@article{DE_1988_24_3_a12,
     author = {A. I. Perov and V. L. Khatskevich},
     title = {Regularity classes of bilinear forms, and nonlinear elliptic boundary value problems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {464--476},
     publisher = {mathdoc},
     volume = {24},
     number = {3},
     year = {1988},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1988_24_3_a12/}
}