# Filled Julia set

The filled-in Julia set $K(f)$ of a polynomial $f$ is :

## Formal definition

The filled-in Julia set $K(f)$ of a polynomial $f$ is defined as the set of all points $z$ of the dynamical plane that have bounded orbit with respect to $f$ $K(f){\overset {\mathrm {def} }{{}={}}}\left\{z\in \mathbb {C} :f^{(k)}(z)\not \to \infty ~{\text{as}}~k\to \infty \right\}$ where :

$\mathbb {C}$ is the set of complex numbers

$f^{(k)}(z)$ is the $k$ -fold composition of $f$ with itself = iteration of function $f$ ## Relation to the Fatou set

The filled-in Julia set is the (absolute) complement of the attractive basin of infinity.
$K(f)=\mathbb {C} \setminus A_{f}(\infty )$ The attractive basin of infinity is one of the components of the Fatou set.
$A_{f}(\infty )=F_{\infty }$ In other words, the filled-in Julia set is the complement of the unbounded Fatou component:
$K(f)=F_{\infty }^{C}.$ ## Relation between Julia, filled-in Julia set and attractive basin of infinity

The Julia set is the common boundary of the filled-in Julia set and the attractive basin of infinity

$J(f)=\partial K(f)=\partial A_{f}(\infty )$ where:
$A_{f}(\infty )$ denotes the attractive basin of infinity = exterior of filled-in Julia set = set of escaping points for $f$ $A_{f}(\infty )\ {\overset {\underset {\mathrm {def} }{}}{=}}\ \{z\in \mathbb {C} :f^{(k)}(z)\to \infty \ as\ k\to \infty \}.$ If the filled-in Julia set has no interior then the Julia set coincides with the filled-in Julia set. This happens when all the critical points of $f$ are pre-periodic. Such critical points are often called Misiurewicz points.

## Spine

The most studied polynomials are probably those of the form $f(z)=z^{2}+c$ , which are often denoted by $f_{c}$ , where $c$ is any complex number. In this case, the spine $S_{c}$ of the filled Julia set $K$ is defined as arc between $\beta$ -fixed point and $-\beta$ ,

$S_{c}=\left[-\beta ,\beta \right]$ with such properties:

• spine lies inside $K$ . This makes sense when $K$ is connected and full
• spine is invariant under 180 degree rotation,
• spine is a finite topological tree,
• Critical point $z_{cr}=0$ always belongs to the spine.
• $\beta$ -fixed point is a landing point of external ray of angle zero ${\mathcal {R}}_{0}^{K}$ ,
• $-\beta$ is landing point of external ray ${\mathcal {R}}_{1/2}^{K}$ .

Algorithms for constructing the spine:

• detailed version is described by A. Douady
• Simplified version of algorithm:
• connect $-\beta$ and $\beta$ within $K$ by an arc,
• when $K$ has empty interior then arc is unique,
• otherwise take the shortest way that contains $0$ .

Curve $R$ :

$R{\overset {\mathrm {def} }{{}={}}}R_{1/2}\cup S_{c}\cup R_{0}$ divides dynamical plane into two components.