@article{10_4064_ap_21_3_305_307,
     author = {F. H. Szafraniec},
     title = {Note on the existence of periodic solutions of a second order differential equation},
     journal = {Annales Polonici Mathematici},
     pages = {305--307},
     publisher = {mathdoc},
     volume = {21},
     number = {3},
     year = {1968},
     doi = {10.4064/ap-21-3-305-307},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-21-3-305-307/}
}