@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/}
}