@article{AIF_1976__26_2_55_0, author = {Helffer, Bernard}, title = {Invariants associ\'es \`a une classe d'op\'erateurs pseudodiff\'erentiels et applications \`a l'hypoellipticit\'e}, journal = {Annales de l'Institut Fourier}, pages = {55--70}, publisher = {Institut Fourier}, volume = {26}, number = {2}, year = {1976}, doi = {10.5802/aif.614}, zbl = {0301.35026}, mrnumber = {54 #1318}, language = {fr}, url = {http://www.numdam.org/articles/10.5802/aif.614/} }