Metadata
Abstract
In this article we show that the braid type of a set of -periodic orbits of a non-degenerate Hamiltonian diffeomorphism on a surface is stable under perturbations which are sufficiently small with respect to the Hofer metric . We call this new phenomenon braid stability for the Hofer metric.
We apply braid stability to study the stability of the topological entropy of Hamiltonian diffeomorphisms on surfaces under small perturbations with respect to . We show that is lower semicontinuous on the space of Hamiltonian diffeomorphisms of a closed surface endowed with the Hofer metric, and on the space of compactly supported diffeomorphisms of the two-dimensional disk endowed with the Hofer metric. This answers the two-dimensional case of a question of Polterovich.
En route to proving the lower semicontinuity of with respect to , we prove that the topological entropy of a diffeomorphism on a compact surface can be recovered from the braid types realized by the periodic orbits of .
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