@article{MAN_1978__235_163144,
     author = {A. Menikoff and J. Sj\"ostrand},
     title = {On the {Eigenvalue} of a {Class} of {Hypoelliptic} {Operators.}},
     journal = {Mathematische Annalen},
     pages = {55--86},
     publisher = {mathdoc},
     volume = {235},
     year = {1978},
     zbl = {0375.35014},
     url = {http://geodesic.mathdoc.fr/item/MAN_1978__235_163144/}
}