@article{DE_1973_9_2_a11,
     author = {V. Ya. Ivrii},
     title = {A certain class of nonstrictly hyperbolic operators},
     journal = {Differencialʹnye uravneni\^a},
     pages = {299--306},
     publisher = {mathdoc},
     volume = {9},
     number = {2},
     year = {1973},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1973_9_2_a11/}
}