Automorphisms of Banach space projective tensor product of C*-algebras

TL;DR

This paper characterizes isometric automorphisms of the Banach space projective tensor product of unital C*-algebras, providing insights into their structure and answering a question about unitaries in this tensor product.

Contribution

It offers a complete characterization of automorphisms of the tensor product of C*-algebras and addresses a specific open question about unitaries.

Findings

Characterization of isometric automorphisms of A⊗^γ B

Identification of unitaries in the tensor product as U(A)⊗U(B)

Description of the relative commutant in the tensor product

Abstract

For unital $C^*$-algebras $A$ and $B$, we completely characterize the isometric ($*$-) automorphisms of their Banach space projective tensor product $A\otimes^\gamma B$. This leads to the characterization of inner and outer isometric $*$-automorphisms of $A\otimes^\gamma B$, as well. As an application, we provide a partial affirmative answer to a question posed by Kaijser and Sinclair, viz., we prove that for unital $C^*$-algebras $A$ and $B$, the set of norm-one unitaries of $A\otimes^\gamma B$ coincides with $U(A) \otimes U(B)$, where $U(A)$ is the unitary group of $A$. We also establish the fact that the relative commutant of $A\otimes^\gamma \mathbb{C} 1$ in $A \otimes^\gamma B$ is same as $Z(A) \otimes^\gamma B$, where $B$ is a subhomogenous unital $C^*$-algebra, and $A$ is any $C^*$-algebra.