Skip to content

Erdős problem 1047

Let fC[x]f\in \mathbb{C}[x] be a monic polynomial with mm distinct roots, and let c>0c>0 be a constant small enough such that {z:f(z)c}\{ z: \lvert f(z)\rvert\leq c\} has mm distinct connected components.

Sources

Browse retained paths and inspect the exact material available for this Problem.

6 retained statements2415f78e850a

Open selected source

FormalConjectures/ErdosProblems/

1047.lean

Retained formal statement1 of 6

Let fC[x]f\in \mathbb{C}[x] be a monic polynomial with mm distinct roots, and let c>0c>0 be a constant small enough such that {z:f(z)c}\{ z: \lvert f(z)\rvert\leq c\} has mm distinct connected components.

Must all these components be convex?

A question of Grunsky, which was reported by Erdős, Herzog, and Piranian [EHP58].

The answer is no, as shown by Pommerenke [Po61], who showed that, if kk is sufficiently large, and f(z)=zk(za)f(z)=z^k(z-a) where aa is sufficiently close to (1+1k)k1k+1(1+\frac{1}{k})k^{\frac{1}{k+1}}, then {z:f(z)1}\{ z: \lvert f(z)\rvert\leq 1\} has two components, and the component which contains 00 is not convex.

FormalConjectures/ErdosProblems/1047.leanErdos1047.erdos_10477 linesExact file
False  ∀ (f : Polynomial ℂ) (m : ℕ) (c : ℝ),    f.Monic      (f.rootSet ℂ).ncard = m        0 < c          (Erdos1047.componentsIn (Erdos1047.sublevelSet f c)).ncard = mtErdos1047.componentsIn (Erdos1047.sublevelSet f c), Convext
SolvedStatement only, no proof

Search problems.science

Find a Problem, Result, source, or page