@article{DAN_1969_187_2_a10,
     author = {E. V. Radkevich},
     title = {A priori estimates and hypoelliptic operators with multiple characteristics},
     journal = {Doklady Akademii Nauk},
     pages = {274--277},
     publisher = {mathdoc},
     volume = {187},
     number = {2},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1969_187_2_a10/}
}