2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/94322After revealing difficulties of the standard time-dependent perturbation theory in quantum mechanics mainly from the viewpoint of practical calculation, we propose a new quasi-canonical perturbation theory. In the new theory, the dynamics of physical observables, instead of that of coefficients of wave-function expansion, is formulated so that the gauge-invariance and correspondence principles are observed naturally.13 pages, No figuresQuantum Physicstext