@article{10_4064_ap_21_2_231_237,
     author = {S. S\k{e}dziwy},
     title = {Periodic solutions of $x'' + f(x)x'^{2n} + g(x) = \ensuremath{\mu}p(t)$},
     journal = {Annales Polonici Mathematici},
     pages = {231--237},
     publisher = {mathdoc},
     volume = {21},
     number = {2},
     year = {1968},
     doi = {10.4064/ap-21-2-231-237},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-21-2-231-237/}
}