Homotopy of product systems and K-theory of Cuntz-Nica-Pimsner algebras

TL;DR

This paper introduces homotopy of product systems and proves that their associated Cuntz-Nica-Pimsner algebras have isomorphic K-theory, with applications to k-graph C*-algebras and their invariance properties.

Contribution

It defines homotopy for product systems and demonstrates K-theory invariance of their Cuntz-Nica-Pimsner algebras, providing new proofs and insights for k-graph C*-algebras.

Findings

K-theory is invariant under homotopy of product systems

K-theory of 2-graph C*-algebras is independent of factorisation rules

K-theory of twisted k-graph C*-algebras is independent of the twisting 2-cocycle

Abstract

We introduce the notion of a homotopy of product systems, and show that the Cuntz-Nica-Pimsner algebras of homotopic product systems over N^k have isomorphic K-theory. As an application, we give a new proof that the K-theory of a 2-graph C*-algebra is independent of the factorisation rules, and we further show that the K-theory of any twisted k-graph C*-algebra is independent of the twisting 2-cocycle. We also explore applications to K-theory for the C*-algebras of single-vertex k-graphs, reducing the question of whether the $K$-theory is independent of the factorisation rules to a question about path-connectedness of the space of solutions to an equation of Yang-Baxter type.