@article{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},
     journal = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     pages = {16--32},
     publisher = {Ecole polytechnique},
     year = {1977},
     mrnumber = {80f:35031},
     language = {fr},
     url = {http://www.numdam.org/item/JEDP_1977____16_0/}
}