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Riddled Basin 📂Dynamics

Riddled Basin

Definition 1

In a dynamical system, assume there are $n$ basins for $R_{1} , \cdots , R_{n}$ different attractors. A set $\mathcal{R}$ is termed a riddled basin if for all $\mathbf{x} \in \mathcal{R}$ and for all $\varepsilon > 0$, the open ball $B \left( \mathbf{x} ; \varepsilon \right)$ is not disjoint from all $R_{1} , \cdots , R_{n}$: $$ \forall \mathbf{x} \in \mathcal{R} , \varepsilon > 0 , k = 1 , \cdots n : B \left( \mathbf{x} ; \varepsilon \right) \cap R_{k} \ne \emptyset \implies \mathcal{R} \text{ is a riddled basin} $$

Example 2

In simple terms, a riddled basin is a basin with a complex and intertwined structure for multiple attractors, such that no matter how small a neighborhood is chosen, it includes parts of all the basins. By definition, even the smallest subset of a riddled basin will continuously exhibit these properties, revealing a connection with fractals.

$$ f : z \mapsto z^{2} - \left( 1 + i a \right) \bar{z} $$ Consider the example of a dynamical system defined by a map on the complex plane as described above. Here, the line $N_{1} = \left\{ z = x + iy : y = a/2 \right\}$ remains invariant under $f$ regardless of $a$, that is, $$ \begin{align*} & f \left( x + i a / 2 \right) \\ =& \left( x + i a / 2 \right)^{2} - \left( 1 + i a \right) \left( x - i {\frac{ a }{ 2 }} \right) \\ =& x^{2} - {\frac{ 3 a^{2} }{ 4 }} - x + i {\frac{ a }{ 2 }} \end{align*} $$ and hence under $f \left( N_{1} \right) \subset N_{1}$. The line rotated around the origin by ${\frac{ 4 \pi }{ 3 }}$ is denoted as $N_{2}$, and another rotation results in $N_{3}$, which are said to indeed be the three attractors under $f$.

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When the basins for the three attractors are visualized in red, green, and blue respectively, they appear as shown above. A riddled basin has overlaps with all three basins regardless of how small a circle is chosen anywhere. From the perspective of each color, no matter where you look, the surface appears riddled with holes, justifying the aptness of the term.


  1. Yorke. (1996). CHAOS: An Introduction to Dynamical Systems: p170. ↩︎

  2. https://brain.cc.kogakuin.ac.jp/~kanamaru/Chaos/e/RiddledBasin/ ↩︎