On approximate Gauss-Lucas theorems

Abstract

The Gauss--Lucas theorem states that any convex set $K\subset\mathbb{C}$ which contains all $n$ zeros of a degree $n$ polynomial $p\in\mathbb{C}[z]$ must also contain all $n-1$ critical points of $p$. In this paper we explore the following question: for which choices of positive integers $n$ and $k$, and positive real number $\epsilon$, will it follow that for every degree $n$ polynomial $p$ with at least $k$ zeros lying in $K$, $p$ will have at least $k-1$ critical points lying in the $\epsilon$-neighborhood of $K$. We supply an inequality relating $n$, $k$, and $\epsilon$ which, when satisfied, guarantees a positive answer to the above question.