@incollection{JEDP_1977____157_0,
     author = {Menikoff, A. and Sj\"ostrand, Johannes},
     title = {The eigenvalues of hypoelliptic operators},
     booktitle = {},
     series = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     pages = {157--163},
     publisher = {Ecole polytechnique},
     year = {1977},
     language = {en},
     url = {http://archive.numdam.org/item/JEDP_1977____157_0/}
}