中国科学院数学与系统科学研究院期刊网
On the Conditions of a Center and General Integrals of Quadratic Differential Systems
Wei Yin YE(1),Yan Qian YE(2)
Acta Mathematica Sinica, Chinese Series . 2003, (2): 309 -316 .  DOI: 10.12386/A2003sxxb0040