We study the multiplicity of periodic solutions of nonautonomous delay differential equations which are asymptotically linear both at zero and at infinity. By making use of a theorem of Benci, some sufficient conditions are obtained to guarantee the existence of multiple periodic solutions.
Huafeng Xiao. Bo Zheng. "The Existence of Multiple Periodic Solutions of Nonautonomous Delay Differential Equations." J. Appl. Math. 2011 1 - 15, 2011. https://doi.org/10.1155/2011/829107