Department of Mathematical Sciences,Sharif University of Technology
In this paper, the third order differential equation x'''+\psi(x')x''+(k^2+\phi(x))x'+f(t,x)=e(t) is considered. Under certain conditions on the functions appearing in the differential equation, the existence of periodic solutions is proved. Similar problems have been treated by authors in [4,6,7]. However, the method employed here is used by Reissig  and the results obtained are, in fact, a generalization of those in [1-3]. The conditions imposed on the nonlinear terms do not require the ultimate boundedness of all solutions.