A methodology for obtaining asymptotic estimates for the exponentially   small splitting of separatrices to whiskered tori with quadratic frequencies
  The aim of this work is to provide asymptotic estimates for the splitting of separatrices in a perturbed 3-degree-of-freedom Hamiltonian system, associated to a 2-dimensional whiskered torus (invariant hyperbolic torus) whose frequency ratio is a quadratic irrational number. We show that the dependence of the asymptotic estimates on the perturbation parameter is described by some functions which satisfy a periodicity property, and whose behavior depends strongly on the arithmetic properties of the frequencies. 
Amadeu Delshams, Marina Gonchenko, Pere Guti\'errez
math.DS