@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},
     publisher = {Gauthier-Villars},
     volume = {15},
     number = {4},
     year = {1998},
     zbl = {0921.35033},
     language = {en},
     url = {http://www.numdam.org/item/AIHPC_1998__15_4_517_0/}
}