On functions of rectangles and their application to analytic functions
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 2, Volume 3 (1934) no. 1, p. 27-32 
@article{ASNSP_1934_2_3_1_27_0,
     author = {Saks, Stanislaw and Zygmund, Antoni},
     title = {On functions of rectangles and their application to analytic functions},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     publisher = {Scuola normale superiore},
     volume = {Ser. 2, 3},
     number = {1},
     year = {1934},
     pages = {27-32},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1934_2_3_1_27_0}
}

Saks, Stanislaw; Zygmund, Antoni. On functions of rectangles and their application to analytic functions. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 2, Volume 3 (1934) no. 1, pp. 27-32. http://www.numdam.org/item/ASNSP_1934_2_3_1_27_0/

A.S. Besicovitch [1], On sufficient conditions for a function to be analytic, and on behaviour of analytic functions in the neighbourhood of non isolated singular points, Proc. of the London Mathem. Soc., 32 (1930), pp. 1-10. | JFM 56.0272.01

H. Hahn [1], Theorie der reellen Funktionen, Erste Auflage, pp. VIII + 600, Berlin, 1921. | JFM 48.0261.09

F. Hausdorff [1], Dimension und äußeres Maß, Math. Annalen, 79 (1919), pp. 157-179. | JFM 46.0292.01

C. De La Vallée-Poussin [1], Intégrales de Lebesgue, fonctions d'ensemble, classes de Baire, pp. VIII + 154, Paris, 1916. | JFM 46.1519.01

L. Tonelli [1], Sul teorema di Green, Atti Accad. naz. Lincei (6), 11 (1925), pp. 482-488. | JFM 51.0239.02