Traces arising from regular inclusions

TL;DR

This paper develops a framework for extending states to tracial states in regular inclusions of $C^*$-algebras, with applications to groupoid algebras and graph $C^*$-algebras, highlighting invariance properties and new operations.

Contribution

It introduces a method to characterize tracial states via invariance properties in regular inclusions, and applies it to describe the trace space of Cuntz-Krieger graph algebras.

Findings

Characterization of tracial states via invariance conditions.

Complete description of trace space for Cuntz-Krieger algebras.

Introduction of graph tightenings connecting to finiteness questions.

Abstract

We study the problem of extending a state on an abelian $C^*$- subalgebra to a tracial state on the ambient $C^*$-algebra. We propose an approach that is well-suited to the case of regular inclusions, in which there is a large supply of normalizers of the subalgebra. Conditional expectations onto the subalgebra give natural extensions of a state to the ambient $C^*$-algebra; we prove that these extensions are tracial states if and only if certain invariance properties of both the state and conditional expectations are satisfied. In the example of a groupoid $C^*$-algebra, these invariance properties correspond to invariance of associated measures on the unit space under the action of bisections. Using our framework, we are able to completely describe the tracial state space of a Cuntz-Krieger graph algebra. Along the way we introduce certain operations called graph tightenings, which both streamline our description and provides connections to related finiteness questions in graph $C^*$-algebras. Our investigation has close connections with the so-called unique state extension property and its variants.