Extensions of quasi-morphisms to the symplectomorphism group of the disk

Abstract

On the group $\mathrm{Symp}(\mathrm{D},\partial \mathrm{D})$ of symplectomorphisms of the disk which are the identity near the boundary, there are homogeneous quasi-morphisms called the Ruelle invariant and Gambaudo-Ghys quasi-morphisms. In this paper, we show that the above homogeneous quasi-morphisms extend to homogeneous quasi-morphisms on the whole group $\mathrm{Symp}(\mathrm{D})$ of symplectomorphisms of the disk. As a corollary, we show that the second bounded cohomology $H_b^2(\mathrm{Symp}(\mathrm{D}))$ is infinite-dimensional.

7 pages