2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/206544We show that a strongly perturbed quantum system, being a semiclassical system characterized by the Wigner-Kirkwood expansion for the propagator, has the same expansion for the eigenvalues as for the WKB series. The perturbation series is rederived by the duality principle in perturbation theory.4 pages, no figures. Accepted for publication in Proceedings of the Royal Society AHigh Energy Physics - TheoryMathematical PhysicsQuantum Physicstext