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Izvestiya Mathematics
Article . 2002 . Peer-reviewed
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On the order of the best approximation in spaces with asymmetric norm and sign-sensitive weight on classes of differentiable functions

On the order of the best approximation in spaces with asymmetric norm and sign-sensitive weight on classes of differentiable functions.
Authors: Kozko, A. I.;

On the order of the best approximation in spaces with asymmetric norm and sign-sensitive weight on classes of differentiable functions

Abstract

Let \(\mathbb{T}\) be the one dimensional torus represented by the interval \([-\pi,\pi]\) with the end points \(-\pi\) and \(\pi\) identified. For \(\mathbf{p}=(p_1,p_2)\) (\(1\leq p_1,p_2\leq +\infty\)), \(\mathbf{\rho}=(\rho_+,\rho_-)\) (a pair of nonnegative functions which may assume infinite values) and \(\psi(u,v)\) (an arbitrary asymmetric and monotone function, such that \(\Psi(u,v)\) is a norm on the plane), the author considers the asymmetric norm defined by \[ \psi_{\mathbf{\rho,p}}(f)=\psi\big(\| \rho_+(\cdot)f^{+}(\cdot)\| _{L_{p_1}(\mathbb{T})}, \| \rho_-(\cdot)f^{-}(\cdot)\| _{L_{p_2}(\mathbb{T})}\big), \] where \(f^+=\max\{f,0\}\) and \(f^-=\max\{-f,0\}\). Let \(L_{\psi_{\mathbf{\rho,p}}}(\mathbb{T})\) be the cone of the \(2\pi\)-periodic Lebesgue measurable functions \(f\) on \(\mathbb{T}\) such that \(\psi_{\mathbf{\rho,p}}(f)<+\infty\) and denote by \(\mathcal{T}_n\), \(n\in\mathbb{N}\), the set of all trigonometric polynomials of degree at most \(n\). For \(r\in\mathbb{N}\), let \(\text{BW}_{\psi_{\mathbf{\rho,p}}}^r(\mathbb{T})\) be the class of all functions \(f\) that have \((r-1)\)th derivative which is absolutely continuous on \(\mathbb{T}\) and \(f^{(r)}\in L_{\psi_{\mathbf{\rho,p}}}(\mathbb{T})\). The main theorem of this paper states that if \(\mathbf{p}=(p_1,p_2)\), \(\mathbf{q}=(q_1,q_2)\), with \(1\leq p_1,p_2,q_1,q_2\leq \infty\), \(\rho=(\rho_+,\rho_-)\), \(\widetilde{\rho}=(\widetilde{\rho}_+,\widetilde{\rho}_-)\) are two pairs of nonnegative functions and \(\psi\) is an asymmetric and monotone norm on the plane such that \(\psi(u,v)=\psi(v,u)\), then \[ \begin{aligned} E_n\Big(\text{BW}_{\psi_{\mathbf{\rho,p}}}^r(\mathbb{T}), L_{\psi_{\mathbf{\widetilde{\rho},q}}}(\mathbb{T})\Big):= &\sup_{f\in\text{BW}_{\psi_{\mathbf{\rho,p}}}^r(\mathbb{T})} E_n\Big(f,L_{\psi_{\mathbf{\widetilde{\rho},q}}}(\mathbb{T})\Big):= \\ &\sup_{f\in\text{BW}_{\psi_{\mathbf{\rho,p}}}^r(\mathbb{T})} \inf_{t\in\mathcal{T}_n}\psi_{\mathbf{\widetilde{\rho},q}}(f-t) \leq 8K_{r}(n+1)^{-r}C_\mathbf{\rho,\widetilde{\rho},p,q}(n).\end{aligned} \] where \(K_r\) is the so-called Favard constant and \(C_\mathbf{\rho,\widetilde{\rho},p,q}(n)\) are constants. Also, in some cases of asymmetric norms with weights \(\rho=(\alpha,\beta)\), where \(\alpha\) and \(\beta\) are real numbers in \([1,+\infty)\), the author finds the rate of decrease of \(E_n(\text{BW}_{\psi_{\mathbf{\rho,p}}}^r(\mathbb{T}), L_{\psi_{\mathbf{\widetilde{\rho},q}}}(\mathbb{T}))\) as \(n\to\infty\) for a fixed number \(r\in\mathbb{N}\).

Related Organizations
Keywords

Trigonometric approximation, trigonometric polynomials, trigonometric approximation, Rate of convergence, degree of approximation, Completeness of sets of functions in one variable harmonic analysis, Abstract approximation theory (approximation in normed linear spaces and other abstract spaces), Approximation with constraints, degree of approximation, asymmetric norms, best approximation, rate of convergence

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
2
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