On conjugation invariant norms, asymptotic cones, metric ultraproducts and contractibility

TL;DR

This paper investigates the contractibility of asymptotic cones and metric ultraproducts for groups with conjugation invariant norms, revealing new results on their algebraic and geometric properties.

Contribution

It introduces novel lemmata on strong contractibility, applies them to various groups, and establishes the contractibility of the asymptotic cone of the infinite symmetric group.

Findings

Asymptotic cone of the infinite symmetric group is contractible and algebraically simple.

Contractibility results extend to ultraproducts of subgroups of linear groups.

New connections between asymptotic cones and algebraic properties of groups.

Abstract

In the present paper we prove lemmata on strong contractibility in asymptotic cones and metric ultraproducts which we apply to both the case of finitely generated word norms and the case of conjugation invariant norms. We recover classically known contractibility results on free products and prove the contractibility of the asymptotic cone of the infinite symmetric group $\Sym_\infty$ equipped with a conjugation invariant norm. Furthermore, we give examples of contractible metric ultraproducts arising from subgroups of general linear groups. Additionally, we discuss algebraic properties of groups arising as asymptotic cones for conjugation invariant norms. For example, we show that the asymptotic cone of the infinite symmetric group is itself an algebraically simple group relating strongly to the universally sophic groups defined by Elek and Szabo.