2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/199050We introduce a time-dependent perturbation method to calculate the number of created particles in a 1D cavity with an oscillating wall of the frequency $Ω. $ This method makes it easy to find the dominant part of the solution which results from the parametric resonance. The maximal number of particles are created at the mode frequency $Ω/2 . $ Using the Floquet theory, we discuss the long-time behavior of the particle creation.11 pages, RevTeX, no figures, Proceedings of 5th Korean-Italian Symposium on Relativistic AstrophysicsQuantum Physicstext