A four dimensional map for escape from resonance: negative energy modes   and nonlinear instability
  Positive definiteness of a Hamiltonian expanded about an equilibrium point provides only a necessary condition for stability, a criterion known as Dirichlet's theorem. The reason that this criterion is not necessary for stability is because of the possible existence of negative energy modes, which are linearly stable modes of oscillation that have negative energy. When such modes are present, the Hamiltonian is, in general, indefinite. Although such systems with negative energy modes are linearly stable (spectral stable), they are unstable to infinitesimal perturbations under the nonlinear dynamics. In the present work we study this kind of nonlinear instability with the simplest nontrivial four dimensional area-preserving map, which has a cubic degree of freedom, that was designed to mimic the behavior of a Hamiltonian system with one positive and one negative energy mode, and a quadratic degree of freedom, that allows eventual escapes in phase space, usually called as Arnold diffusion. 
Caroline G. L. Martins, P. J. Morison and C. Curry
physics.plasm-ph nlin.CD