Are parabolic points in the Mandelbrot set algebraic numbers

algebraic-numberscomplex-dynamicsminimal-polynomials

Define the iterated quadratic polynomial:

$$
\begin{aligned}
f_c^0(z) &= 0 \\
f_c^{n+1}(z) &= f_c^n(z)^2+c
\end{aligned}
$$

The $c$ for which $f_c^n(0)$ remains bounded form the Mandelbrot set, which has hyperbolic components (cardioid- and disc-like regions) with a rich structure of periodic attractors.

Points $c$ on the boundary of a period $P$ component at the bond point with a period $Pp$ child component satisfy (for some $z$):

$$
\begin{aligned}
f_c^P(z) &= z \\
\frac{\partial}{\partial z}f_c^P(z) &= e^{2 \pi i \frac{q}{p}} \\
\gcd(q,p) &= 1
\end{aligned}
$$

Are all algebraic numbers? (I have found some that are.)

Are any algebraic integers? (I have found some that are not.)

What can be said about their degrees (as algebraic numbers) in relation to the $P$ and $p$?

Does the minimal polynomial remain the same when changing (only) $q$ while finding $c$ on the boundary of the same component?

Do points on the boundaries of different components of the same period have different minimal polynomial, even if $p$ and $q$ are the same?

Best Answer

Edit: previous answer was misleading, sorry.

Here's how you can do it: consider the function $$P_n(c)=\prod_{z=f_c^n(z)} \left( (f_c^n)'(z)-e^{2i\pi p/q} \right)$$ Then clearly a parameter $c$ satisfies your property iff $P_n(c)=0$.

Now it is not clear at first glance that $P_n$ is a polynomial (or even continuous), but it is. Let $A:=\overline{\mathbb{Q}}$ be the field of algebraic numbers. Observe that $f_c^n(z)-z$ can be seen as a polynomial in $z$ with coefficients in $A[c]$ (actually even in $\mathbb Z[c]$), and let $Q_1$ denote that polynomial. Then $P_n(c)$ is the resultant of $Q_1$ and of the polynomial $Q_2(z):=(f_c^n)'(z)-e^{2i\pi p/q}$, also with coefficients in $A[c]$.

Therefore, by properties of the resultant, $P_n(c) \in A[c]$, so the zeroes of $P_n$ are algebraic numbers.

In fact, this should also prove that parameters with an $n$-cycle of multiplier 1 are algebraic integers.