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JOURNALS // Izvestiya Vysshikh Uchebnykh Zavedenii. Matematika // Archive

Izv. Vyssh. Uchebn. Zaved. Mat., 2020 Number 6, Pages 65–72 (Mi ivm9583)

This article is cited in 5 papers

Approximation of functions of a complex variable by Fourier sums in orthogonal systems in $L_2$

M. Sh. Shabozova, M. S. Saidusaynovb

a Tajik National University, Dushanbe, 734025 Republic of Tajikistan
b University of Central Asia, Dushanbe, SPCE, 734013 Republic of Tajikistan

Abstract: The sharp inequalities of Jackson-Stechkin type inequalities between the best approximation $E_{n-s-1}(f^{(s)}) (s=\overline{0,r}, r\in\mathbb{N})$ of successive derivatives $f^{(s)} (s=\overline{0,r}, r\in\mathbb{N})$ of analytic functions $f\in L_{2}(U)$ in the disk $U:=\left\{z: |z|<1\right\}$ as for special module of continuity $\Omega_{m}$ of $m$th order satisfying the condition
$$\Omega_{m}\left(f^{(r)},t\right)_{2}\leq\Phi(t), 0<t<1,$$
where $\Phi$ is give majorant and also for Peetre $\mathscr{K}$-functional satisfying the constraint
$$\mathscr{K}_{m}\left(f^{(r)},t^{m}\right)\leq\Phi(t^{m}), 0<t<1,$$
were obtained.

Keywords: the generalized module of continuity, generalized translation operator, orthonormal system of functions, Jackson–Stechkin inequality, $\mathscr{K}$-functional.

UDC: 517.5

Received: 25.06.2019
Revised: 31.07.2019
Accepted: 25.09.2019

DOI: 10.26907/0021-3446-2020-6-65-72


 English version:
Russian Mathematics (Izvestiya VUZ. Matematika), 2020, 64:6, 56–62

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© Steklov Math. Inst. of RAS, 2025