@article{MZM_1981_29_6_a3,
     author = {M. Yu. Zvyagin},
     title = {Classification of {B\"acklund} transformations of second-order partial differential equations},
     journal = {Matemati\v{c}eskie zametki},
     pages = {829--842},
     year = {1981},
     volume = {29},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1981_29_6_a3/}
}