@article{CMH_1941__14_138788,
     author = {Heinz Hopf},
     title = {Fundamentalgruppe und zweite {Bettische} {Gruppe.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {257--309},
     publisher = {mathdoc},
     volume = {14},
     year = {1941},
     zbl = {0027.09503},
     url = {http://geodesic.mathdoc.fr/item/CMH_1941__14_138788/}
}