On the periodic solution of neutral functional differential equations
中立型泛函微分方程的周期解

In this paper, we prove that the ultimate boundedness of solutions implies the existence of periodic solutions for periodic functional differential equations of neutral type, which is an extension of Yoshizawa's theorem on the existence of periodic solutions for ordinary differential equations to neutral functional differential equations. Then by employing the results, we obtain sufficient conditions which guarantee the unique existence and attractivity of periodic solutions for neutral functional differential equations arising from electric systems.