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JOURNALS // Zapiski Nauchnykh Seminarov POMI // Archive

Zap. Nauchn. Sem. POMI, 2024 Volume 537, Pages 151–177 (Mi znsl7522)

Inverse theorem of polynomial approximation on an elliptic curve

M. A. Shagay

National Research University Higher School of Economics, St. Petersburg School of Economics and Management

Abstract: Let $\wp(z)$ be a doubly periodic Weierstrass function with periods $2\omega_1$, $2\omega_2$, let $Q$ be a parallelogram with vertexes $0, 2\omega_1, 2\omega_2, 2(\omega_1+\omega_2)$, and let $s_k$, $1\leq k\leq m$, be pairwise disjoint segments, $s_k=[a_k,b_k]\subset Q$, $1\leq k\leq m$. We choose numbers $\varepsilon_{kn}>0$ satisfying the condition $\sum\limits_{k=1}^m \overset{\infty} {\underset{n=1}{\sum}}\varepsilon^2_{kn}<\infty$. We denote by $g(z)$ the Green functions of the region $\mathbb{C}\setminus \bigcup\limits_{k=1}^{m} s_k$ with the logarithmic pole at $\infty$ and put $L_h=\{z\in Q\setminus \bigcup\limits_{k=1}^{m} s_k: g(z)=h\}$, $0<h<{\underset{z\in\overline{Q}}{\max g(z)}}, \rho_h(z)={\rm dist}(z,L_h)$. Let $T(z)=(\wp(z),\wp'(z)), z\in Q$,
\begin{equation*} d_{kn}(z)=1+\dfrac{1}{2^n\sqrt{\delta(T(z),T(a_k))\cdot\delta(T(z),T(b_k))}}, z\in s_k, \end{equation*}

\begin{equation*} \delta((\zeta,w),(\zeta',w'))=\sqrt{|\zeta-\zeta'|^2+|w-w'|^2} .\end{equation*}

We prove the following claim.
Theorem 1$'$. Suppose that $2\leq p_k<\infty, 1\leq k\leq m, f_k\in C(s_k)$. Assume that there exist polynomials $\mathsf{P}_{2^n}(u,v), \deg\mathsf{P}_{2^n}\leq 2^n$, and a constant $C_*$ such that for $n=1,2,...$ one has the estimate
\begin{equation*} \sum\limits_{k=1}^m \int\limits_{s_k}\displaystyle\left|\frac{f_k(z) -\mathsf{P}_{2^n}(\wp(z),\wp'(z))}{\varepsilon_{kn}\rho_{2^{-n}}(z)}\right|^{p_k}d_{kn}(z)|dz|\leq C_{*}. \end{equation*}
Then $f_k'(z)\in L^{p_k}(s_k), 1\leq k\leq m$.

Key words and phrases: doubly periodic Weierstrass functions, approximation in the mean, inverse theorems.

UDC: 517.547

Received: 16.06.2024



© Steklov Math. Inst. of RAS, 2025