2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/57184We characterise the set of periods for which number theoretical obstructions prevent us from finding periodic solutions of the Schroedinger equation on a two dimensional torus as well as the asymptotic occurrence of possible resonances.7 pages, revised versionMathematical PhysicsNumber Theory81Q05, 11K60text