@article{JEDP_1980____A2_0,
     author = {Helffer, Bernard and Nourrigat, Jean},
     title = {Hypoellipticit\'e maximale pour des op\'erateurs polyn\^omes de champs de vecteurs},
     journal = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {2},
     pages = {1--2},
     publisher = {Ecole polytechnique},
     year = {1980},
     zbl = {0454.58019},
     language = {fr},
     url = {http://www.numdam.org/item/JEDP_1980____A2_0/}
}