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This article is cited in 18 scientific papers (total in 18 papers)
Sharp Jackson–Stechkin inequality in $L^2$ for multidimensional spheres
A. G. Babenko Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences
Abstract:
In this paper we prove the Jackson–Stechkin inequality
$$
E_{n-1}(f)<\omega_r(f,2\tau_{n,\lambda}), \qquad n\ge1, \quad m\ge5, \quad r\ge1,
$$
$f\in L^2(\mathbb S^{m-1})$, $f\not\equiv\textrm{const}$, which is sharp for each $n=2,3,\dots$; here $E_{n-1}(f)$ is the best approximation of a function $f$ by spherical polynomials of degree $\le n-1$, $\omega_r(f,\tau)$ is the $r$th modulus of continuity of $f$ based on the translations
$$
s_tf(x)=\frac 1{|\mathbb S^{m-2}|}\int_{\mathbb S^{m-2}}f(x\cos t+\xi\sin t)\,d\xi, \qquad
t\in\mathbb R, \quad x\in\mathbb S^{m-1},
$$
$\mathbb S^{m-2}=\mathbb S^{m-2}_x=\bigl\{\xi\in \mathbb S^{m-1}:x\cdot\xi=0\bigr\}$,
$|\mathbb S^{m-2}|$ is the measure of the unit Euclidean sphere $\mathbb S^{m-2}$,
$\lambda=(m-2)/2$ and $\tau_{n,\lambda}$ is the first positive zero of the Gegenbauer cosine polynomial $C^\lambda_n(\cos t)$ .
Received: 04.04.1994 Revised: 18.06.1996
Citation:
A. G. Babenko, “Sharp Jackson–Stechkin inequality in $L^2$ for multidimensional spheres”, Mat. Zametki, 60:3 (1996), 333–355; Math. Notes, 60:3 (1996), 248–263
Linking options:
https://www.mathnet.ru/eng/mzm1834https://doi.org/10.4213/mzm1834 https://www.mathnet.ru/eng/mzm/v60/i3/p333
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