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JOURNALS // Matematicheskii Sbornik // Archive

Mat. Sb., 1992 Volume 183, Number 6, Pages 97–110 (Mi sm1048)

This article is cited in 1 paper

$L_p$ extensions of Gonchar's inequality for rational functions

A. L. Levin, E. B. Saff


Abstract: Given a condenser $~(E,\, F)$ in the complex plane, let $~C(E,\, F)$ denote its capacity and let $~\mu^*=\mu_E^*-\mu_F^*$ be the (signed) equilibrium distribution for $~(E,\, F)$. Given a finite positive measure $\mu$ on $E\cup F$, let
$$ G(\mu_E')=\exp\biggl(\,\int\log(d\mu/d\mu_E^*)\,d\mu_E^*\biggr),\quad G(\mu_F')=\exp\biggr(\,\int\log(d\mu/d\mu_F^*)\,d\mu_F^*\biggr). $$
We show that for $0<p,q<\infty$ and for any rational function $r_n$ of order $n$
\begin{equation} \|r_n\|_{L_p(d\mu,E)}\|1/r_n\|_{L_q(d\mu,F)}\geqslant e^{-n/C(E,F)}G^{1/p}(\mu_E') G^{1/q}(\mu_E'), \tag{1} \end{equation}
which extends a classical result due to A. A. Gonchar. For a symmetric condenser we also obtain a sharp lower bound for $\|r_n-\lambda\|_{L_p(d\mu,\,E\cup F)}$, where $\lambda=\lambda(z)$ is equal to $0$ on $E$ and $1$ on $F$. The question of exactness of (1) and the relation to certain $n$-widths are also discussed.

UDC: 517.5

MSC: Primary 30A10, 30C85; Secondary 31A15

Received: 12.06.1991


 English version:
Russian Academy of Sciences. Sbornik. Mathematics, 1993, 76:1, 199–210

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