@incollection{JEDP_1978____A1_0, author = {Helffer, Bernard and Nourrigat, Jean}, title = {Hypoellipticit\'e pour des op\'erateurs diff\'erentiels sur des groupes de {Lie} nilpotents}, booktitle = {}, series = {Journ\'ees \'equations aux d\'eriv\'ees partielles}, eid = {1}, pages = {1--1}, publisher = {Ecole polytechnique}, year = {1978}, mrnumber = {58 #24395}, language = {fr}, url = {http://archive.numdam.org/item/JEDP_1978____A1_0/} }