A Family of Quasimorphism Constructions

TL;DR

This paper introduces a new principle for constructing quasimorphisms based on local data from group actions on structured spaces, simplifying their creation and understanding across various groups.

Contribution

It proposes a family of spaces that facilitate direct quasimorphism construction and demonstrates their application to countable groups, circle actions, and diffeomorphism groups.

Findings

Provides a new principle for quasimorphism construction

Demonstrates examples for countable and circle-related groups

Establishes an embedding of countable groups into quasi-isometry groups

Abstract

In this work we present a principle which says that quasimorphisms can be obtained via "local data" of the group action on certain appropriate spaces. In a rough manner the principle says that instead of starting with a given group and try to build or study its space of quasimorphisms, we should start with a space with a certain structure, in such a way that groups acting on this space and respect this structure will automatically carry quasimorphisms, where these are suppose to be better understood. In this paper we suggest such a family of spaces and give demonstrating examples for countable groups, groups that relate to action on the circle as well as outline construction for diffeomorphism groups. A distinctive advantage of this principle is that it allows the construction of the quasimorphism in a quite direct way. Further, we prove a lemma which besides serving as a platform for the construction of quasimorphisms on countable groups, bare interest by itself. Since it provides us with an embedding of any given countable group as a group of quasi-isometries of a universal space, where this space of embeddings is in bijection with the projective space of the homogeneous quasimorphism space of the group.