@article{SEDP_1974-1975____A21_0,
     author = {Helffer, B.},
     title = {Sur une classe d'op\'erateurs pseudodiff\'erentiels hypoelliptiques \`a caract\'eristiques doubles, d'apr\`es {L.} {H\"ormander}},
     journal = {S\'eminaire \'Equations aux d\'eriv\'ees partielles (Polytechnique) dit aussi "S\'eminaire Goulaouic-Schwartz"},
     note = {talk:22},
     pages = {1--17},
     publisher = {Ecole Polytechnique, Centre de Math\'ematiques},
     year = {1974-1975},
     mrnumber = {390839},
     zbl = {0306.35033},
     language = {fr},
     url = {http://www.numdam.org/item/SEDP_1974-1975____A21_0/}
}