@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/}
}