1. Alt, W.: Some periodicity criteria for functional differential equations. Manuscripta math. 23, 295–318 (1978)
2. an der Heiden, U.: Periodic solutions of a nonlinear second order differential equation with delay. J. Math. An. Appl. (in press)
3. Banks, H. T.: Delay systems in biological models: approximation techniques. In: Nonlinear systems and applications. Proc. Conf. Arlington 1976, V. Lakshmikantham ed., New York: Academic Press, 1977
4. Bellman, R., Cooke, K. L.: Differential-Difference Equations. New York-San FranciscoLondon: Acad. Press, 1963
5. Chow, S.-N.: Existence of periodic solutions of autonomous functional differential equations. J. Diff. Eq. 15, 350–378 (1974)