@article{05237105, title = {Periodic solutions of second-order nonautonomous dynamical systems.}, journal = {Boundary Value Problems [electronic only]}, volume = {2006}, year = {2006}, doi = {10.1155/BVP/2006/25104}, zbl = {1140.34366}, language = {en}, url = {http://dml.mathdoc.fr/item/05237105} }
Schechter, Martin. Periodic solutions of second-order nonautonomous dynamical systems.. Boundary Value Problems [electronic only], Tome 2006 (2006) . doi : 10.1155/BVP/2006/25104. http://gdmltest.u-ga.fr/item/05237105/