Hofer Geometry of a Subset of a Symplectic Manifold

To every closed subset $X$ of a symplectic manifold $(M,\omega )$ we associate a natural group of Hamiltonian diffeomorphisms $Ham(X,\omega )$. We equip this group with a semi-norm $\mathrm{\parallel }\cdot {\mathrm{\parallel }}^{X,\omega }$, generalizing the Hofer norm. We discuss $Ham(X,\omega )$ and $\mathrm{\parallel }\cdot {\mathrm{\parallel }}^{X,\omega }$ if $X$ is a symplectic or isotropic submanifold. The main result involves the relative Hofer diameter of $X$ in $M$. Its first part states that for the unit sphere in $R^{2n}$ this diameter is bounded below by $\frac{\pi }{2}$, if $n\ge 2$. Its second part states that for $n\ge 2$ and $d\ge n+1$ there exists a compact set in $R^{2n}$ of Hausdorff dimension at most $d$, with relative Hofer diameter bounded below by $\pi \mathrm{/}k(n,d)$, where $k(n,d)$ is an explicitly defined integer.