@article{MAN_1877__11_156759,
     author = {B\"acklund},
     title = {Ueber {Systeme} partieller {Differentialgleichungen} erster {Ordnung}},
     journal = {Mathematische Annalen},
     pages = {412--433},
     publisher = {mathdoc},
     volume = {11},
     year = {1877},
     zbl = {09.0276.01},
     url = {http://geodesic.mathdoc.fr/item/MAN_1877__11_156759/}
}