Weak* tensor products for von Neumann algebras

TL;DR

This paper introduces the concept of weak* tensor products for von Neumann algebras, exploring their properties and demonstrating the existence of many non-equivalent completions, especially for abelian algebras like $L^\infty(\\mathbb R)$.

Contribution

It defines weak* tensor products in the von Neumann algebra category and characterizes when these have unique completions, revealing a rich structure of multiple non-equivalent tensor products.

Findings

M has a unique weak* tensor product completion with every N iff M is completely atomic.

Even abelian von Neumann algebras can have multiple weak* tensor product completions.

There are 2^{ c} non-equivalent weak* tensor products of L^( R)  L^( R).

Abstract

The category of $C^*$-algebras is blessed with many different tensor products. In contrast, virtually the only tensor product ever used in the category of von Neumann algebras is the normal spatial tensor product. We propose a definition of what a generic tensor product in this category should be. We call these weak* tensor products. For von Neumann algebras $M$ and $N$, there are, in general, many choices of weak* tensor completions of the algebraic tensor product $M\odot N$. In fact, we demonstrate that $M$ has the property that $M\odot N$ has a unique weak* tensor product completion for every von Neumann algebra $N$ if and only if $M$ is completely atomic, i.e., is a direct product of type I factors. This in particular implies that even abelian von Neumann algebras need not have this property. As an application of the theory developed throughout the paper, we construct $2^{\mathfrak c}$ nonequivalent weak* tensor product completions of $L^\infty(\mathbb R)\odot L^\infty(\mathbb R)$.