A computation with the Connes-Thom isomorphism

TL;DR

This paper explores how the Connes-Thom isomorphism interacts with automorphisms induced by matrices on $C^*$-algebras, revealing a determinant-dependent commutation property in $K$-theory.

Contribution

It establishes the precise relationship between automorphisms and the Connes-Thom map in $K$-theory, depending on the sign of the determinant of the matrix.

Findings

$ au$ commutes with the Connes-Thom map if $ ext{det}(A)>0$

$ au$ anticommutes with the Connes-Thom map if $ ext{det}(A)<0$

Recomputed $K$-groups of Cuntz-Li algebra for integer dilation matrices

Abstract

Let $A \in M_{n}(\mathbb{R})$ be an invertible matrix. Consider the semi-direct product $\mathbb{R}^{n} \rtimes \mathbb{Z}$ where $\mathbb{Z}$ acts on $\mathbb{R}^{n}$ by matrix multiplication. Consider a strongly continuous action $(\alpha,\tau)$ of $\mathbb{R}^{n} \rtimes \mathbb{Z}$ on a $C^{*}$-algebra $B$ where $\alpha$ is a strongly continuous action of $\mathbb{R}^{n}$ and $\tau$ is an automorphism. The map $\tau$ induces a map $\widetilde{\tau}$ on $B \rtimes_{\alpha} \mathbb{R}^{n}$. We show that, at the $K$-theory level, $\tau$ commutes with the Connes-Thom map if $\det(A)>0$ and anticommutes if $\det(A)<0$. As an application, we recompute the $K$-groups of the Cuntz-Li algebra associated to an integer dilation matrix.