@article{00733112, title = {On the periodic boundary-value problem for systems of second-order nonlinear ordinary differential equations.}, journal = {Georgian Mathematical Journal}, volume = {2}, year = {1995}, pages = {21-36}, doi = {10.1007/BF02257731}, zbl = {0824.34027}, language = {en}, url = {http://dml.mathdoc.fr/item/00733112} }
Gaprindashvili, G. On the periodic boundary-value problem for systems of second-order nonlinear ordinary differential equations.. Georgian Mathematical Journal, Tome 2 (1995) pp. 21-36. doi : 10.1007/BF02257731. http://gdmltest.u-ga.fr/item/00733112/