We show the following dichotomy for a linear parabolic Z2-action ρL on the torus with at least one step-2 generator: (i) Any affine Z2-action with linear part ρL has a ℤ-factor that is either identity or genuinely parabolic, and is thus not KAM-rigid, or (ii) Almost every affine Z2-action with linear part ρL is KAM-rigid under volume preserving perturbations.

We prove an analog of the Livshits theorem for real-analytic families of cocycles over an integrable system with values in a Banach algebra (Formula Presented) or a Lie group. Namely, we consider an integrable dynamical system (Formula Presented), and a real-analytic family of cocycles (Formula Presented) indexed by a complex parameter ε in an open ball (Formula Presented). We show that if ηε is close to identity and has trivial periodic data, i.e., (Formula Presented) for each periodic point p = fn p and each (Formula Presented), then there exists a real-analytic family of maps (Formula Presented) satisfying the coboundary equation (Formula Presented) for all (Formula Presented) and (Formula Presented). We also show that if the coboundary equation above with an analytic left-hand side ηε has a solution in the sense of formal power series in ε, then it has an analytic solution.

Consider an analytic Hamiltonian system near its analytic invariant torus T-0 carrying zero frequency. We assume that the Birkhoff normal form of the Hamiltonian at T-0 is convergent and has a particular form: it is an analytic function of its non-degenerate quadratic part. We prove that in this case there is an analytic canonical transformation-not just a formal power series-bringing the Hamiltonian into its Birkhoff normal form.

Any Cd conservative map f of the d-dimensional unit ball Bd, d ≥ 2, can be realized by renormalized iteration of a Cd perturbation of identity: there exists a conservative diffeomorphism of Bd, arbitrarily close to identity in the Cd topology, that has a periodic disc on which the return dynamics after a Cd change of coordinates is exactly f. 

We present examples of nearly integrable analytic Hamiltonian systems with several strong diffusion properties: topological weak mixing and diffusion at all times. These examples are obtained by AbC constructions with several frequencies.

We show that one-dimensional random walks in a quasi-periodic environment with Liouville frequency generically have an erratic statistical behavior. In the recurrent case we show that neither quenched nor annealed limit theorems hold and both drift and variance exhibit wild oscillations, being logarithmic at some times and almost linear at other times. In the transient case we show that the annealed Central Limit Theorem fails generically. These results are in stark contrast with the Diophantine case where the Central Limit Theorem with linear drift and variance was established by Sinai.

We construct on ℝ2d, for any d ≥ 3, smooth Hamiltonians having an elliptic equilibrium with an arbitrary frequency, that is not accumulated by a positive measure set of invariant tori. For d ≥ 4, the Hamiltonians we construct have not any invariant torus of dimension d. Our examples are obtained by a version of the successive conjugation scheme à la Anosov-Katok.

The main model studied in this paper is a lattice of pendula with a nearest-neighbor coupling. If the coupling is weak, then the system is near-integrable and KAM tori fill most of the phase space. For all KAM trajectories the energy of each pendulum stays within a narrow band for all time. Still, we show that for an arbitrarily weak coupling of a certain localized type, the neighboring pendula can exchange energy. In fact, the energy can be transferred between the pendula in any prescribed way.

The famous ergodic hypothesis suggests that for a typical Hamiltonian on a typical energy surface nearly all trajectories are dense. KAM theory disproves it. Ehrenfest (The Conceptual Foundations of the Statistical Approach in Mechanics. Ithaca, NY: Cornell University Press, 1959) and Birkhoff (Collected Math Papers. Vol 2, New York: Dover, pp 462-465, 1968) stated the quasi-ergodic hypothesis claiming that a typical Hamiltonian on a typical energy surface has a dense orbit. This question is wide open. Herman (Proceedings of the International Congress of Mathematicians, Vol II (Berlin, 1998). Doc Math 1998, Extra Vol II, Berlin: Int Math Union, pp 797-808, 1998) proposed to look for an example of a Hamiltonian near with a dense orbit on the unit energy surface. In this paper we construct a Hamiltonian which has an orbit dense in a set of maximal Hausdorff dimension equal to 5 on the unit energy surface.