@article{DE_1998_34_6_a16,
     author = {U. D. Bekbaev},
     title = {On the equivalence and differential rational invariants of second-order linear partial differential equations of two variables under a change of variables},
     journal = {Differencialʹnye uravneni\^a},
     pages = {835--837},
     publisher = {mathdoc},
     volume = {34},
     number = {6},
     year = {1998},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1998_34_6_a16/}
}