@article{MAN_1878__13_156811,
     author = {B\"acklund},
     title = {Zur {Theorie} der {Charakteristiken} der partiellen {Differentialgleichungen} zweiter {Ordnung}},
     journal = {Mathematische Annalen},
     pages = {411--428},
     publisher = {mathdoc},
     volume = {13},
     year = {1878},
     zbl = {10.0260.02},
     url = {http://geodesic.mathdoc.fr/item/MAN_1878__13_156811/}
}