@article{CMH_1944__17_138862,
     author = {Heinz Hopf},
     title = {Beitr\"age zur {Homotopietheorie.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {307--326},
     year = {1944},
     volume = {17},
     zbl = {0061.40704},
     url = {http://geodesic.mathdoc.fr/item/CMH_1944__17_138862/}
}