@article{GDZPPN002202085,
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},
volume = {346},
year = {1984},
pages = {139-152},
zbl = {0543.22007},
url = {http://dml.mathdoc.fr/item/GDZPPN002202085}
}
Corwin, Lawrence. Necessary and sufficient conditions for hypoelliptic of certain left invariant operators on nilpotent Lie groups II.. Journal für die reine und angewandte Mathematik, Tome 346 (1984) pp. 139-152. http://gdmltest.u-ga.fr/item/GDZPPN002202085/