Invariant quasimorphisms for groups acting on the circle and non-equivalence of SCL

TL;DR

This paper constructs invariant quasimorphisms for groups acting on the circle, relates them to the Poincaré translation number, and demonstrates non-bi-Lipschitz equivalence of stable commutator lengths in specific groups, including surface groups and hyperbolic mapping tori.

Contribution

It introduces a new method to construct invariant quasimorphisms, provides criteria for their non-extendability, and applies these to show non-equivalence of stable commutator lengths in certain finitely generated groups.

Findings

Invariant quasimorphisms relate to Poincaré translation number.

Stable commutator length and stable mixed commutator length are not bi-Lipschitzly equivalent.

Examples include surface groups and hyperbolic mapping tori.

Abstract

We construct invariant quasimorphisms for groups acting on the circle. Furthermore, we provide a criterion for the non-extendablity of the resulting quasimorphisms and an explicit formula which relates the values of our quasimorphisms to those of the Poincar\'{e} translation number. By using them, we show that the stable commutator length $\mathrm{scl}_G$ and the stable mixed commutator length $\mathrm{scl}_{G,N}$ are not bi-Lipschitzly equivalent for the surface group $G=\pi_1(\Sigma_{\ell})$ of genus at least $2$ and its commutator subgroup $N = [\pi_1(\Sigma_{\ell}), \pi_1(\Sigma_{\ell})]$. We also show the non-equivalence for a pair $(G,N)$ such that $G$ is the fundamental group of a $3$-dimensional closed hyperbolic mapping torus. These pairs serve as the first family of examples of such $(G,N)$ in which $G$ is finitely generated.