  We try to generalize a result of M. Willem on forced periodic oscillations
which required the assumption that the forced potential is periodic on spatial
variables. In this paper, we only assume its integral on the time variable is
periodic, and so we extend the result to cover the forced pendulum equation. We
apply the direct variational minimizing method and Rabinowtz's saddle point
theorem to study the periodic solution when the integral of the potential on
the time variable is periodic.
