Higher index theory for certain expanders and Gromov monster groups I

TL;DR

This paper investigates higher index theory for expanders with increasing girth, demonstrating injectivity but not surjectivity of the coarse Baum-Connes assembly map, and explores implications for Gromov monster groups.

Contribution

It establishes new results on the injectivity and non-surjectivity of the Baum-Connes assembly map for expanders and Gromov monster groups, linking geometric properties to algebraic K-theory.

Findings

Coarse Baum-Connes assembly map is injective but not surjective for certain expanders.

Girth tending to infinity is crucial for the non-surjectivity result.

Baum-Connes assembly map with specific coefficients is injective but not surjective for Gromov monster groups.

Abstract

In this paper, the first of a series of two, we continue the study of higher index theory for expanders. We prove that if a sequence of graphs is an expander and the girth of the graphs tends to infinity, then the coarse Baum-Connes assembly map is injective, but not surjective, for the associated metric space $X$. Expanders with this girth property are a necessary ingredient in the construction of the so-called `Gromov monster' groups that (coarsely) contain expanders in their Cayley graphs. We use this connection to show that the Baum-Connes assembly map with certain coefficients is injective but not surjective for these groups. Using the results of the second paper in this series, we also show that the maximal Baum-Cones assembly map with these coefficients is an isomorphism.