no. 1
Fractional powers of elliptic differential operators
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 3, Tome 22 (1968) no. 1, pp. 113-132. 
@article{ASNSP_1968_3_22_1_113_0,
     author = {Burak, Tamar},
     title = {Fractional powers of elliptic differential operators},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {113--132},
     publisher = {Scuola normale superiore},
     volume = {Ser. 3, 22},
     number = {1},
     year = {1968},
     zbl = {0159.15601},
     mrnumber = {236767},
     language = {en},
     url = {archive.numdam.org/item/ASNSP_1968_3_22_1_113_0/}
}
Burak, Tamar. Fractional powers of elliptic differential operators. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 3, Tome 22 (1968) no. 1, pp. 113-132. http://archive.numdam.org/item/ASNSP_1968_3_22_1_113_0/

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