@article{MAN_1939__116_159993,
     author = {H. Hopf and H. Schilt},
     title = {\"Uber {Isometrie} und stetige {Verbiegung} von {Fl\"achen}},
     journal = {Mathematische Annalen},
     pages = {58--75},
     year = {1939},
     volume = {116},
     zbl = {0019.28001},
     url = {http://geodesic.mathdoc.fr/item/MAN_1939__116_159993/}
}