@article{JEDP_1977____88_0,
     author = {Helffer, Bernard},
     title = {Quelques exemples d'op\'erateurs pseudodiff\'erentiels localement r\'esolubles},
     journal = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     pages = {88--116},
     publisher = {Ecole polytechnique},
     year = {1977},
     mrnumber = {81g:58035},
     zbl = {0388.47031},
     language = {fr},
     url = {http://www.numdam.org/item/JEDP_1977____88_0/}
}