An Invitation to Coarse Groups

TL;DR

This paper develops a foundational theory of coarse groups, exploring their algebraic properties, actions, and connections to classical mathematical subjects, thereby extending geometric group theory into a coarse, large-scale framework.

Contribution

It introduces the concept of coarse groups and actions, proves coarse analogs of classical theorems, and links the theory to various areas like number theory and topological groups.

Findings

Coarse versions of the Isomorphism Theorems established.

Development of coarse homomorphisms, quotients, and subgroups.

Connections made between coarse groups and classical mathematical subjects.

Abstract

In this monograph we lay the foundation for a theory of coarse groups and coarse actions. Coarse groups are group objects in the category of coarse spaces, and can be thought of as sets with operations that satisfy the group axioms "up to uniformly bounded error". In the first part of this work, we develop the theory of coarse homomorphisms, quotients, and subgroups, and prove that coarse versions of the Isomorphism Theorems hold true. We also initiate the study of coarse actions and show how they relate to the fundamental observation of Geometric Group Theory. In the second part we explore a selection of specialized topics, such as the study of coarse group structures on set-groups, groups of coarse automorphisms and spaces of controlled maps. Here the main aim is to show how the theory of coarse groups connects with classical subjects. These include: number theory; the study of bi-invariant metrics on groups; quasimorphisms and stable commutator length; ${\rm Out}(F_n)$; topological group actions.