@article{DE_1976_12_2_a14,
     author = {A. A. Iosif'yan},
     title = {Characteristic problems with a principal part that contains mixed derivatives},
     journal = {Differencialʹnye uravneni\^a},
     pages = {309--324},
     publisher = {mathdoc},
     volume = {12},
     number = {2},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1976_12_2_a14/}
}