@article{ASENS_1927_3_44__317_0,
     author = {Cerf, G.},
     title = {Sur les caract\'eristiques des \'equations et syst\`emes aux d\'eriv\'ees partielles du premier ordre},
     journal = {Annales scientifiques de l'\'Ecole Normale Sup\'erieure},
     pages = {317--343},
     publisher = {Elsevier},
     volume = {3e s{\'e}rie, 44},
     year = {1927},
     doi = {10.24033/asens.780},
     language = {fr},
     url = {http://www.numdam.org/articles/10.24033/asens.780/}
}