# Operator Algebrae/Printable version

Operator Algebrae

The current, editable version of this book is available in Wikibooks, the open-content textbooks collection, at
https://en.wikibooks.org/wiki/Operator_Algebrae

Permission is granted to copy, distribute, and/or modify this document under the terms of the Creative Commons Attribution-ShareAlike 3.0 License.

# Von Neumann algebrae

## The operator algebra

Definition (operator algebra):

Let ${\displaystyle X}$ be a Banach space over the field ${\displaystyle \mathbb {K} =\mathbb {R} }$ or ${\displaystyle \mathbb {C} }$. Consider the set ${\displaystyle B(X)}$ of bounded and linear functions from ${\displaystyle X}$ to itself. This

## Operator topologies

### Topologies on a Banach space

Definition (weak topology):

Let ${\displaystyle X}$ be a Banach space, and let ${\displaystyle X^{*}}$ be its dual space. The weak topology on ${\displaystyle X}$ is defined to be the initial topology with respect to the maps ${\displaystyle x\mapsto x^{*}(x)}$, where ${\displaystyle x^{*}}$ ranges over ${\displaystyle X^{*}}$.

Theorem (properties of the weak topology):

### Topologies exclusively for operator spaces

Proposition (bounded operators on a normed space form a Banach space under norm topology):

Let ${\displaystyle X}$ be a Banach space, and equip the space ${\displaystyle B(X)}$ with

Definition (uniform topology):

## Von Neumann algebrae, basic constructions

Definition (von Neumann algebra):

A von Neumann algebra is a subalgebra ${\displaystyle A\leq B(H)}$ which is closed under the weak operator topology.