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