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This article is cited in 1 scientific paper (total 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.
Received: 12.06.1991
Citation:
A. L. Levin, E. B. Saff, “$L_p$ extensions of Gonchar's inequality for rational functions”, Russian Acad. Sci. Sb. Math., 76:1 (1993), 199–210
Linking options:
https://www.mathnet.ru/eng/sm1048https://doi.org/10.1070/SM1993v076n01ABEH003408 https://www.mathnet.ru/eng/sm/v183/i6/p97
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