@article{DAN_1973_210_3_a4,
     author = {V. V. Lychagin},
     title = {Local classification of first-order nonlinear partial differential equations},
     journal = {Doklady Akademii Nauk},
     pages = {525--528},
     publisher = {mathdoc},
     volume = {210},
     number = {3},
     year = {1973},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1973_210_3_a4/}
}