A tracial characterization of Furstenberg's $\times p,\times q$ conjecture

Abstract

We investigate almost minimal actions of abelian groups and their crossed products. As an application, given multiplicatively independent integers $p$ and $q$, we show that Furstenberg's $\times p,\times q$ conjecture holds if and only if the canonical trace is the only faithful extreme tracial state on the $C^{\ast }$-algebra of the group $\mathbb{Z}[\frac{1}{pq}]⋊{\mathbb{Z}}^2$. We also compute the primitive ideal space and K-theory of $C^{\ast }(\mathbb{Z}[\frac{1}{pq}]⋊{\mathbb{Z}}^2)$.

12 pages. Fixed a gap in the proof of Theorem 3.10. Accepted in the Canadian Mathematical Bulletin