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@article{MZ_1967__98_170745, author = {Klaus Schmitt}, title = {Periodic {Solutions} of {Nonlinear} {Second} {Order} {Differential} {Equations.}}, journal = {Mathematische Zeitschrift}, pages = {200--207}, publisher = {mathdoc}, volume = {98}, year = {1967}, zbl = {0153.12501}, url = {http://geodesic.mathdoc.fr/item/MZ_1967__98_170745/} }
Klaus Schmitt. Periodic Solutions of Nonlinear Second Order Differential Equations.. Mathematische Zeitschrift, Tome 98 (1967), pp. 200-207. http://geodesic.mathdoc.fr/item/MZ_1967__98_170745/