@article{10_4064_ap_7_3_309_319,
     author = {Z. Opial},
     title = {Sur les solutions p\'eriodiques et presque-p\'eriodiques de l'\'equation diff\'erentielle x'' + kf(x)x' + g(x) = kp(t)},
     journal = {Annales Polonici Mathematici},
     pages = {309--319},
     publisher = {mathdoc},
     volume = {7},
     number = {3},
     year = {1959},
     doi = {10.4064/ap-7-3-309-319},
     language = {fr},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-7-3-309-319/}
}