# Intermediate subalgebras of commutative crossed products of discrete group actions.

### Tattwamasi Amrutam (Ben-Gurion University)

*Thursday, October 21, 2021,
11:10 – 12:00*,
**Building 34, room 14**

**Please Note the Unusual Place!**

**Abstract:**

In this talk, we shall focus our attention on intermediate subalgebras of $C(X)\rtimes_r\Gamma$ (and $L^{\infty}(X,\nu)\ltimes\Gamma$). We begin by describing the construction of the commutative crossed product $C(X)\rtimes_r\Gamma$ and how the group contributes to its structure. We shall talk about various (generalized) averaging properties in this context. As a first application, we will show that every intermediate $C^*$-subalgebra $\mathcal{A}$ of the form $C(Y)\rtimes_r\Gamma\subseteq\mathcal{A}\subseteq C(X)\rtimes_r\Gamma$ is simple for an inclusion $C(Y)\subset C(X)$ of minimal $\Gamma$-spaces whenever $C(Y)\rtimes_r\Gamma$ is simple. We shall also show that, for a large class of actions of $C^*$-simple groups $\Gamma\curvearrowright X$, including non-faithful action of any hyperbolic group with trivial amenable radical, every intermediate $C^*$-algebra $\mathcal{A}$, $C_r^*(\Gamma)\subset \mathcal{A}\subset C(X)\rtimes_r\Gamma$ is a crossed product of the form $C(Y)\rtimes_r\Gamma$, $C(Y)\subset C(X)$ is an inclusion of $\Gamma$-$C^*$-algebras.