@article{MAN_1976__221_162841,
     author = {A. Menikoff},
     title = {Some {Examples} of {Hypoelliptic} {Partial} {Differential} {Equations.}},
     journal = {Mathematische Annalen},
     pages = {167--182},
     year = {1976},
     volume = {221},
     zbl = {0323.35019},
     url = {http://geodesic.mathdoc.fr/item/MAN_1976__221_162841/}
}