@article{CMJ_2002__52_4_a13,
     author = {Zhao, Cheng-Dong and He, Qi-Min},
     title = {On the center of the generalized {Li\'enard} system},
     journal = {Czechoslovak Mathematical Journal},
     pages = {817--832},
     publisher = {mathdoc},
     volume = {52},
     number = {4},
     year = {2002},
     mrnumber = {1940062},
     zbl = {1021.34023},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMJ_2002__52_4_a13/}
}