The similarity degree of an operator algebra

G. Pisier

Algebra i analiz, Tome 10 (1998) no. 1, pp. 132-186.

Résumé

Let $A$ be a unital operator algebra having the property that every bounded unital homomorphism $u\colon A\to B(H)$ is similar to a contractive one. Let $\operatorname{Sim}(u)=\inf\{\|S\|\,\|S^{-1}\|\}$, where the infimum runs over all invertible operators $S\colon H\to H$ such that the “conjugate” homomorphism $a\mapsto S^{-1}u(a)S$ is contractive. Now for all $c>1$, let $\Phi(c)=\sup\operatorname{Sim}(u)$, where the supremum runs over all unital homomorphism $u\colon A\to B(H)$ with $\|u\|\le c$. Then there is $\alpha\ge 0$ such that for some constant $K$ we have: $$ \Phi(c)\le Kc^{\alpha},\qquad c>1. $$ Moreover, the infimum of such $\alpha$'s is an integer (denoted by $d(A)$ and called the similarity degree of $A$), and (*) is still true for some $K$ when $\alpha=d(A)$. Among the applications of these results, new characterizations are given of proper uniform algebras on one hand, and of nuclear $C^*$-algebras on the other. Moreover, a characterization of amenable groups is obtained, which answers (at least partially) a question on group representations going back to a 1950 paper of Dixmier.

Détail

Keywords: Similarity problem, similarity degree, completely bounded map, operator space, operator algebra, group representation, uniform algebra.