@article{JRAM_1930__163_149731,
     author = {Heinz Hopf},
     title = {Zur {Algebra} der {Abbildungen} von {Mannigfaltigkeiten.}},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {71--88},
     publisher = {mathdoc},
     volume = {163},
     year = {1930},
     zbl = {56.0501.03},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1930__163_149731/}
}