Periodic Perturbations of Non-Conservative Second Order Differential Equations

Consider 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) = \epsilon h(t)$. {\it Key Words:} perturbed systems, Lienard equation, polynomial systems.