@article{MZ_1939__44_168818,
     author = {E., Digel},
     title = {\"Uber die {Bedingungen} der {Existenz} der {Integrale} partieller {Differentialgleichungen} erster {Ordnung}},
     journal = {Mathematische Zeitschrift},
     pages = {445--451},
     publisher = {mathdoc},
     volume = {44},
     year = {1939},
     zbl = {64.0449.04},
     url = {http://geodesic.mathdoc.fr/item/MZ_1939__44_168818/}
}