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