Abstract
We introduce the notion of \(\bar \Psi \)-integrals of 2π-periodic summable functions f, f ε L, on the basis of which the space L is decomposed into subsets (classes) \(L^{\bar \Psi } \). We obtain integral representations of deviations of the trigonometric polynomials U n(f;x;Λ) generated by a given Λ-method for summing the Fourier series of functions \(f{\text{ }}\varepsilon {\text{ }}L^{\bar \Psi } \). On the basis of these representations, the rate of convergence of the Fourier series is studied for functions belonging to the sets \(L^{\bar \Psi } \) in uniform and integral metrics. Within the framework of this approach, we find, in particular, asymptotic equalities for upper bounds of deviations of the Fourier sums on the sets \(L^{\bar \Psi } \), which give solutions of the Kolmogorov-Nikol'skii problem. We also obtain an analog of the well-known Lebesgue inequality.
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Institute of Mathematics, Ukrainian Academy of Sciences, Kiev. Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 49, No. 8, pp. 1069–1113, August, 1997.
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Stepanets, A.I. Rate of convergence of Fourier series on the classes of \(\bar \Psi \)-integrals-integrals. Ukr Math J 49, 1201–1251 (1997). https://doi.org/10.1007/BF02487549
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DOI: https://doi.org/10.1007/BF02487549