@article{1206139730,
author = {Urabe, Minoru},
title = {The potential force yielding a periodic motion whose period is an arbitrary continuously differentiable function of the amplitude},
journal = {J. Sci. Hiroshima Univ. Ser. A-I Math.},
volume = {26},
number = {2},
year = {1962},
pages = { 93-109},
language = {en},
url = {http://dml.mathdoc.fr/item/1206139730}
}
Urabe, Minoru. The potential force yielding a periodic motion whose period is an arbitrary continuously differentiable function of the amplitude. J. Sci. Hiroshima Univ. Ser. A-I Math., Tome 26 (1962) no. 2, pp. 93-109. http://gdmltest.u-ga.fr/item/1206139730/