# Completely bounded maps into certain Hilbertian operator spaces

Research paper by Gilles Pisier

Indexed on: 07 Jul '04Published on: 07 Jul '04Published in: Mathematics - Operator Algebras

#### Abstract

We prove a factorization of completely bounded maps from a $C^*$-algebra $A$ (or an exact operator space $E\subset A$) to $\ell_2$ equipped with the operator space structure of $(C,R)_\theta$ ($0<\theta<1$) obtained by complex interpolation between the column and row Hilbert spaces. More precisely, if $F$ denotes $\ell_2$ equipped with the operator space structure of $(C,R)_\theta$, then $u: A \to F$ is completely bounded iff there are states $f,g$ on $A$ and $C>0$ such that $\forall a\in A\quad \|ua\|^2\le C f(a^*a)^{1-\theta}g(aa^*)^{\theta}.$ This extends the case $\theta=1/2$ treated in a recent paper with Shlyakhtenko. The constants we obtain tend to 1 when $\theta \to 0$ or $\theta\to 1$. We use analogues of "free Gaussian" families in non semifinite von Neumann algebras. As an application, we obtain that, if $0<\theta<1$, $(C,R)_\theta$ does not embed completely isomorphically into the predual of a semifinite von Neumann algebra. Moreover, we characterize the subspaces $S\subset R\oplus C$ such that the dual operator space $S^*$ embeds (completely isomorphically) into $M_*$ for some semifinite von neumann algebra $M$: the only possibilities are $S=R$, $S=C$, $S=R\cap C$ and direct sums built out of these three spaces. We also discuss when $S\subset R\oplus C$ is injective, and give a simpler proof of a result due to Oikhberg on this question. In the appendix, we present a proof of Junge's theorem that $OH$ embeds completely isomorphically into a non-commutative $L_1$-space. The main idea is similar to Junge's, but we base the argument on complex interpolation and Shlyakhtenko's generalized circular systems (or generalized free Gaussian"), that somewhat unifies Junge's ideas with those of our work with Shlyakhtenko.