@article{JRAM_1984__355_152690,
     author = {Lawrence Corwin},
     title = {Necessary and sufficient conditions for hypoelliptic of certain left invariant operators on nilpotent {Lie} groups {II.}},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {139--152},
     year = {1984},
     volume = {355},
     zbl = {0543.22007},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1984__355_152690/}
}