@article{MAN_1900__53_157973,
     author = {J. Sommer},
     title = {Focaleigenschaften quadratischer {Mannigfaltigkeiten} im vierdimensionalen {Raum}},
     journal = {Mathematische Annalen},
     pages = {113--160},
     publisher = {mathdoc},
     volume = {53},
     year = {1900},
     zbl = {31.0637.01},
     url = {http://geodesic.mathdoc.fr/item/MAN_1900__53_157973/}
}