@article{MAN_1879__15_156847,
     author = {B\"acklund},
     title = {Zur {Theorie} der partiellen {Differentialgleichungen} zweiter {Ordnung.}},
     journal = {Mathematische Annalen},
     pages = {39--85},
     publisher = {mathdoc},
     volume = {15},
     year = {1879},
     zbl = {11.0257.04},
     url = {http://geodesic.mathdoc.fr/item/MAN_1879__15_156847/}
}