The Atiyah conjecture and Artinian rings

TL;DR

This paper investigates the Atiyah conjecture for groups with bounded finite subgroups, showing that under certain conditions, the division closure of the group ring forms a matrix ring over a skew field, extending understanding of algebraic structures related to the conjecture.

Contribution

It proves that the division closure of the group ring in the algebra of affiliated operators forms a matrix ring over a skew field, assuming the Atiyah conjecture and specific group properties.

Findings

Division closure D(KG,U(G)) is a matrix ring over a skew field under certain conditions.

The structure of D(KG,U(G)) depends on the presence of finite normal subgroups in G.

Results extend the algebraic understanding related to the Atiyah conjecture.

Abstract

Let G be a group such that its finite subgroups have bounded order, let d denote the lowest common multiple of the orders of the finite subgroups of G, and let K be a subfield of C that is closed under complex conjugation. Let U(G) denote the algebra of unbounded operators affiliated to the group von Neumann algebra N(G), and let D(KG,U(G)) denote the division closure of KG in U(G); thus D(KG,U(G)) is the smallest subring of U(G) containing KG that is closed under taking inverses. Suppose n is a positive integer, and \alpha \in \Mat_n(KG). Then \alpha induces a bounded linear map \alpha: l^2(G)^n \to \l^2(G)^n, and \ker\alpha has a well-defined von Neumann dimension \dim_{N(G)} (\ker\alpha). This is a nonnegative real number, and one version of the Atiyah conjecture states that d \dim_{N(G)}(\ker\alpha) \in Z. Assuming this conjecture, we shall prove that if G has no nontrivial finite normal subgroup, then D(KG,U(G)) is a d \times d matrix ring over a skew field. We shall also consider the case when G has a nontrivial finite normal subgroup, and other subrings of U(G) that contain KG.