1. Bai, D., Xu, Y.: Periodic solutions of first order functional differential equations with periodic deviations. Comput. Math. Appl. 53, 1361–1366 (2007)
2. Deimling, K.: Nonlinear Functional Analysis. Springer, Berlin (1985)
3. Jiang, D., Wei, J.J.: Existence of positive periodic solutions for non-autonomous functional differential equations with delay. Chin. Ann. Math. 20A, 715–720 (1999). (in Chinese)
4. Jiang, D., Wei, J., Jhang, B.: Positive periodic solutions of functional differential equations and population models. Electron. J. Differ. Equ. 2002(71), 1–13 (2002)
5. Jiang, D., O’Regan, D., Agrawal, R.P., Xu, X.J.: On the number of positive periodic solutions of functional differential equations and population models. Math. Models Meth. Appl. Sci. 15, 555–573 (2005)