Irreducible representations of Bost–Connes systems

Abstract

The classification problem of Bost–Connes systems was studied by Cornelissen and Marcolli partially, but still remains unsolved. In this paper, we give a representation-theoretic approach to this problem. We generalize the result of Laca and Raeburn, which is concerned with the primitive ideal space of the Bost–Connes system for $\mathbb{Q}$. As a consequence, the Bost–Connes $C^{\ast }$-algebra for a number field $K$ has $h_K^1$-dimensional irreducible representations and does not have finite-dimensional irreducible representations for the other dimensions, where $h_K^1$ is the narrow class number of $K$. In particular, the narrow class number is an invariant of Bost–Connes $C^{\ast }$-algebras.