Quantifying metric approximations of discrete groups
Annales de la Faculté des sciences de Toulouse : Mathématiques, Serie 6, Volume 33 (2024) no. 3, pp. 831-877.

We introduce and systematically study a profile function whose asymptotic behaviour quantifies the dimension or the size of a metric approximation of a finitely generated group G by a family of groups ={(G α ,d α ,k α ,ε α )} αI, where each group G α is equipped with a bi-invariant metric d α and a dimension k α , for strictly positive real numbers ε α such that inf α ε α >0. Through the notion of a residually amenable profile that we introduce, our approach generalises classical isoperimetric (aka Følner) profiles of amenable groups and recently introduced functions quantifying residually finite groups. Our viewpoint is much more general and covers hyperlinear and sofic approximations as well as many other metric approximations such as weakly sofic, weakly hyperlinear, and linear sofic approximations.

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DOI: 10.5802/afst.1788
Classification: 20E26, 20F69, 20C99, 03C20
Keywords: Residually finite groups, sofic and hyperlinear groups, metric ultraproducts, amenable groups, full residual finiteness growth, Følner function.

Goulnara Arzhantseva 1; Pierre-Alain Cherix 2

1 Universität Wien, Fakultät für Mathematik, Oskar-Morgenstern-Platz 1, 1090 Wien, Austria.
2 Université de Genève, Section de Mathématiques, Uni Dufour, 24 rue du Général Dufour, Case postale 64, 1211 Genève 4, Switzerland.
License: CC-BY 4.0
Copyrights: The authors retain unrestricted copyrights and publishing rights
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Goulnara Arzhantseva; Pierre-Alain Cherix. Quantifying metric approximations of discrete groups. Annales de la Faculté des sciences de Toulouse : Mathématiques, Serie 6, Volume 33 (2024) no. 3, pp. 831-877. doi : 10.5802/afst.1788. https://afst.centre-mersenne.org/articles/10.5802/afst.1788/

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