@article{DE_2000_36_2_a6,
     author = {V. G. Bardakov},
     title = {The classification of differential equations with two independent variables on the basis of the principal part},
     journal = {Differencialʹnye uravneni\^a},
     pages = {187--197},
     year = {2000},
     volume = {36},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_2000_36_2_a6/}
}