2017
Том 69
№ 7

All Issues

Converse theorems of the theory of approximation of functions in complex regions

Dzyadyk V. K.

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Abstract

An inequality is established for the modulus of the derivative of the algebraic polynomial $P_n(z)$ of degree $n$ to the effect that if, on an analytic arc $C$ on a piecewise-smooth boundary $C$ of a simply connected region $G$, $P_n(z)$ satisfies the condition $$|P_n(z)| \leq [\varrho_{l+1/n} (z)]^s \omega|\varrho_{l+1/n}(z)|, \quad(1)$$ where $\omega(t)$ is some modulus of continuity, $\varrho_{l+1/n}(z)$ is the distance from $z \in C$ to the $n$th line of level $C_n$ (i.e. to the line $\Phi(z) = R\left(1 + \cfrac1n\right)$, where $\Phi(z)$ is the mapping function of the outside $C$ on the outside of a unit circle, and $R$ is the conforming radius, $G$ and $s \leq 0$ then $$|P^1_n(z)| \leq A[\varrho_{l+1/n}(z)]^{s-1}\omega [\varrho_{l+1/n}(z)], A = const \quad(2)$$ After thjs an estimate is given of the continuity modulus of the rth derivative ($z$ is a whole number $\leq 0$) of the function $f(z)$ on $C$ under the condition that with each natural $n$ a polynomial $P,(z)$ can be found for it, such that $$|f(z) — P^1-n(z)| \leq [\varrho_{l+1/n}(z)]^r\omega [\varrho_{l+1/n}(z)]\quad(3)$$

Citation Example: Dzyadyk V. K. Converse theorems of the theory of approximation of functions in complex regions // Ukr. Mat. Zh. - 1963. - 15, № 4. - pp. 365-375.

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