Continuation theorems for Ambrosetti-Prodi type periodic problems

We study the existence of periodic solutions u(.) for a class of nonlinear ordinary differential equations depending on a real parameter s and obtain the existence of closed connected branches of solution pairs (u, s) to vari ous classes of problems, including some cases, like the superlinear one, wh ere there is a lack of a priori bounds. The results are obtained as a conse quence of a new continuation theorem for the coincidence equation Lu. = N(u , s) in normed spaces. Among the applications, we discuss also an example o f existence of global branches of periodic solutions for the Ambrosetti-Pro di type problem u" + g(u) = s + p(t), with g satisfying some asymmetric con ditions.