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JOURNALS // Trudy Instituta Matematiki i Mekhaniki UrO RAN // Archive

Trudy Inst. Mat. i Mekh. UrO RAN, 2017 Volume 23, Number 3, Pages 292–299 (Mi timm1459)

Uniform Lebesgue constants of local spline approximation

V. T. Shevaldin

Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences, Ekaterinburg

Abstract: Let a function $\varphi\in C^1[-h,h]$ be such that $\varphi(0)=\varphi'(0)=0$, $\varphi(-x)=\varphi(x)$ for $x\in [0;h])$, and $\varphi(x)$ is nondecreasing on $[0;h]$. For any function $f:\ \mathbb R\to \mathbb R$, we consider local splines of the form
$$S(x)=S_{\varphi}(f,x)=\sum_{j\in \mathbb Z} y_j B_{\varphi}\Big( x+\frac{3h}{2}-jh\Big)\quad (x\in \mathbb R),$$
where $y_j=f(jh)$, $m(h)>0$, and
$$B_{\varphi}(x)=m(h)\left\{
\begin{array}{cl}\varphi(x),& x\in [0;h],\\ 2\varphi(h)-\varphi(x-h)-\varphi(2h-x),& x\in [h;2h], \\ \varphi(3h-x),& x\in [2h;3h],\\ 0, & x\not\in [0;3h]. \end{array}
\right.$$
These splines become parabolic, exponential, trigonometric, etc., under the corresponding choice of the function $\varphi$. We study the uniform Lebesgue constants $L_{\varphi}=\|S\|_C^C$ (the norms of linear operators from $C$ to $C$) of these splines as functions depending on $\varphi$ and $h$. In some cases, the constants are calculated exactly on the axis $\mathbb R$ and on a closed interval of the real line (under a certain choice of boundary conditions from the spline $S_{\varphi}(f,x)$).

Keywords: Lebesgue constants, local splines, three-point system.

UDC: 519.65

MSC: 41A15

Received: 02.06.2017

DOI: 10.21538/0134-4889-2017-23-3-292-299


 English version:
Proceedings of the Steklov Institute of Mathematics (Supplement Issues), 2018, 303, suppl. 1, S196–S202

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