@incollection{JEDP_1977____16_0, author = {Bolley, Pierre and Camus, Jacques and Helffer, Bernard}, title = {Hypoellipticit\'e pour une \'equation d'\'evolution abstraite du second ordre}, booktitle = {}, series = {Journ\'ees \'equations aux d\'eriv\'ees partielles}, pages = {16--32}, publisher = {Ecole polytechnique}, year = {1977}, mrnumber = {80f:35031}, language = {fr}, url = {http://archive.numdam.org/item/JEDP_1977____16_0/} }