The Role of Resonances in the Stochastic Limit
  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. 
F. Gomez