@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://archive.numdam.org/item/JEDP_1974____A3_0/} }