Maximal ideals of reduced group C*-algebras and Thompson's groups

Abstract

Given a conditional expectation $P$ from a C*-algebra $B$ onto a C*-subalgebra $A$, we observe that induction of ideals via $P$, together with a map which we call co-induction, forms a Galois connection between the lattices of ideals of $A$ and $B$. Using properties of this Galois connection, we show that, given a discrete group $G$ and a stabilizer subgroup $G_x$ for the action of $G$ on its Furstenberg boundary, induction gives a bijection between the set of maximal co-induced ideals of $C^{\ast }(G_x)$ and the set of maximal ideals of $C_r^{\ast }(G)$. As an application, we prove that the reduced C*-algebra of Thompson's group $T$ has a unique maximal ideal. Furthermore, we show that, if Thompson's group $F$ is amenable, then $C_r^{\ast }(T)$ has infinitely many ideals.

16 pages. Added Example 2.5 and a more direct proof of Theorem 3.1. Accepted in TAMS