@article{ELMA_1967__22_140841,
     author = {Arnold Kirsch},
     title = {Eine geometrische {Charakterisierung} der {Differenzierbarkeit} f\"ur {Funktionen} zweier {Ver\"anderlicher.}},
     journal = {Elemente der Mathematik},
     pages = {27--34},
     publisher = {mathdoc},
     volume = {22},
     year = {1967},
     zbl = {0145.06404},
     url = {http://geodesic.mathdoc.fr/item/ELMA_1967__22_140841/}
}