@article{AM2_1997__53_1-2_137687,
     author = {Josef Hainzl},
     title = {On the number of limit cycles in perturbations of a two dimensional nonlinear differential system.},
     journal = {Aequationes mathematicae},
     pages = {254--274},
     publisher = {mathdoc},
     volume = {53},
     number = {1-2},
     year = {1997},
     zbl = {0871.34018},
     url = {http://geodesic.mathdoc.fr/item/AM2_1997__53_1-2_137687/}
}