The non-hyperbolicity of irrational invariant curves for twist maps and   all that follows
  The key result of this article is key lemma: if a Jordan curve $\gamma$ is invariant by a given C 1+$\alpha$ -diffeomorphism f of a surface and if $\gamma$ carries an ergodic hyperbolic probability $\mu$, then $\mu$ is supported on a periodic orbit. From this Lemma we deduce three new results for the C 1+$\alpha$ symplectic twist maps f of the annulus: 1. if $\gamma$ is a loop at the boundary of an instability zone such that f |$\gamma$ has an irrational rotation number, then the convergence of any orbit to $\gamma$ is slower than exponential; 2. if $\mu$ is an invariant probability that is supported in an invariant curve $\gamma$ with an irrational rotation number, then $\gamma$ is C 1 $\mu$-almost everywhere; 3. we prove a part of the so-called "Greene criterion", introduced by J. M. Greene in [16] in 1978 and never proved: assume that (pn qn) is a sequence of rational numbers converging to an irrational number $\omega$; let (f k (x n)) 1$\le$k$\le$qn be a minimizing periodic orbit with rotation number pn qn and let us denote by R n its mean residue R n = |1/2 -- Tr(Df qn (x n))/4| 
M.-C Arnaud (LMA), P Berger (LAGA)
math.DS