@article{10_4064_ap_11_2_149_159,
     author = {Z. Opial},
     title = {Sur l'existence des solutions p\'eriodiques de l'\'equation diff\'erentielle x'' + f(x,x')x' + g(x) = p(t)},
     journal = {Annales Polonici Mathematici},
     pages = {149--159},
     publisher = {mathdoc},
     volume = {11},
     number = {2},
     year = {1961},
     doi = {10.4064/ap-11-2-149-159},
     language = {fr},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-11-2-149-159/}
}