On mixing diffeomorphisms of the disk
We prove that a real analytic pseudo-rotation $f$ of the disc or the sphere is never topologically mixing. When the rotation number of $f$ is of Brjuno type, the latter follows from a KAM theorem of Rüssmann on the stability of real analytic elliptic fixed points. In the non-Brjuno case, we prove that a pseudo-rotation of class $C^k$, $k\geq 2$, is $C^{k-1}$-rigid using the simple observation, derived from Franks' Lemma on free discs, that a pseudo-rotation with small rotation number compared to its $C^1$ (or Hölder) norm must be close to Identity. From our result and a structure theorem by Franks and Handel (on zero entropy surface diffeomorphisms) it follows that an analytic conservative diffeomorphism of the disc or the sphere that is topologically mixing must have positive topological entropy. In our proof we need an a priori limit on the growth of the derivatives of the iterates of a pseudo-rotation that we obtain via an effective finite information version of the Katok closing lemma for an area preserving surface diffeomorphism $f$, that provides a controlled gap in the possible growth of the derivatives of $f$ between exponential and sub-exponential.