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2. A. Zygmund, Trigonometric Series [Russian translation], Vol. 1, Mir, Moscow (1965).
3. A. I. Stepanets, Classes of Periodic Functions and Approximation of Their Elements by Fourier Sums [in Russian], Preprint No. 83.10, Institute of Mathematics, Ukrainian Academy of Sciences, Kiev (1983).
4. A. I. Stepanets, “Deviations of Fourier sums on classes of infinitely differentiable functions,” Ukr. Mat. Zh., 36, No. 6, 750–758 (1984).
5. A. I. Stepanets, Classification and Approximation of Periodic Functions [in Russian], Naukova Dumka, Kiev (1987).