@article{ASNSP_1977_4_4_4_689_0, author = {Menikoff, A.}, title = {On hypoelliptic operators with double characteristics}, journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze}, pages = {689--724}, publisher = {Scuola normale superiore}, volume = {Ser. 4, 4}, number = {4}, year = {1977}, mrnumber = {473490}, zbl = {0376.35013}, language = {en}, url = {http://www.numdam.org/item/ASNSP_1977_4_4_4_689_0/} }