中国科学院数学与系统科学研究院期刊网
On the Existence of Exactly One and Two Periodic Solutions of Liénard Equation
Hong Bin CHEN(1),Kai Tai LI(2)
Acta Mathematica Sinica, Chinese Series . 2004, (3): 417 -424 .  DOI: 10.12386/A2004sxxb0053