The similarity degree of an operator algebra

Let $A$ be a unital operator algebra. Let us assume that every {\it bounded\/} unital homomorphism $u\colon \ A\to B(H)$ is similar to a {\it contractive\/} one. Let $\text{\rm Sim}(u) = \inf\{\|S\|\, \|S^{-1}\|\}$ where the infimum runs over all invertible operators $S\colon \ H\to H$ such that the ``conjugate'' homomorphism $a\to S^{-1}u(a)S$ is contractive. Now for all $c>1$, let $Φ(c) = \sup\text{\rm Sim}(u)$ where the supremum runs over all unital homomorphism $u\colon\ A\to B(H)$ with $\|u\|\le c$. Then, there is $α\ge 0$ such that for some constant $K$ we have: $$Φ(c) \le Kc^α.\leqno (*)\qquad \forall c>1$$ Moreover, the smallest $α$ for which this holds is an integer, denoted by $d(A)$ (called the similarity degree of $A$) and $(*)$ still holds for some $K$ when $α=d(A)$. Among the applications of these results, we give new characterizations of proper uniform algebras on one hand, and of nuclear $C^*$-algebras on the other. Moreover, we obtain a characterization of amenable groups which answers (at least partially) a question on group representations going back to a 1950 paper of Dixmier.

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