@article{JEDP_1978____A1_0,
     author = {Helffer, Bernard and Nourrigat, Jean},
     title = {Hypoellipticit\'e pour des op\'erateurs diff\'erentiels sur des groupes de {Lie} nilpotents},
     journal = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {1},
     pages = {1--1},
     publisher = {Ecole polytechnique},
     year = {1978},
     mrnumber = {58 #24395},
     language = {fr},
     url = {http://archive.numdam.org/item/JEDP_1978____A1_0/}
}