@article{JMPA_1918_8_1__309_0,
     author = {Cerf, G.},
     title = {Sur les transformations des \'equations aux d\'eriv\'ees partielles d'ordre quelconque \`a deux variables ind\'ependantes},
     journal = {Journal de Math\'ematiques Pures et Appliqu\'ees},
     pages = {309--412},
     publisher = {Gauthier-Villars},
     volume = {8e s{\'e}rie, 1},
     year = {1918},
     language = {fr},
     url = {http://www.numdam.org/item/JMPA_1918_8_1__309_0/}
}