-groups, and universality conditions, especially for finite-dimensional wandering spaces.

Findings

Complete list of irreducible representations

Explicit faithful representation under denseness assumption

K-theory computations for specific cases

Abstract

Let \( m, n \in \mathbb{N}_0 \), and let \( X \) be a closed subset of \( \mathbb{T}^{\binom{m+n}{2}} \). We define \( C^{m,n}_X \) to be the universal \( C^* \)-algebra among those generated by \( m \) unitaries and \( n \) isometries satisfying doubly twisted commutation relations with respect to a twist \( \mathcal{U} = \{U_{ij}\}_{1 \leq i < j \leq m+n} \) of commuting unitaries having joint spectrum \( X \). We provide a complete list of the irreducible representations of \( C^{m,n}_X \) up to unitary equivalence and, under a denseness assumption on \( X \), explicitly construct a faithful representation of \( C^{m,n}_X \). Under the same assumption, we also give a necessary and sufficient condition on a fixed tuple \( \mathcal{U} \) of commuting unitaries with joint spectrum \( X \) for the existence of a universal tuple of \( \mathcal{U} \)-doubly twisted isometries. For \( X = \mathbb{T}^{\binom{m+n}{2}} \), we compute the \( K \)-groups of \( C^{m,n}_X \). We further classify the \( C^* \)-algebras generated by a pair of doubly twisted isometries with a fixed parameter \( \theta \in \mathbb{R} \setminus \mathbb{Q} \), whose wandering spaces are finite-dimensional. Finally, for a fixed unitary \( U \), we classify all the \( C^* \)-algebras generated by a pair of \( U \)-doubly twisted isometries with finite-dimensional wandering spaces.