Ping-pong in the projective plane over a nonarchimedean field

Abstract

We show that any lattice in ${\mathrm{SL}}_3(k)$, where $k$ is a nonarchimedean local field, contains an undistorted subgroup isomorphic to the free product ${\mathbb{Z}}^2\ast \mathbb{Z}$. To our knowledge, the subgroups we construct give the first examples in the literature of finitely generated discrete subgroups of nonarchimedean Lie groups that are not virtually isomorphic to lattices in such Lie groups. Our result is in contrast to the case of ${\mathrm{SL}}_3(\mathbb{Z})$, in which the existence of a ${\mathbb{Z}}^2\ast \mathbb{Z}$ subgroup remains open.

6 pages. Minor edits; it was pointed out by a referee that if k' is a finite extension of a local field k, then a lattice in SL(3,k) remains k'-dense in SL(3,k'), and the previous draft did not account for examples of infinite-covolume discrete subgroups of nonarchimedean Lie groups that arise in this fashion