@book{MSM_1925__6__1_0,
     author = {Goursat, E.},
     title = {Le probl\`eme de {B\"acklund}},
     series = {M\'emorial des sciences math\'ematiques},
     publisher = {Gauthier-Villars},
     number = {6},
     year = {1925},
     language = {fr},
     url = {http://www.numdam.org/item/MSM_1925__6__1_0/}
}