We establish the exact-order estimates for the best orthogonal trigonometric approximations of the Nikol’skii–Besov classes \( {B}_{1,\theta}^r \)(\( \mathbbm{T} \)d), 1 ≤ θ ≤ ∞, of periodic functions of one and many variables with predominant mixed derivative in the space Bθ, 1 (\( \mathbbm{T} \)d). In the multidimensional case, d ≥ 2, we establish the exact-order estimates for the approximations of the indicated classes of functions by their step-hyperbolic Fourier sums and determine the orders of orthoprojection widths in the same space. The behaviors of the corresponding approximation characteristics of the Sobolev classes \( {W}_{1,\alpha}^r \)(\( \mathbbm{T} \)d) with d ∈ {1, 2} are also investigated.
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Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 74, No. 6, pp. 844–855, June, 2022. Ukrainian DOI: https://doi.org/10.37863/umzh.v74i6.7141.
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Romanyuk, A.S., Yanchenko, S.Y. Approximation of the Classes of Periodic Functions of One and Many Variables from the Nikol’skii–Besov and Sobolev Spaces. Ukr Math J 74, 967–980 (2022). https://doi.org/10.1007/s11253-022-02110-5
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DOI: https://doi.org/10.1007/s11253-022-02110-5