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I want to show that the energy displacement of $Z^{2n}(r)$ is at most $\pi r^2$.

In the textbook of Mcduff and Salamon they write that I should identify the two dimensional ball (with radius $r$) with a square of the same area, and then calculate Hofer's metric, $d_H(Id,\phi)$ where $\phi$ is a translation s.t $\phi(B^2(r)\times K) \cap (B^2(r)\times K)=\emptyset$.

I don't know how to calculate Hofer's metric, I mean it depends on the Hamiltonian here, and I don't know how does it look here?

Thanks in advance. Edit: $K$ is a compact subset of $R^{2n-2}$.

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