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. 
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     author = {Lewy, Hans},
     title = {On analyticity in homogeneous first order partial differential equations},
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     publisher = {Scuola normale superiore},
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     number = {4},
     year = {1976},
     mrnumber = {430503},
     zbl = {0335.35025},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1976_4_3_4_719_0/}
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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/

[1] W. Rudin, Lectures on the edge-of-the wedge Theorem, Regional Conference Series in Mathematics 1971, p. 10. | MR | Zbl

[2] M. Sato - T. Kawai - N. Kashiwara, Hyperfunctions and Pseudodifferential Equations, Lectures Notes 287 (Springer 1973), p. 265 ff. | MR | Zbl