Bent walls for random groups in the square and hexagonal model

TL;DR

This paper investigates phase transitions for Kazhdan's Property (T) in random groups within square and hexagonal models, introducing new geometric tools and establishing sharp density thresholds.

Contribution

It establishes a sharp density threshold for Property (T) at 1/3 and introduces novel geometric methods for analyzing random groups.

Findings

Threshold for Property (T) at density 1/3

Groups without Property (T) below density 3/8

New isoperimetric inequality and geometric tools

Abstract

We consider two random group models: the hexagonal model and the square model, defined as the quotient of a free group by a random set of reduced words of length four and six respectively. Our first main result is that in this model there exists a sharp density threshold for Kazhdan's Property (T) and it equals 1/3. Our second main result is that for densities < 3/8 a random group in the square model with overwhelming probability does not have Property (T). Moreover, we provide a new version of the Isoperimetric Inequality that concerns non-planar diagrams and we introduce new geometrical tools to investigate random groups: trees of loops, diagrams collared by a tree of loops and specific codimension one structures in the Cayley complex, called bent hypergraphs.