Flexibility and analytic smoothing in averaging theory

Abstract

Using a new strategy, we extend the classical Nekhoroshev's estimates to the case of H\"older regular steep near-integrable hamiltonian systems, the stability times being polynomially long in the inverse of the size of the perturbation. We prove that the stability exponents can be taken to be $(\ell-1)/(2n\alpha_1...\alpha_{n-2})$ for the time of stability and $1/(2n\alpha_1...\alpha_{n-1})$ for the radius of stability, $\ell >n+1$ being the regularity and the $\alpha_i$'s being the indices of steepness. Our strategy consists in deriving a perturbation theory which exploits a sharp analytic smoothing theorem to approximate any H\"older function by an analytic one. In addition, an appropriate choice of the free parameters in the problem enables us to have a first grasp on the relation connecting the time and radius of stability to the threshold that the size of the perturbation must satisfy in order for the theorem to apply. Particular attention is payed to a geometric presentation of the construction of the so-called "resonant blocks", in order to shed a definitive light on the nature of the steepness condition. We also investigate the convex setting, using a similar approach.