Inverse theorem of approximation by entire functions of exponential type
O. V. Silvanovicha,
N. A. Shirokovb a Saint-Petersburg State Mining Institute
b St. Petersburg Department of Steklov Mathematical Institute of Russian Academy of Sciences
Abstract:
Let $I_k=[a_k,b_k],\ J_k=[b_k,a_{k+1}],\ b_k<a_{k+1},\ k\in\mathbb{Z}$, be segments on the real axis tending to
$+\infty$ and to
$-\infty$. Assume that these segments satisfy the conditions
$|I_k|=2^{-n\alpha}$ if
$I_k\subset [2^{n},2^{n+1}] $ or
$I_k\subset [-2^{n+1},-2^{n}] $,
$\alpha>0$ being fixed,
$n\geq n_0$. Assume also that there exists a constant
$c_1>0$ such that $ 2^{n_0}\cdot 2^{-n\alpha}\leq|J_k|\leq c_1\cdot2^{n_0}\cdot 2^{-n\alpha}$ if
$J_k\subset [2^{n},2^{n+1}]$ or
$J_k\subset [-2^{n+1},-2^{n}],k\in\mathbb{Z}$. Put
$E=\bigcup\limits_{k\in\mathbb{Z}}J_k$. Denote by
$f_{E,1}(z)$ a function subharmonic on
$\mathbb{C}$ and satisfying the conditions
$f_{E,1}(x)=0,x\in E, f_{E,1}(z)$ is harmonic on
$\mathbb{C}\setminus E$, $ \underset{z\rightarrow\infty}{\varlimsup} \dfrac{f_{E,1}(z)}{|z|}=1$, and
$g(z)\leq f_{E,1}(z),\ z\in\mathbb{C}$, for every function
$g$ subharmonic on
$\mathbb{C}$ and such that
$g(x)\leq 0$,
$x\in E$, and $ \underset{z\rightarrow\infty}{\varlimsup}\dfrac{g(z)}{|z|}\leq1.$ We define sets
$L_t(E)$ as follows:
$$L_t=\{z\in\mathbb{C}: f_{E,1}(z)=t\}$$
and put
$\rho_t(x)=\text{dist}(x,L_t(E)),\ x\in E$. Let
$T_{\sigma}$ be the set of entire functions of exponential type satisfying the condition $|F_{\sigma}(z)|\leq c_{F_{\sigma}}\exp(\sigma|\text{Im}z|),z\in\mathbb{C}, F_{\sigma}\in T_{\sigma}.$ Denote by
$\Lambda^s(E)$ the
$s$-Hölder class on
$E,\ 0<s<1,$ of functions bounded on the set
$E$.
We prove the following result.
Theorem 1.
Assume that for a function $f$ defined on $ E$ there exist functions $F_{\sigma}\in T_{\sigma}$ such that \begin{equation}{\notag} |f(x)-F_{\sigma}(x)|\leq c_f\rho^s_{\frac{1}{\sigma}}(x),\ x\in E,\ \sigma \geq 1. \end{equation}
Then $f \in \Lambda^s(E)$.
Key words and phrases:
entire functions of exponential type, Hölder classes, approximation.
UDC:
517.574 Received: 17.06.2024