1. Nussbaum, R. D., Periodic solution of special equations: An example in nonlinear functional analysis,Proc. Royzl. Soc. Edinburgh., Ser. A, 1978, 81: 131.
2. Ge Weigao, On periodic solutions of differential-delay equations with several lags,Acta. Math. Appl. Sinica (in Chinese), 1994, 17(2): 173.
3. Li Jibin, Liu Zhenrong,Hamilton Systems and Periodic Solutions to DDEs (in Chinese), Beijing: Science Press, 1996.