@article{JMPA_1942_9_21__141_0,
     author = {Humbert, Pierre},
     title = {G\'eom\'etrie plane dans l'espace attach\'e \`a l'op\'erateur $\Delta_3$},
     journal = {Journal de Math\'ematiques Pures et Appliqu\'ees},
     pages = {141--153},
     publisher = {Gauthier-Villars},
     volume = {9e s{\'e}rie, 21},
     year = {1942},
     zbl = {0027.09302},
     language = {fr},
     url = {http://www.numdam.org/item/JMPA_1942_9_21__141_0/}
}