@article{DAN_1977_234_2_a4,
     author = {Yu. V. Egorov},
     title = {On solvability conditions for equations with double characteristics},
     journal = {Doklady Akademii Nauk},
     pages = {280--282},
     year = {1977},
     volume = {234},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1977_234_2_a4/}
}