Given the Hamiltonian
of a completely integrable system, the
is described by the Hamiltonian
is a function whose numerical value is of the same order of
<< 1. The perturbed problem thus differs slightly from the unperturbed one, but unfortunately the same is not true for the solution: a small perturbation can give rise to
effects, i.e., to a slow but progressive wandering from the unperturbed, and known for infinite time, solution.