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This article is cited in 12 scientific papers (total in 12 papers)
Lower bounds for the modulus of the logarithmic derivative of a polynomial
N. V. Govorov, Yu. P. Lapenko Kuban State University
Abstract:
Estimates are given for the measure of a section of an arbitrary straight line of the set
$$
E_\delta=\{z:|P'(z)/(nP(z))|\le\delta\}\quad(\delta>0),
$$
where $P(z)$ is a polynomial of degree $n$.
THEOREM. {\em Suppose $P(x)=(x-x_1)\dots(x-x_n)$ is a polynomial with real zeros. Then, for any $\delta>0$, on any interval $a\le x\le b$, containing all of the $x_k$ $(k=1,2,\dots,n)$, outside an exceptional set $E_\delta\subset[a,b]$ such that
$$
\operatorname{mes}E_\delta\le(\sqrt{1+\delta^2(b-a)^2}-1)/\delta,
$$
we have the inequality}
$$
|P'(x)/(nP(x))|>\delta.
$$
A similar estimate is given for polynomials whose roots lie either in $\operatorname{Im}z\ge0$ or in $\operatorname{Im}z\le0$.
Received: 24.01.1977
Citation:
N. V. Govorov, Yu. P. Lapenko, “Lower bounds for the modulus of the logarithmic derivative of a polynomial”, Mat. Zametki, 23:4 (1978), 527–535; Math. Notes, 23:4 (1978), 288–292
Linking options:
https://www.mathnet.ru/eng/mzm8168 https://www.mathnet.ru/eng/mzm/v23/i4/p527
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Abstract page: | 294 | Full-text PDF : | 135 | First page: | 1 |
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