@article{CMH_1933__5_138593,
     author = {Heinz Hopf},
     title = {Die {Klassen} der {Abbildungen} der n-dimensionalen {Polyeder} auf die n-dimensionale {Sph\"are.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {39--54},
     year = {1933},
     volume = {5},
     zbl = {0005.31304},
     url = {http://geodesic.mathdoc.fr/item/CMH_1933__5_138593/}
}