%0 Journal Article %A Lars Hörmander %T A Class of Hypoelliptic Pseudodifferential Operators with Double Characteristics. %J Mathematische Annalen %D 1975 %P 165-188 %V 217 %I mathdoc %U http://geodesic.mathdoc.fr/item/MAN_1975__217_162768/ %F MAN_1975__217_162768