Approximation of functions in variable-exponent Lebesgue and Sobolev spaces by de la Vall\'ee-Poussin means

I. I. Sharapudinov

Sbornik. Mathematics, Tome 207 (2016) no. 7, pp. 1010-1036

Résumé

We consider the space $L^{p(\cdot)}_{2\pi}$ formed by $2\pi$-periodic real measurable functions $f$ for which the integral $\displaystyle\int_{-\pi}^{\pi}|f(x)|^{p(x)}\,dx$ exists and is finite, where $p(x)$, $1\leqslant p(x)$, is a $2\pi$-periodic measurable function (a variable exponent). If $p(x)\leqslant \overline p\infty$, then the space $L^{p(\cdot)}_{2\pi}$ can be endowed with the structure of Banach space with the norm $$ \|f\|_{p(\cdot)}=\inf\biggl\{\alpha>0:\int_{-\pi}^{\pi}\biggl|\frac{f(x)}{\alpha}\biggr|^{p(x)}\,dx\leqslant1\biggr\}. $$ In the space $L^{p(\cdot)}_{2\pi}$ we distinguish a subspace $W^{r,p(\cdot)}_{2\pi}$ of Sobolev type. We investigate the approximation properties of the de la Vallée-Poussin means for trigonometric Fourier sums for functions in the space $W^{r,p(\cdot)}_{2\pi}$. In particular, we prove that if the variable exponent $p=p(x)$ satisfies the Dini-Lipschitz condition $|p(x)-p(y)|\ln\frac{2\pi}{|x-y|}\leqslant c$ and if $f\in W^{r,p(\cdot)}_{2\pi}$, then the de la Vallée-Poussin means $V_m^n(f)=V_m^n(f,x)$ with $n\leqslant am$ satisfy $$ \|f-V_m^n(f)\|_{p(\cdot)}\leqslant \frac{c_r(p,a)}{n^r}\Omega\biggl(f^{(r)}, \frac1n\biggr)_{p(\cdot)}, $$ where $\Omega(g,\delta)_{p(\cdot)}$ is a modulus of continuity of the function $g\in L^{p(\cdot)}_{2\pi}$ defined in terms of the Steklov functions. It is proved that if $1$, $r\geqslant1$, $f\in W^{r,p(\cdot)}_{2\pi}$ and the Dini-Lipschitz condition holds, then $$ |f(x)-V_m^n(f,x)|\leqslant\frac{c_r(p)}{m+1}\sum_{k=n}^{n+m}\frac{E_k(f^{(r)})_{p(\cdot)}}{(k+1)^{r-{{1}/{p(x)}}}}, $$ where $E_k(g)_{p(\cdot)}$ stands for the best approximation to $g\in L^{p(\cdot)}_{2\pi}$ by trigonometric polynomials of order $k$. Bibliography: 19 titles.

Détail

Keywords: Lebesgue and Sobolev spaces with variable exponents, approximation of functions by de la Vallée-Poussin means.