@article{DAN_1971_198_6_a1,
     author = {Yu. V. Egorov},
     title = {On the structure of subelliptic operators},
     journal = {Doklady Akademii Nauk},
     pages = {1259--1262},
     year = {1971},
     volume = {198},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1971_198_6_a1/}
}