@phdthesis{BJHTUP11_1976__0034__P0_0,
     author = {Helffer, Bernard},
     title = {Hypoellipticit\'e pour des classes d'op\'erateurs pseudodiff\'erentiels \`a caract\'eristiques multiples},
     series = {Th\`eses d'Orsay},
     publisher = {Universit\'e de Paris-Sud Centre d'Orsay},
     number = {34},
     year = {1976},
     language = {fr},
     url = {http://www.numdam.org/item/BJHTUP11_1976__0034__P0_0/}
}