A theorem due to G. D. Birkhoff states that every essential curve which is invariant under a symplectic twist map of the annulus is the graph of a Lipschitz map. We prove: if the graph of a Lipschitz map h:T→R is invariant under a symplectic twist map, then h is a little bit more regular than simply Lipschitz (Theorem 1); we deduce that there exists a Lipschitz map h:T→R whose graph is invariant under no symplectic twist map (Corollary 2).Assuming that the dynamic of a twist map restricted to a Lipschitz graph is bi-Lipschitz conjugate to a rotation, we obtain that the graph is even C 1 (Theorem 3).Then we consider the case of the C 0 integrable symplectic twist maps and we prove that for such a map, there exists a dense G δ subset of the set of its invariant curves such that every curve of this G δ subset is C 1 (Theorem 4).
@article{PMIHES_2009__109__1_0, author = {Arnaud, M.-C.}, title = {Three results on the regularity of the curves that are invariant by an exact symplectic twist map}, journal = {Publications Math\'ematiques de l'IH\'ES}, volume = {110}, year = {2009}, pages = {1-17}, doi = {10.1007/s10240-009-0017-8}, mrnumber = {2511585}, zbl = {1177.53070}, language = {en}, url = {http://dml.mathdoc.fr/item/PMIHES_2009__109__1_0} }
Arnaud, M.-C. Three results on the regularity of the curves that are invariant by an exact symplectic twist map. Publications Mathématiques de l'IHÉS, Tome 110 (2009) pp. 1-17. doi : 10.1007/s10240-009-0017-8. http://gdmltest.u-ga.fr/item/PMIHES_2009__109__1_0/
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