@incollection{JEDP_1974____A3_0,
     author = {Bolley, Pierre and Camus, Jacques and Helffer, Bernard},
     title = {Op\'erateurs pseudo-diff\'erentiels \`a valeurs vectorielles. {Application} \`a l'\'etude de l'hypoellipticit\'e de certains op\'erateurs},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {3},
     pages = {1--9},
     publisher = {\'Ecole polytechnique},
     year = {1974},
     language = {fr},
     url = {http://geodesic.mathdoc.fr/item/JEDP_1974____A3_0/}
}