The eigenvalues of hypoelliptic operators
Journées équations aux dérivées partielles (1977), pp. 157-163. 
@article{JEDP_1977____157_0,
     author = {Menikoff, A. and Sj\"ostrand, Johannes},
     title = {The eigenvalues of hypoelliptic operators},
     journal = {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/}
}
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Menikoff, A.; Sjöstrand, Johannes. The eigenvalues of hypoelliptic operators. Journées équations aux dérivées partielles (1977), pp. 157-163. http://archive.numdam.org/item/JEDP_1977____157_0/

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