@phdthesis{THESE_1920__19__1_0,
     author = {Janet, Maurice},
     title = {Sur les syst\`emes d'\'equations aux d\'eriv\'ees partielles},
     series = {Th\`eses de l'entre-deux-guerres},
     number = {19},
     year = {1920},
     language = {fr},
     url = {http://archive.numdam.org/item/THESE_1920__19__1_0/}
}