The author studies the existence and asymptotic stability of a time periodic solution for the evolution equation
where is a positive-definite selfadjoint operator on a Hilbert space , is a nonlinear mapping which is -periodic in , and are positive constants.
Essential conditions on the nonlinearity are given, which guarantee that the equation has co-periodic solutions or an asymptotically stable co-periodic solution. The author improves and extends some previous results on the given topic by a different method. In this way, some milder conditions can be used, and the specific condition can be deleted. The results are based on analytic semigroups and on an integral inequality of Bellman type with delays. At the end, periodic solutions of delay parabolic equations are treated.