no. 4
On analyticity in homogeneous first order partial differential equations
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 3 (1976) no. 4, pp. 719-723. 
@article{ASNSP_1976_4_3_4_719_0,
     author = {Lewy, Hans},
     title = {On analyticity in homogeneous first order partial differential equations},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {719--723},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 3},
     number = {4},
     year = {1976},
     zbl = {0335.35025},
     mrnumber = {430503},
     language = {en},
     url = {www.numdam.org/item/ASNSP_1976_4_3_4_719_0/}
}
Lewy, Hans. On analyticity in homogeneous first order partial differential equations. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 3 (1976) no. 4, pp. 719-723. http://www.numdam.org/item/ASNSP_1976_4_3_4_719_0/

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