Estimates for Approximations by Fourier Sums, Best Approximations and Best Orthogonal Trigonometric Approximations of the Classes of (ψ, β) - Differentiable Functions

Bulletin de la Société des Science et des Lettres de Łódź. Serie: Recherches sur les Déformations, Tom 66, Numer 2 (2016) s. 35-43
Anatoly S. Serdyuk, Tetiana A. Stepanyuk