中国科学院数学与系统科学研究院期刊网
AMS
Liouvillian and Analytic Integrability of the Quadratic Vector Fields Having an Invariant Ellipse
Jaume LLIBRE, Claudia VALLS
Acta Mathematica Sinica . 2014, (3): 453 -466 .  DOI: 10.1007/s10114-014-2484-1