Entropy and quasimorphisms

Abstract

Let $S$ be a compact oriented surface. We construct homogeneous quasimorphisms on $Diff(S,area)$, on $Dif{f}_0(S,area)$ and on $Ham(S)$ generalizing the constructions of Gambaudo-Ghys and Polterovich. We prove that there are infinitely many linearly independent homogeneous quasimorphisms on $Diff(S,area)$, on $Dif{f}_0(S,area)$ and on $Ham(S)$ whose absolute values bound from below the topological entropy. In case when $S$ has a positive genus, the quasimorphisms we construct on $Ham(S)$ are $C^0$-continuous. We define a bi-invariant metric on these groups, called the entropy metric, and show that it is unbounded. In particular, we reprove the fact that the autonomous metric on $Ham(S)$ is unbounded.

arXiv comment23 pages, one figure. In this version the main results are proved for all compact oriented surfaces. To appear in Journal of Modern Dynamics