  In the stochastic limit the resonances play a fundamental role because they
determine the generalized susceptivities which are the building blocks of all
the physical information which survives in this limit. There are two sources of
possible divergences: one related to the singularities of the form factor;
another to the chaoticity of the spectrum. The situation will be illustrated
starting from the example of the discrete part of the hydrogen atom in
interaction with the electromagnetic field.
