@article{MAN_1931__104_159489,
     author = {H. Hopf},
     title = {\"Uber die {Abbildungen} der dreidimensionalen {Sph\"are} auf die {Kugelfl\"ache}},
     journal = {Mathematische Annalen},
     pages = {637--665},
     publisher = {mathdoc},
     volume = {104},
     year = {1931},
     zbl = {0001.40703},
     url = {http://geodesic.mathdoc.fr/item/MAN_1931__104_159489/}
}