@article{CMH_1932__4_138586,
     author = {Ludwig Bieberbach},
     title = {Eine singularit\"atenfreie {Fl\"ache} konstanter negativer {Kr\"ummung} im {Hilbertschen} {Raum.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {248--255},
     publisher = {mathdoc},
     volume = {4},
     year = {1932},
     zbl = {0005.08202},
     url = {http://geodesic.mathdoc.fr/item/CMH_1932__4_138586/}
}