@article{GDZPPN002271060, title = {Vektorfelder in n-dimensionalen-Mannigfaltigkeiten}, journal = {Mathematische Annalen}, volume = {97}, year = {1927}, pages = {225-250}, zbl = {52.0571.01}, url = {http://dml.mathdoc.fr/item/GDZPPN002271060} }
Hopf, H. Vektorfelder in n-dimensionalen-Mannigfaltigkeiten. Mathematische Annalen, Tome 97 (1927) pp. 225-250. http://gdmltest.u-ga.fr/item/GDZPPN002271060/