no. 3
C ∞ regularity of solutions of a quasilinear equation related to the Levi operator
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 23 (1996) no. 3, pp. 483-529. 
@article{ASNSP_1996_4_23_3_483_0,
     author = {Citti, G.},
     title = {$C^\infty $ regularity of solutions of a quasilinear equation related to the {Levi} operator},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {483--529},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 23},
     number = {3},
     year = {1996},
     zbl = {0872.35018},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1996_4_23_3_483_0/}
}
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Citti, G. $C^\infty $ regularity of solutions of a quasilinear equation related to the Levi operator. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 23 (1996) no. 3, pp. 483-529. http://www.numdam.org/item/ASNSP_1996_4_23_3_483_0/

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