no. 4
C ∞ regularity of solutions of the Levi equation
Annales de l'I.H.P. Analyse non linéaire, Tome 15 (1998) no. 4, pp. 517-534 
@article{AIHPC_1998__15_4_517_0,
     author = {Citti, G.},
     title = {$C^\infty $ regularity of solutions of the {Levi} equation},
     journal = {Annales de l'I.H.P. Analyse non lin\'eaire},
     pages = {517--534},
     year = {1998},
     publisher = {Gauthier-Villars},
     volume = {15},
     number = {4},
     zbl = {0921.35033},
     language = {en},
     url = {https://www.numdam.org/item/AIHPC_1998__15_4_517_0/}
}
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Citti, G. $C^\infty $ regularity of solutions of the Levi equation. Annales de l'I.H.P. Analyse non linéaire, Tome 15 (1998) no. 4, pp. 517-534. https://www.numdam.org/item/AIHPC_1998__15_4_517_0/

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