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2. M. I. D’yachenko, “Trigonometric Series with Generalized-Monotone Coefficients,” Izv. Vyssh. Uchebn. Zaved. Mat., No. 7, 39–50 (1986) (Soviet Mathematics (Iz. VUZ) 30 (7) 37–48 (1986).
3. A. F. Andersen, “Comparison Theorems in the Theory of Cesaro Summability,” Proc. London Math. Soc. Ser. 2 27(1), 39–71, (1927).
4. N. K. Bari, Trigonometric Series, (Fizmatlit, Moscow, 1961) [in Russian].