@article{SEDP_1976-1977____A18_0,
     author = {Beals, R.},
     title = {Op\'erateurs invariants hypoelliptiques sur un groupe de {Lie} nilpotent},
     journal = {S\'eminaire \'Equations aux d\'eriv\'ees partielles (Polytechnique) dit aussi "S\'eminaire Goulaouic-Schwartz"},
     note = {talk:19},
     pages = {1--8},
     publisher = {Ecole Polytechnique, Centre de Math\'ematiques},
     year = {1976-1977},
     mrnumber = {494313},
     zbl = {0359.43006},
     language = {fr},
     url = {http://archive.numdam.org/item/SEDP_1976-1977____A18_0/}
}