# Vanishing of $L^{2}$-Betti numbers and failure of acylindrical hyperbolicity of matrix groups over rings

## TL;DR

This paper demonstrates the vanishing of certain $L^{2}$-Betti numbers for matrix groups over rings and shows that these groups are not acylindrically hyperbolic when the matrix size is at least four, using the concept of $n$-rigid rings.

## Contribution

It establishes new vanishing results for $L^{2}$-Betti numbers of matrix groups over rings and proves their non-acylindrical hyperbolicity for larger matrices, introducing the notion of $n$-rigid rings.

## Key findings

- $L^{2}$-Betti numbers $b_{i}^{(2)}(G)$ vanish for $i=0,1, o,n-2$.
- Groups are not acylindrically hyperbolic for $n extgreater=4$.
- Results extend to certain noncommutative rings.

## Abstract

Let $R$ be an infinite commutative ring with identity and $n\geq 2$ be an integer. We prove that for each integer $i=0,1,\cdots ,n-2,$ the $L^{2}$-Betti number $b_{i}^{(2)}(G)=0,$ $\ $when $G=\mathrm{GL}_{n}(R)$ the general linear group, $\mathrm{SL}_{n}(R)$ the special linear group, $% E_{n}(R)$ the group generated by elementary matrices. When $R$ is an infinite principal ideal domain, similar results are obtained for $\mathrm{Sp}_{2n}(R)$ the symplectic group, $\mathrm{ESp}_{2n}(R)$ the elementary symplectic group, $\mathrm{O}(n,n)(R)$ the split orthogonal group or $\mathrm{EO}(n,n)(R)$ the elementary orthogonal group. Furthermore, we prove that $G$ is not acylindrically hyperbolic if $n\geq 4$. We also prove similar results for a class of noncommutative rings. The proofs are based on a notion of $n$-rigid rings.