no. 4
On hypoelliptic operators with double characteristics
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 4 (1977) no. 4, pp. 689-724. 
@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},
     zbl = {0376.35013},
     mrnumber = {473490},
     language = {en},
     url = {archive.numdam.org/item/ASNSP_1977_4_4_4_689_0/}
}
Menikoff, A. On hypoelliptic operators with double characteristics. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 4 (1977) no. 4, pp. 689-724. http://archive.numdam.org/item/ASNSP_1977_4_4_4_689_0/

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