@incollection{MSMF_1977__51-52__13_0,
     author = {Helffer, Bernard},
     title = {Sur l'hypoellipticit\'e des op\'erateurs pseudodiff\'erentiels \`a caract\'eristiques multiples (perte de 3/2 d\'eriv\'ees)},
     booktitle = {Contributions \`a l'\'etude des op\'erateurs elliptiques et hypoelliptiques},
     series = {M\'emoires de la Soci\'et\'e Math\'ematique de France},
     pages = {13--61},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {51-52},
     year = {1977},
     doi = {10.24033/msmf.233},
     mrnumber = {57 #855},
     zbl = {0374.35012},
     url = {http://archive.numdam.org/articles/10.24033/msmf.233/}
}