On the distribution function of a system in a thermal bath and in the field of a harmonic external force

A system in a steady state in a thermal bath and in the field of an external harmonic force is considered. Model calculations lead to the suggestion that there is an excess population in the regions of the phase space of the classical system where the frequency of the normal modes is equal to the external frequency or on the levels of the quantum system that are in resonance with the external frequency. The part played by the quasienergy is discussed.