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