no. 3
A priori estimates for hypoelliptic differential equations in a half-space
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 3, Tome 22 (1968) no. 3, pp. 409-424. 
@article{ASNSP_1968_3_22_3_409_0,
     author = {Arkeryd, Leif},
     title = {A priori estimates for hypoelliptic differential equations in a half-space},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {409--424},
     publisher = {Scuola normale superiore},
     volume = {Ser. 3, 22},
     number = {3},
     year = {1968},
     zbl = {0162.41002},
     mrnumber = {236518},
     language = {en},
     url = {archive.numdam.org/item/ASNSP_1968_3_22_3_409_0/}
}
Arkeryd, Leif. A priori estimates for hypoelliptic differential equations in a half-space. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 3, Tome 22 (1968) no. 3, pp. 409-424. http://archive.numdam.org/item/ASNSP_1968_3_22_3_409_0/

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