On random presentations with fixed relator length

Abstract

The standard $(n,k,d)$ model of random groups is a model where the relators are chosen randomly from the set of cyclically reduced words of length $k$ on an $n$-element generating set. Gromov's density model of random groups considers the case where $n$ is fixed, and $k$ tends to infinity. We instead fix $k$, and let $n$ tend to infinity. We prove that for all $k\ge 2$ at density $d>1/2$ a random group in this model is trivial or cyclic of order two, whilst for $d<\frac{1}{2}$ such a random group is infinite and hyperbolic. In addition we show that for $d<\frac{1}{k}$ such a random group is free, and that this threshold is sharp. These extend known results for the triangular ($k=3$) and square ($k=4)$ models of random groups.