中国科学院数学与系统科学研究院期刊网
AMS
Periodic Orbits for Some Systems of Delay Differential Equations
Jaume LLIBRE, Alexandrina–Alina TARŢA
Acta Mathematica Sinica . 2008, (2): 267 -274 .  DOI: 10.1007/s10114-007-0983-z