@article{MAN_1975__217_162768,
     author = {Lars H\"ormander},
     title = {A {Class} of {Hypoelliptic} {Pseudodifferential} {Operators} with {Double} {Characteristics.}},
     journal = {Mathematische Annalen},
     pages = {165--188},
     publisher = {mathdoc},
     volume = {217},
     year = {1975},
     zbl = {0306.35032},
     url = {http://geodesic.mathdoc.fr/item/MAN_1975__217_162768/}
}