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Sharp Kolmogorov-type inequalities for moduli of continuity and best approximations by trigonometric polynomials and splines
O. L. Vinogradov,
V. V. Zhuk Saint-Petersburg State University
Abstract:
In what follows
$C$ is the space of
$2\pi$-periodic continuous functions;
$P$ is a seminorm defined on
$C$, shift-invariant, and majorized by the uniform norm;
$\omega_m(f, h)_P$ is the
$m$th modulus of continuity of a function
$f$ with step
$h$ and calculated with respect to
$P$; $\mathscr K_r=\frac4\pi\sum\limits^{\infty}_{l=0}\frac{(-1)^{l(r+1)}}{(2l+1)^{r+1}}$, $B_r(x)=-\frac{r!}{2^{r-1}\pi^r}\sum\limits^{\infty}_{k-1}\frac{\cos(2k\pi x-r\pi/2)}{k^r}$
$(r\in\mathbb N)$,
$B_0(x)=1$,
$\gamma_r=\frac{B_r(\frac12)}{r!}$;
$(k)=k_1+\cdots+k_m$,
\begin{gather*}
K_{r,m}=\{k\in\mathbb Z^m_+:0\le k_{\nu}\le r+\nu-2-k_1-\dots-k_{\nu-1}\},
\\
A_{r,0}=\frac2{r!}\int^{1/2}_0\left|B_r(t)-B_r\left(\frac12\right)\right|\,dt,
\\
A_{r, m}=\sum_{k\in K_{r,m}}\left(\prod^m_{j=1}|\gamma_{k_j}|\right)A_{r+m-(k), 0}, \quad \Sigma_{r, m}=\sum^{m-1}_{\nu=0}2^{\nu}A_{r,\nu},
\\
M_{r, m}(f, h)_P=\begin{cases}
\Sigma^{-1}_{r,m}\sum\limits^{m-1}_{\nu=0}A_{r,\nu}\omega_{\nu}(f,h)_P,&\text{</nomathmode><mathmode>
$r$ is even},
\Sigma^{-1}_{r, m}(\dfrac{A_{r, 0}}2\omega_1(f, h)_P+\sum\limits^{m-1}_{\nu=1}A_{r,\nu}\omega_{\nu}(f,h)_P),&\text{
$r$ is odd}.
\end{cases}
\end{gather*}
</mathmode><nomathmode>
Theorem 1. \textit{Let
$r,m\in\mathbb N$,
$n,\lambda>0$,
$f\in C^{(r+m)}$. Then}
$$
\begin{gathered}
P(f^{(m)})\le\lambda^r\left\{\Sigma_{r, m}+2^m\sum_{k\in K_{r, m}}\left(\prod^m_{j=1}|\gamma_{k_j}|\right)\frac{\mathscr K_{r+m-(k)}}{\lambda^{r+m-(k)}}\right\}
\\
\times\max\left\{\left(\frac{\omega_m(f,\tfrac{\lambda}n)_P}{\mathscr K_{r+m}2^m}\right)^{\frac r{r+m}}M^{\frac m{r+m}}_{r, m},\left(f^{(r+m)},\frac{\lambda}n\right), \frac{n^m\omega_m(f,\frac{\lambda}n)_P}{\mathscr K_{r+m}2^m}\right\}.
\end{gathered}
$$
For some values of
$\lambda$ and seminorms related to best approximations by trigonometric polynomials and splines in the uniform and integral metrics, the inequalities are sharp.
UDC:
517.5
Received: 22.10.2002