@article{MZ_1976__149_172380,
     author = {Gudlaugur Thorbergsson},
     title = {Vierscheitelsatz auf {Fl\"achen} nichtpositiver {Kr\"ummung.}},
     journal = {Mathematische Zeitschrift},
     pages = {47--56},
     publisher = {mathdoc},
     volume = {149},
     year = {1976},
     language = {de},
     url = {http://geodesic.mathdoc.fr/item/MZ_1976__149_172380/}
}