@article{DE_1975_11_1_a13,
     author = {N. I. Popivanov},
     title = {Boundary value problems for degenerate multidimensional hyperbolic equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {116--126},
     publisher = {mathdoc},
     volume = {11},
     number = {1},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1975_11_1_a13/}
}