@article{SEDP_1978-1979____A5_0,
     author = {Helffer, B. and Nourrigat, J.},
     title = {Op\'erateurs diff\'erentiels hypoelliptiques et groupes nilpotents},
     journal = {S\'eminaire \'Equations aux d\'eriv\'ees partielles (Polytechnique) dit aussi "S\'eminaire Goulaouic-Schwartz"},
     note = {talk:5},
     pages = {1--16},
     publisher = {Ecole Polytechnique, Centre de Math\'ematiques},
     year = {1978-1979},
     mrnumber = {557516},
     zbl = {0411.35028},
     language = {fr},
     url = {http://www.numdam.org/item/SEDP_1978-1979____A5_0/}
}