A radius 1 irreducibility criterion for lattices in products of trees

Abstract

Let $T_1,T_2$ be regular trees of degrees $d_1,d_2\ge 3$. Let also $\Gamma \le \mathrm{Aut}(T_1)\times \mathrm{Aut}(T_2)$ be a group acting freely and transitively on $V{T}_1\times V{T}_2$. For $i=1$ and $2$, assume that the local action of $\Gamma$ on $T_i$ is $2$-transitive; if moreover $d_i\ge 7$, assume that the local action contains $\mathrm{Alt}(d_i)$. We show that $\Gamma$ is irreducible, unless $(d_1,d_2)$ belongs to an explicit small set of exceptional values. This yields an irreducibility criterion for $\Gamma$ that can be checked purely in terms of its local action on a ball of radius~$1$ in $T_1$ and $T_2$. Under the same hypotheses, we show moreover that if $\Gamma$ is irreducible, then it is hereditarily just-infinite, provided the local action on $T_i$ is not the affine group ${\mathbf{F}}_5⋊{\mathbf{F}}_5^{\ast }$. The proof of irreducibility relies, in several ways, on the Classification of the Finite Simple Groups.

arXiv commentMinor corrections and improvements of the exposition. Referees' comments taken into account