A perturbative expansion of the evolution operator associated with   time-dependent quantum Hamiltonians
  A novel expansion -- which generalizes Magnus expansion -- of the evolution operator associated with a (in general, time-dependent) perturbed Hamiltonian is introduced. It is shown that it has a wide range of possible solutions that can be fitted according to computational convenience. The time-independent and the adiabatic case are studied in detail. 
P. Aniello