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JOURNALS // Zapiski Nauchnykh Seminarov POMI // Archive

Zap. Nauchn. Sem. POMI, 2023 Volume 527, Pages 242–255 (Mi znsl7398)

Polynomial approximation by doubly periodic Weierstrass functions on disjoint segments in the $L^P$ metric

M. A. Shagaya, N. A. Shirokovb

a National Research University Higher School of Economics, St. Petersburg School of Economics and Management
b St. Petersburg Department of Steklov Mathematical Institute of Russian Academy of Sciences

Abstract: Let $s_k, 1\leqslant k\leqslant m, m\geqslant 2$, be disjoint segments lying in a parallelogram $Q$. We denote by $\wp(z)$ a doubly periodic Weierstrass function with the fundamental parallelogram $Q$. Let $f_k:s_k\rightarrow\mathbb{C}$ be functions, and let $f_k'\in L^{p_k}(s_k), 1\leqslant k\leqslant m, 1<p_k<\infty$.
Consider the Green function $G(z)$ of the domain $\mathbb{C}\backslash\overset{m}{\underset{k=1}{\cup}} s_k$ with the pole at infinity and define
$$ L_h\stackrel{\rm def}{=} \{\ \zeta: \zeta\in\mathbb{C}\backslash\overset{m}{\underset{k=1}{\cup}} s_k, G(\zeta)=\log(1+h) \}, h>0; \rho_h(\zeta)\stackrel{\rm def}{=} \mathrm{dist}(\zeta,L_h). $$
Theorem. There exist polynomials $P_n(u,v), \deg P_n\leqslant n, n=1,2,\cdots$, such that
$$ \overset{m}{\underset{k=1}{\sum}}{\underset{s_k}{\int}}\left|\frac{f_k(\zeta)-P_n(\wp(\zeta),\wp'(\zeta))}{\rho_{\frac1n}(\zeta)}\right|^{p_k}|d\zeta|\leqslant c. $$


Key words and phrases: Weierstrass doubly periodic functions, approximation, polynomials.

UDC: 517.547

Received: 23.09.2023



© Steklov Math. Inst. of RAS, 2024