@article{DAN_1976_227_3_a5,
     author = {N. H. Ibragimov and R. L. Anderson},
     title = {Groups of {Lie{\textendash}B\"acklund} contact transformations},
     journal = {Doklady Akademii Nauk},
     pages = {539--542},
     year = {1976},
     volume = {227},
     number = {3},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1976_227_3_a5/}
}