@article{700060,
title = {Periodic solutions of $\ddot{x}=f(x,\dot{x})$
},
booktitle = {Proceedings of Equadiff III},
series = {GDML\_Books},
publisher = {Univ. J. E. Purkyn\v e - P\v r\'\i rodov\v edeck\'a fakulta},
address = {Brno},
year = {1973},
pages = {127-138},
mrnumber = {MR0348194},
zbl = {0331.34038},
url = {http://dml.mathdoc.fr/item/700060}
}
Ráb, Miloš. Periodic solutions of $\ddot{x}=f(x,\dot{x})$
, dans Proceedings of Equadiff III, GDML_Books, (1973), pp. 127-138. http://gdmltest.u-ga.fr/item/700060/