Sofic approximations and quantitative measure couplings

Abstract: Measure equivalence was introduced by Gromov as a measured analogue of quasi-isometry. Unlike the latter, measure equivalence does not preserve the large scale geometry of groups and happens to be very flexible in the amenable world. Indeed the Ornstein-Weiss theorem shows that all infinite countable amenable groups are measure equivalent to the group of integers. To refine this equivalence relation and make it responsive to geometry, Delabie, Koivisto, Le Ma^itre and Tessera introduced a quantitative version of measure equivalence. They also defined a relaxed version of this notion called quantitative measure subgroup coupling. In this article we offer to answer the inverse problem of the quantification (find a group admitting a measure subgroup coupling with a prescribed group with prescribed quantification) in the case of the lamplighter group.