@article{JEDP_1979____A10_0,
     author = {Helffer, Bernard and Nourrigat, Jean},
     title = {Hypoellipticit\'e pour des op\'erateurs quasi-homog\`enes \`a coefficients polynomiaux},
     journal = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {10},
     pages = {1--5},
     publisher = {Ecole polytechnique},
     year = {1979},
     mrnumber = {83c:35026},
     zbl = {0405.35024},
     language = {fr},
     url = {http://archive.numdam.org/item/JEDP_1979____A10_0/}
}