2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/61138Consider the Lienard system $ u'' + f(u) u' + g(u) = 0$ with a center at the origin 0. In the case where the period function $T$ is monotonic, we examine periodic solutions of the perturbed equation $ u'' + a(u)u' + f(u) = εh(t)$. {\it Key Words:} perturbed systems, Lienard equation, polynomial systems.7 pagesDynamical SystemsClassical Analysis and ODEs34C25, 34C35text