@article{10_4153_S0008439519000626,
     author = {Llibre, Jaume and Mart{\'\i}nez, Paulina and Vidal, Claudio},
     title = {Limit {Cycles} of a {Perturbation} of a {Polynomial} {Hamiltonian} {Systems} of {Degree} 4 {Symmetric} with {Respect} to the {Origin}},
     journal = {Canadian mathematical bulletin},
     pages = {547--561},
     year = {2020},
     volume = {63},
     number = {3},
     doi = {10.4153/S0008439519000626},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/S0008439519000626/}
}