1. Kaplan J L, Yorke J A, Ordinary differential equations which yield periodic solutions of differential delay equations,J Math Anal Appl, 1974,48: 317–324.
2. Nussbaum R D, Periodic solutions of special differential delay equations: An example in nonlinear analysis,Proc Royal Soc Edingbourgh, 1978,81A: 131–151.
3. Wen Lizhi, On the existence of periodic solutions of a type of differential difference equations,Chin Ann Math, 1989,10A3: 249–254.
4. Ge Weigao, Existence of many and infinitely many periodic solutions of some classes of differential delay equations,J Beijing Inst Tech, 1993,2(1): 5–14.
5. Ge Weigao, Number of simple periodic solutions of differential delay equation $$\dot x(t) = - f(x(t - 1))$$ , submitted.