@incollection{AST_1974__19__79_0,
     author = {Helffer, Bernard},
     title = {Sur l'hypoellipticit\'e d'une classe d'op\'erateurs paraboliques d\'eg\'en\'er\'es},
     booktitle = {Sur quelques \'equations aux d\'eriv\'ees partielles singuli\`eres},
     series = {Ast\'erisque},
     pages = {79--105},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {19},
     year = {1974},
     mrnumber = {364843},
     zbl = {0291.35014},
     language = {fr},
     url = {http://archive.numdam.org/item/AST_1974__19__79_0/}
}