中国科学院数学与系统科学研究院期刊网
Existence of Periodic Solutions for Second Order Differential Equations with Asymmetric Nonlinearity
Xiao Mei ZHANG(1),Ding Bian QI
Acta Mathematica Sinica, Chinese Series . 2003, (5): 1017 -102 .  DOI: 10.12386/A2003sxxb0135