A simple proof of a theorem of Kirchberg and related results on $C^*$-norms

TL;DR

This paper provides a simple proof of Kirchberg's theorem that the full $C^*$-algebra of a free group tensorized with the bounded operators on a Hilbert space has a unique $C^*$-norm, extending understanding of $C^*$-norms and nuclearity.

Contribution

The authors present a simplified proof of Kirchberg's theorem on the equality of minimal and maximal tensor norms for $C^*$-algebras of free groups with $B(H)$, and extend related results.

Findings

Proved $C^*(F) ensor_{min} B(H) = C^*(F) ensor_{max} B(H)$ for free groups.

Extended Kirchberg's results on $C^*$-norms and nuclearity.

Provided a simpler proof of a key theorem in $C^*$-algebra theory.

Abstract

Recently, E.\ Kirchberg [K1--2] revived the study of pairs of $C^*$-algebras $A,B$ such that there is only one $C^*$-norm on the algebraic tensor product $A\otimes B$, or equivalently such that $A \otimes_{\rm min}B = A\otimes_{\rm max}B$. Recall that a $C^*$-algebra is called nuclear cf.\ [L, EL] if this happens for any $C^*$-algebra $B$. Kirchberg [K1] constructed the first example of a non-nuclear $C^*$-algebra such that $A\otimes_{\rm min} A^{op} = A \otimes_{\rm max} A^{op}$. He also proved the following striking result [K2] for which we give a very simple proof and which we extend. \proclaim Theorem 0.1. {\bf (Kirchberg [K2]).} Let $F$ be any free group and let $C^*(F)$ be the (full) $C^*$-algebra of $F$, then $$C^*(F) \otimes_{\rm min} B(H) = C^*(F) \otimes_{\rm max} B(H).$$