@article{10_4064_ap_30_2_165_174,
     author = {G. J. Butler},
     title = {The existence of periodic solutions of an autonomous second order non-linear differential equation},
     journal = {Annales Polonici Mathematici},
     pages = {165--174},
     publisher = {mathdoc},
     volume = {30},
     number = {2},
     year = {1974},
     doi = {10.4064/ap-30-2-165-174},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-30-2-165-174/}
}