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Bounding the covolume of lattices in products

Part of: Structure and classification of infinite or finite groups Locally compact groups and their algebras Lie groups

Published online by Cambridge University Press:  11 October 2019

Pierre-Emmanuel Caprace  and
Adrien Le Boudec
Pierre-Emmanuel Caprace
Affiliation:
UCLouvain, 1348 Louvain-la-Neuve, Belgium email [email protected]
Adrien Le Boudec
Affiliation:
UMPA – ENS Lyon, France email [email protected]
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Abstract

We study lattices in a product $G=G_{1}\times \cdots \times G_{n}$ of non-discrete, compactly generated, totally disconnected locally compact (tdlc) groups. We assume that each factor is quasi just-non-compact, meaning that $G_{i}$ is non-compact and every closed normal subgroup of $G_{i}$ is discrete or cocompact (e.g. $G_{i}$ is topologically simple). We show that the set of discrete subgroups of $G$ containing a fixed cocompact lattice $\unicode[STIX]{x1D6E4}$ with dense projections is finite. The same result holds if $\unicode[STIX]{x1D6E4}$ is non-uniform, provided $G$ has Kazhdan’s property (T). We show that for any compact subset $K\subset G$, the collection of discrete subgroups $\unicode[STIX]{x1D6E4}\leqslant G$ with $G=\unicode[STIX]{x1D6E4}K$ and dense projections is uniformly discrete and hence of covolume bounded away from $0$. When the ambient group $G$ is compactly presented, we show in addition that the collection of those lattices falls into finitely many $\operatorname{Aut}(G)$-orbits. As an application, we establish finiteness results for discrete groups acting on products of locally finite graphs with semiprimitive local action on each factor. We also present several intermediate results of independent interest. Notably it is shown that if a non-discrete, compactly generated quasi just-non-compact tdlc group $G$ is a Chabauty limit of discrete subgroups, then some compact open subgroup of $G$ is an infinitely generated pro-$p$ group for some prime $p$. It is also shown that in any Kazhdan group with discrete amenable radical, the lattices form an open subset of the Chabauty space of closed subgroups.

MSC classification

Primary: 20E08: Groups acting on trees 20E32: Simple groups 22D05: General properties and structure of locally compact groups 22E40: Discrete subgroups of Lie groups
Type
Research Article
Information
Compositio Mathematica , Volume 155 , Issue 12 , December 2019 , pp. 2296 - 2333
Copyright
© The Authors 2019 

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Footnotes

The first author is an F.R.S.-FNRS Senior Research Associate. The second author is a CNRS Researcher. This work was partially carried out when the second author was an F.R.S.-FNRS Post-Doctoral Researcher at UCLouvain. This work was partially supported by ANR-14-CE25-0004 GAMME.

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