no. 1
Integral representation of solutions of first-order linear partial differential equations, I
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 3 (1976) no. 1, pp. 1-35. 
@article{ASNSP_1976_4_3_1_1_0,
     author = {Treves, Fran\c cois},
     title = {Integral representation of solutions of first-order linear partial differential equations, I},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {1--35},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 3},
     number = {1},
     year = {1976},
     mrnumber = {399635},
     language = {en},
     url = {www.numdam.org/item/ASNSP_1976_4_3_1_1_0/}
}
Treves, François. Integral representation of solutions of first-order linear partial differential equations, I. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 3 (1976) no. 1, pp. 1-35. http://www.numdam.org/item/ASNSP_1976_4_3_1_1_0/

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