@article{MAN_1956__131_160495,
     author = {H. Lenz},
     title = {Zur {Definition} der {Fl\"achen} zweiter {Ordnung.}},
     journal = {Mathematische Annalen},
     pages = {385--389},
     year = {1956},
     volume = {131},
     zbl = {0070.15702},
     url = {http://geodesic.mathdoc.fr/item/MAN_1956__131_160495/}
}