In order to define the Mandelbrot set $\mathbb M$, one looks at the sequences $$ s = f(0),(f\circ f)(0), (f\circ f \circ f)(0),\ldots, $$ where $f:\mathbb C \to \mathbb C,~z\mapsto z^2+c$ for different $c\in\mathbb C$. Now if $c$ is such that the resulting sequence $s$ is bounded, then $c\in\mathbb M$, otherwise $c\notin \mathbb M$. Does anyone know if there are characterizations or (even better) visualizations of the following related sets? I'd be grateful for a reference.
$$\begin{aligned} \mathbb M_\ell & :=\{c\in \mathbb C, s\text{ is a periodic sequence with period $\ell$.}\}\\ \mathbb M_p & := \bigcup_{\ell \in \mathbb N}\mathbb M_{\ell}\\ \mathbb M_{np} & := \mathbb M\setminus \mathbb M_p\\ \mathbb M_{\text{con}} & := \{c \in \mathbb C, s\text{ converges.}\} \end{aligned}$$
The sets $M_l$ are finite (they have $2^l$ elements). They are "in the center" of the connected components of the interior of the Mandelbrot set, and are not on its boundary.
The set $M_p$ is therefore countable, consisting of points all in the interior of $M$; but its closure contains the boundary of $M$.
$M_{np}$ is too big to have a simple description: after all, you only removed countably many points from $M$.
Presumably, $M_{con}$ is exactly the interior of the main cardioid (plus the parameter $c=\frac{1}{4}$, together with a countable collection of points corresponding to parameters for which the sequence $s$ is eventually constant. Those countable parameters are located at certain tips on the boundary of the Mandelbrot set.