Hamiltonian systems with widely separated frequencies

In this paper we study two degrees of freedom Hamiltonian systems and applications to nonlinear wave equations. Near the origin, we assume that the linearized system has purely imaginary eigenvalues: +/-iomega(1) and +/-iomega(2), with 0 < w(2)/w(1) much less than 1 or omega(2)/omega(1) much greater than 1, which is interpreted as a perturbation of a problem with double zero eigenvalues. Using the averaging method, we compute the normal form and show that the dynamics differs from the usual one for Hamiltonian systems at higher-order resonances. Under certain conditions, the normal form is degenerate which forces us to normalize to higher degree. The asymptotic character of the normal form and the corresponding invariant tori is validated using the Kolmogorov-Arnold-Moser theory. This analysis is then applied to widely separated mode-interaction in a family of nonlinear wave equations containing various degeneracies.