@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},
     address = {Grenoble},
     volume = {26},
     number = {2},
     year = {1976},
     doi = {10.5802/aif.614},
     mrnumber = {54 #1318},
     zbl = {0301.35026},
     language = {fr},
     url = {http://archive.numdam.org/articles/10.5802/aif.614/}
}