English

Seemingly injective von Neumann algebras

Operator Algebras 2023-04-05 v4 Functional Analysis

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 MM IdM=vu:M\buildreluB(H)\buildrelvMId_M=vu: M{\buildrel u\over\longrightarrow} B(H) {\buildrel v\over\longrightarrow} M with uu normal, unital, positive and vv completely contractive. As a corollary, if MM has a separable predual, MM is isomorphic (as a Banach space) to B(2)B(\ell_2). For instance this applies (rather surprisingly) to the von Neumann algebra of any free group. Nevertheless, since B(H)B(H) fails the approximation property (due to Szankowski) there are MM's (namely B(H)B(H)^{**} and certain finite examples defined using ultraproducts) that are not seemingly injective. Moreover, for MM to be seemingly injective it suffices to have the above factorization of IdMId_M through B(H)B(H) with u,vu,v positive (and uu still normal).

Keywords

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

R2 v1 2026-06-23T19:39:40.806Z