@article{10_4064_ap_10_1_49_72,
     author = {Z. Opial},
     title = {Sur les p\'eriodes des solutions de l'\'equation diff\'erentielle x'' + g(x) = 0},
     journal = {Annales Polonici Mathematici},
     pages = {49--72},
     publisher = {mathdoc},
     volume = {10},
     number = {1},
     year = {1961},
     doi = {10.4064/ap-10-1-49-72},
     language = {fr},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-10-1-49-72/}
}