@article{MZ_1929__29_168075,
     author = {Eberhard Hopf},
     title = {Kurze {Bemerkung} \"uber wiederholte {Quadraturen}},
     journal = {Mathematische Zeitschrift},
     pages = {373--374},
     year = {1929},
     volume = {29},
     zbl = {54.0259.02},
     url = {http://geodesic.mathdoc.fr/item/MZ_1929__29_168075/}
}