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Let $f:\mathbb C\to \mathbb C$ be a rational map and let $J(f)$ and $F(f)$ denote the Julia and Fatou sets of $f$, respectively.

Let $\mathcal S$ be the set of all boundaries of Fatou components. Assume that each member of $\mathcal S$ is a simple closed curve, and that $J(f)$ is locally connected. Some examples of rational Julia sets with this property are the circle (e.g. $f(z)=z^2$), Sierpinski carpet ($f(z)=z^2+\frac{1}{z^2}$), and Sierpinski triangle ($f(z)=z^2-\frac{1}{z}$).

Question. For all (distinct) $S_1,S_2\in \mathcal S$ is it necessarily true that $S_1\cap S_2$ is finite? Is the cardinality of all such intersections bounded by a finite constant $n$?

Of course this is trivial if the Julia set is a circle. For the Sierpinski carpet the intersections are empty. And for the Sierpinski triangle each intersection has at most 3 points.

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An answer of "no" is provided by the Julia set for Newton's method applied to $f(z) = z^3 - 1$.

The Julia set for Newton's method applied to f(z) = z^3 - 1.

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  • $\begingroup$ This is perhaps just a terminology question, but do the basins count as components? I had interpreted component to mean a connected component of the basin. $\endgroup$ May 21, 2023 at 10:56
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    $\begingroup$ This is a counterexample in either interpretation. However, I interpreted the question as asking about intersections between connected components of the complement of the Julia set. $\endgroup$
    – Sam Nead
    May 21, 2023 at 19:59
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    $\begingroup$ To make sure I have it right: here the intersection between two connected components is either empty or countably infinite? $\endgroup$ May 22, 2023 at 20:07
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    $\begingroup$ Or one point. For example, the largest red region (immediately to the right of the origin) and the medium red region (immediately to the left of the origin) meet only at the origin. $\endgroup$
    – Sam Nead
    May 22, 2023 at 20:35

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