@article{DAN_1976_229_6_a6,
     author = {Yu. V. Egorov},
     title = {On solvability conditions for differential equations with simple characteristics},
     journal = {Doklady Akademii Nauk},
     pages = {1310--1312},
     publisher = {mathdoc},
     volume = {229},
     number = {6},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1976_229_6_a6/}
}