@incollection{MSMF_1977__51-52__5_0,
     author = {Helffer, Bernard},
     title = {Hypoellipticit\'e microlocale : pr\'esentation historique},
     booktitle = {Contributions \`a l'\'etude des op\'erateurs elliptiques et hypoelliptiques},
     series = {M\'emoires de la Soci\'et\'e Math\'ematique de France},
     pages = {5--12},
     publisher = {Soci\'et\'e math\'ematique de France},
     number = {51-52},
     year = {1977},
     doi = {10.24033/msmf.232},
     mrnumber = {57 #854},
     zbl = {0373.35058},
     url = {http://www.numdam.org/articles/10.24033/msmf.232/}
}