@incollection{JEDP_1976____A5_0,
     author = {Pierre Bolley and Jacques Camus and Bernard Helffer},
     title = {Remarques sur l'hypoellipticit\'e},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {5},
     pages = {1--16},
     publisher = {\'Ecole polytechnique},
     year = {1976},
     mrnumber = {54 #13290},
     language = {fr},
     url = {https://proceedings.centre-mersenne.org/item/JEDP_1976____A5_0/}
}