@article{MAN_1928__99_159268,
     author = {K. Lachmann},
     title = {Beitrag zum {Schwingungsproblem} von {Duffing}},
     journal = {Mathematische Annalen},
     pages = {479--492},
     year = {1928},
     volume = {99},
     zbl = {54.0477.03},
     url = {http://geodesic.mathdoc.fr/item/MAN_1928__99_159268/}
}