A Hofer-like metric on the group of symplectic diffeomorphisms

Using a “Hodge decomposition” of symplectic isotopies on a compact symplectic manifold $(M,\omega )$, we construct a norm on the identity component in the group of all symplectic diffeomorphisms of $(M,\omega )$ whose restriction to the group $Ham(M,\omega )$ of hamiltonian diffeomorphisms is bounded from above by the Hofer norm. Moreover, $Ham(M,\omega )$ is closed in $Symp(M,\omega )$ equipped with the topology induced by the extended norm. We give an application to the $C^0$ symplectic topology. We also discuss extensions of Oh’s spectral distance.