Seemingly injective von Neumann algebras
Abstract
We show that a QWEP von Neumann algebra has the weak* positive approximation property if and only if it is seemingly injective in the following sense: there is a factorization of the identity of with normal, unital, positive and completely contractive. As a corollary, if has a separable predual, is isomorphic (as a Banach space) to . For instance this applies (rather surprisingly) to the von Neumann algebra of any free group. Nevertheless, since fails the approximation property (due to Szankowski) there are 's (namely and certain finite examples defined using ultraproducts) that are not seemingly injective. Moreover, for to be seemingly injective it suffices to have the above factorization of through with positive (and still normal).
Cite
@article{arxiv.2010.13743,
title = {Seemingly injective von Neumann algebras},
author = {Gilles Pisier},
journal= {arXiv preprint arXiv:2010.13743},
year = {2023}
}
Comments
A confusion between weak* separability and separability of the predual for a general von Neumann algebra has been corrected in the two places where it occured, namely Remark 1.3 and the proof of Proposition 7.1