The definitions of approximate lattices

Abstract

Approximate lattices of locally compact groups were first studied in a seminal monograph of Yves Meyer and were subsequently used in the theory of aperiodic order to model objects such as Pisot numbers, quasi-cristals or aperiodic tilings. Meyer studied approximate lattices of Euclidean spaces - now dubbed Meyer sets. A fascinating feature of this theory is the wealth of natural and equivalent definitions of Meyer sets that are available. The study of approximate lattices in non-commutative groups has recently attracted more attention. They are meant as generalisation of lattices of locally compact groups (i.e. discrete subgroup of finite co-volume) and, as such, are roughly defined as approximate subgroups of locally compact groups that are both discrete and have finite co-volume. In this generalized framework, the correct notion of `finite co-volume' is however up for interpretation. A number of definitions recently arose following work of Bj\"{o}rklund and Hartnick, Hrushovski, and the author. In stark contrast with the Euclidean framework, little is known of the relations between these notions. Our main objective in this paper is to address this issue. We establish clear relations between these definitions and show that, in fact, these definitions are not all equivalent. We prove nevertheless that all the notions mentioned are related to one another and simply represent varying degrees of generality. Among the original results we prove here, we extend a cornerstone theorem of Meyer and show that all strong approximate lattices are laminar; we build measures with weak invariance properties on the invariant hull of approximate lattices; we use quasi-models to show that any BH-approximate lattice is contained in an approximate lattice; we prove that even laminar approximate lattices can be arbitrarily far from strong approximate lattices and model sets.