@article{MZ_1949__51_169070,
     author = {H., Seifert},
     title = {Periodische {Bewegungen} mechanischer {Systeme}},
     journal = {Mathematische Zeitschrift},
     pages = {197--216},
     year = {1949},
     volume = {51},
     zbl = {0030.22103},
     url = {http://geodesic.mathdoc.fr/item/MZ_1949__51_169070/}
}