# nLab von Neumann algebra factor

Contents

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Definition

For $A$ a von Neumann algebra write $A'$ for its commutant in the ambient algebra $B(\mathcal{H})$ of bounded operators.

###### Definition

A von Neumann algebra $A$ is called a factor if its center is trivial

$Z(A) := A \cap A' = \mathbb{C}1 \,.$

Equivalently: if $A$ and its commutant $A'$ generate the full algebra of bounded operators $B(\mathcal{H})$.

## Properties

Every von Neumann algebra may be written as a direct integral? over factors. (von Neumann 49)

## Classification

Factors are classified in terms of the K-theory of their categories of finite W*-modules?. A W*-module? over a factor $A$ is finite if it is not isomorphic to its proper submodule.

### Type I

Type I factors are characterized by the condition that the K-theory of finite modules is isomorphic to $\mathbf{Z}$, the group of integers. The only factors of this type are of the from $B(H)$, bounded operators on a Hilbert space $H$.

### Type II

Type II factors are characterized by the condition that the K-theory of finite modules is isomorphic to $\mathbf{R}$, the group of real numbers.

Type II factors are subdivided into two classes: type II$_1$ factors are characterized by the condition that $A$ is a finite $A$-module, whereas for a type II$_\infty$ factor $A$ is not a finite $A$-module.

### Type III

Type III factors are characterized by the condition that the K-theory of finite modules is trivial, i.e., only the zero module is finite.

Type III factors are further subdivided into three classes, according to the structure of the center of their modular algebra?, which is a commutative von Neumann algebra graded by purely imaginary numbers, whose graded components are noncommutative L^p-spaces?.

By the von Neumann duality? for commutative von Neumann algebras, the spectrum of this center is a measurable space equipped with a σ-ideal of negligible sets and the grading yields an action of $\mathbf{R}$, the group of real numbers. This object is known as the noncommutative flow of weights?.

If the center is trivial (so the spectrum is a point), the factor has type III$_1$. If the action of $\mathbf{R}$ is not periodic, then the factor has type III$_0$. If the action is periodic with period $\lambda$, a positive real number, then the factor has type III$_{\exp(-\lambda)}$.

### General

The original sources are

Lecture notes include

• V.S. Sunder, von Neumann algebras, $II_1$-factors, and their subfactors (pdf)

• Hideki Kosaki, Type III factors and index theory (1993) (pdf)

### Subfactors

The mathematics of inclusions of subfactors is giving deep structural insights. See also at planar algebra.

• Index for subfactors, Invent. Math. 72, I (I983);

A polynomial invariant for links via von Neumann algebras, Bull. AMS 12, 103 (1985);

Hecke algebra representations of braid groups and link polynomials, Ann. Math. 126, 335 (1987)

• Vaughan Jones, Scott Morrison, Noah Snyder, The classification of subfactors of index at most 5 (arXiv:1304.6141)

• Vaughan F. R. Jones, David Penneys, Infinite index subfactors and the GICAR categories, arxiv/1410.0856

Symmetries of depth two inclusions of subfactors may be described via associative bialgebroids,

• Lars Kadison, Kornél Szlachányi, Bialgebroid actions on depth two extensions and duality, Adv. Math. 179:1 (2003) 75-121 doi