@article{MAN_1927__96_159166,
     author = {H. Hopf},
     title = {Vektorfelder in {n-dimensionalen-Mannigfaltigkeiten}},
     journal = {Mathematische Annalen},
     pages = {225--250},
     publisher = {mathdoc},
     volume = {96},
     year = {1927},
     zbl = {52.0571.01},
     url = {http://geodesic.mathdoc.fr/item/MAN_1927__96_159166/}
}