@article{MAN_1923__90_159015,
     author = {C. Russyan},
     title = {Les caract\'eristiques du premier ordre de l'\'equation aux d\'eriv\'ees partielles du second ordre},
     journal = {Mathematische Annalen},
     pages = {46--54},
     publisher = {mathdoc},
     volume = {90},
     year = {1923},
     zbl = {49.0334.02},
     url = {http://geodesic.mathdoc.fr/item/MAN_1923__90_159015/}
}