@article{SM_1969_8_2_a3,
     author = {E. V. Radkevich},
     title = {Hypoelliptic operators with multiple characteristics},
     journal = {Sbornik. Mathematics},
     pages = {181--205},
     publisher = {mathdoc},
     volume = {8},
     number = {2},
     year = {1969},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1969_8_2_a3/}
}