@article{DE_1990_26_2_a15,
     author = {S. S. Kharibegashvili},
     title = {A characterization problem for a class of second-order hyperbolic systems with a characteristic line of degeneration},
     journal = {Differencialʹnye uravneni\^a},
     pages = {305--313},
     publisher = {mathdoc},
     volume = {26},
     number = {2},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1990_26_2_a15/}
}