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A STABILITY VERSION OF THE GAUSS–LUCAS THEOREM AND APPLICATIONS

Part of: Geometric function theory Polynomials, rational functions

Published online by Cambridge University Press:  05 September 2019

STEFAN STEINERBERGER Open the ORCID record for STEFAN STEINERBERGER [Opens in a new window]
STEFAN STEINERBERGER*
Affiliation:
Department of Mathematics, Yale University, New Haven, CT 06511, USA email [email protected]
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Abstract

Let $p:\mathbb{C}\rightarrow \mathbb{C}$ be a polynomial. The Gauss–Lucas theorem states that its critical points, $p^{\prime }(z)=0$, are contained in the convex hull of its roots. We prove a stability version whose simplest form is as follows: suppose that $p$ has $n+m$ roots, where $n$ are inside the unit disk,

$$\begin{eqnarray}\max _{1\leq i\leq n}|a_{i}|\leq 1~\text{and}~m~\text{are outside}~\min _{n+1\leq i\leq n+m}|a_{i}|\geq d>1+\frac{2m}{n};\end{eqnarray}$$
then $p^{\prime }$ has $n-1$ roots inside the unit disk and $m$ roots at distance at least $(dn-m)/(n+m)>1$ from the origin and the involved constants are sharp. We also discuss a pairing result: in the setting above, for $n$ sufficiently large, each of the $m$ roots has a critical point at distance ${\sim}n^{-1}$.

Keywords

Gauss–Lucas theorempolynomial dynamicsrootszerosderivativescritical points

MSC classification

Primary: 26C10: Polynomials
Secondary: 30C15: Zeros of polynomials, rational functions, and other analytic functions (e.g. zeros of functions with bounded Dirichlet integral)
Type
Research Article
Information
Journal of the Australian Mathematical Society , Volume 109 , Issue 2 , October 2020 , pp. 262 - 269
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Copyright
© 2019 Australian Mathematical Publishing Association Inc.

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Footnotes

The author was supported by the NSF (DMS-1763179) and the Alfred P. Sloan Foundation.

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