Let UF be the Floquet operator of a time periodic hamiltonian H(t).
For each positive and discrete observable A (which we call a {\em probe
energy}), we derive a formula for the Laplace time average of its expectation
value up to time T in terms of its eigenvalues and Green functions at the
circle of radius e1/T. Some simple applications are provided which support
its usefulness.Comment: 31 page