@incollection{AST_1973__2-3__152_0,
     author = {Egorov, Ju. V.},
     title = {Op\'erateurs subelliptiques},
     booktitle = {Colloque International C.N.R.S. sur les \'equations aux d\'eriv\'ees partielles lin\'eaires},
     series = {Ast\'erisque},
     pages = {152--159},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {2-3},
     year = {1973},
     zbl = {0266.35059},
     mrnumber = {404838},
     language = {fr},
     url = {http://www.numdam.org/item/AST_1973__2-3__152_0/}
}