Levitan, B. M. Outline of the history of the theory of almost periodic functions. (Russian) Zbl 0468.01007 Istor.-Mat. Issled. 25, 156-166 (1980). MSC: 01A60 42A75 43A60 42-03 43-03 PDFBibTeX XMLCite \textit{B. M. Levitan}, Istor.-Mat. Issled. 25, 156--166 (1980; Zbl 0468.01007)
Paplauskas, A. B. Fragen der Summierbarkeit und der Divergenz trigonometrischer Reihen. (Russian) Zbl 0135.00406 Istor.-Mat. Issled. 15, 129-170 (1963). MSC: 01A05 42-03 42A24 PDFBibTeX XMLCite \textit{A. B. Paplauskas}, Istor.-Mat. Issled. 15, 129--170 (1963; Zbl 0135.00406)