@article{10_14708_cm_v30i2_6316,
     author = {H. Gacki and I. Janus},
     title = {On periodic solutions of second order nonlinear differentia equations with singularities},
     journal = {Commentationes Mathematicae},
     pages = { 293--299},
     publisher = {mathdoc},
     volume = {30},
     number = {2},
     year = {1991},
     doi = {10.14708/cm.v30i2.6316},
     language = {pl},
     url = {http://geodesic.mathdoc.fr/articles/10.14708/cm.v30i2.6316/}
}