@article{MZ_1934__38_168504,
     author = {Urban. H.},
     title = {Zur affinen {Differentialgeometrie} zweier komplexer {Ver\"anderlicher:} {Fl\"achen} im vierdimensionalen {Raume}},
     journal = {Mathematische Zeitschrift},
     pages = {338--374},
     publisher = {mathdoc},
     volume = {38},
     year = {1934},
     zbl = {0008.32404},
     url = {http://geodesic.mathdoc.fr/item/MZ_1934__38_168504/}
}