2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/91987A 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.34 pages, no figuresQuantum Physicstext