@article{DE_1978_14_2_a11,
     author = {N. I. Popivanov},
     title = {Equations of mixed type with two lines of degeneracy in unbounded domains. {I.~Imbedding} theorem. {Uniqueness} theorem},
     journal = {Differencialʹnye uravneni\^a},
     pages = {304--317},
     publisher = {mathdoc},
     volume = {14},
     number = {2},
     year = {1978},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1978_14_2_a11/}
}