Copyright © 2011 Jian Zu. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
We study periodic solutions for nonlinear second-order ordinary differential problem . By constructing upper and lower boundaries and using
Leray-Schauder degree theory, we present a result about the existence and uniqueness of a periodic solution for second-order ordinary differential equations with some