Number of components of polynomial lemniscates: a problem of Erd\"os, Herzog, and Piranian

Abstract

Let $K\subset\mathbb{C}$ be a compact set in the plane whose logarithmic capacity $c(K)$ is strictly positive. Let $\mathscr{P}_n(K)$ be the space of monic polynomials of degree $n,$ \emph{all} of whose zeros lie in $K.$ For $p\in \mathscr{P}_n(K),$ its filled \emph{unit leminscate} is defined by $\Lambda_p = \{z: |p(z)| < 1\}.$ Let $\mathcal{C}(\Lambda_p) $ denote the number of connected components of the open set $\Lambda_p,$ and define $\mathscr{C}_n(K) = \max_{p\in \mathscr{P}_n(K)}\mathcal{C}(\Lambda_p).$ In this paper we show that the quantity \[M(K) = \limsup_{n\to\infty}\dfrac{\mathscr{C}_n(K)}{n},\] satisfies $M(K) < 1$ when the logarithmic capacity $c(K) < 1,$ and $M(K) = 1$ when $c(K)\geq 1.$ In particular, this answers a question of Erd\"os et. al. posed in $1958$. In addition, we show that for nice enough compact sets whose capacity is strictly bigger than $\frac{1}{2}$, the quantity $m(K) = \liminf_{n\to\infty}\dfrac{\mathscr{C}_n(K)}{n} > 0.$