@article{SEDP_1977-1978____A5_0,
     author = {Helffer, B. and Nourrigat, F.},
     title = {Op\'erateurs hypoelliptiques sur des groupes nilpotents},
     journal = {S\'eminaire \'Equations aux d\'eriv\'ees partielles (Polytechnique) dit aussi "S\'eminaire Goulaouic-Schwartz"},
     note = {talk:4},
     pages = {1--11},
     publisher = {Ecole Polytechnique, Centre de Math\'ematiques},
     year = {1977-1978},
     mrnumber = {504133},
     zbl = {0378.35018},
     language = {fr},
     url = {http://www.numdam.org/item/SEDP_1977-1978____A5_0/}
}