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Title: A STABILITY VERSION OF THE GAUSS–LUCAS THEOREM AND APPLICATIONS
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 ${\sim}n^{-1}$ .
Authors:
Award ID(s):
1763179
Publication Date:
NSF-PAR ID:
10133832
Journal Name:
Journal of the Australian Mathematical Society
Page Range or eLocation-ID:
1 to 8
ISSN:
1446-7887
Sponsoring Org:
National Science Foundation
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