A class of pseudo differential operators with multiple non-involutive characteristics
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Volume 8 (1981) no. 4, p. 575-603 
@article{ASNSP_1981_4_8_4_575_0,
     author = {Mascarello Rodino, Maria and Rodino, Luigi},
     title = {A class of pseudo differential operators with multiple non-involutive characteristics},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 8},
     number = {4},
     year = {1981},
     pages = {575-603},
     zbl = {0441.35069},
     mrnumber = {656001},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1981_4_8_4_575_0}
}

Mascarello Rodino, Maria; Rodino, Luigi. A class of pseudo differential operators with multiple non-involutive characteristics. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Volume 8 (1981) no. 4, pp. 575-603. http://www.numdam.org/item/ASNSP_1981_4_8_4_575_0/

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