### Introduction to von Neumann algebras, Lecture 4 (group von Neumann algebras)

#### by Orr Shalit

As the main reference for this lecture we use (more-or-less) Section 1.3 in the notes by Anantharaman and Popa (here is a link to the notes on Popa’s homepage).

As for exercises:

**Exercise A: **Prove that has the ICC property.

**Exercise B: **Prove that there is an increasing sequence of von Neumann subalgebras of , such that is *-isomorphic to and such that .

**Exercise C: **Prove that the free group () has the ICC property.

**Exercise D: **Prove that . What can you say about ? (May require more advanced material: What can you say about , where is a countable discrete abelian group?).

**Exercise E:** We will later see that is not isomorphic to . It might be a nice exercise to think about it now (it might also be not a nice exercise, take your chances).

**Exercise F: **Let be a left convolver, and let be the corresponding convolution operator. Find the adjoint .

**Exercise G: **Prove that is a commutative group, if and only if (or ) is commutative, and that this happens if and only if .

**Exercise H:** Prove that (where is the usual trace) is the unique linear functional on that satisfies and for all .

[…] hyperfinite factor is hyperfinite, by construction. By Exercise B in Lecture 4, is also hyperfinite (and also a factor). The reason that is called THE hyperfinite factor, is […]