Research ContributionArnold Mathematical Journal

Received: 15 October 2015 / Revised: 9 February 2016 / Accepted: 7 April 2016

Internal Addresses of the Mandelbrot Set and Galois Groups of Polynomials

Dierk Schleicher dierk@jacobs-university.de
Jacobs University, Research I, Postfach 750 561 28725 Bremen Germany

Abstract

We describe an interesting interplay between symbolic dynamics, the structure of the Mandelbrot set, permutations of periodic points achieved by analytic continuation, and Galois groups of certain polynomials. Internal addresses are a convenient and efficient way of describing the combinatorial structure of the Mandelbrot set, and of giving geometric meaning to the ubiquitous kneading sequences in human-readable form (Sects. 3 and 4). A simple extension, angled internal addresses , distinguishes combinatorial classes of the Mandelbrot set and in particular distinguishes hyperbolic components in a concise and dynamically meaningful way. This combinatorial description of the Mandelbrot set makes it possible to derive existence theorems for certain kneading sequences and internal addresses in the Mandelbrot set (Sect. 6) and to give an explicit description of the associated parameters. These in turn help to establish some algebraic results about permutations of periodic points and to determine Galois groups of certain polynomials (Sect. 7). Through internal addresses, various areas of mathematics are thus related in this manuscript, including symbolic dynamics and permutations, combinatorics of the Mandelbrot set, and Galois groups.

Keywords

Mathematics Subject Classification

1. Introduction

The combinatorial structure of the Mandelbrot set has been studied by many people, notably in terms of quadratic minor laminations by Thurston ([Thurston2009]), pinched disks by Douady ([Douady1993]), or orbit portraits by Milnor ([Milnor2000a]). All of these results are modeled on parameter rays of the Mandelbrot set at periodic angles, as well as their landing properties: parameter rays at periodic angles of fixed period are known to land together in pairs, so these pairs subdivide the complex plane into finitely many components. Internal addresses are a natural way to distinguish these components, and thus to describe the combinatorial structure of the Mandelbrot set in an efficient way.

One feature of internal addresses is that they provide a good language to describe the combinatorial structure of $ \M$ (Sects. 3, 4, and 5). Internal addresses also allow us to decide which combinatorial data are realized in $ \M$: we describe a particularly relevant kind of hyperbolic components that we call purely narrow , and we give a necessary and sufficient description of the associated combinatorics (Sect. 6).

Finally, we address an apparently rather different topic: when periodic points are continued analytically across parameter space along closed loops, which permutations can be so achieved? This question can be interpreted as the determination of certain Galois groups of dynamically defined polynomials, or by the structure of the ramified cover over parameter space given by periodic points of given period. We explicitly determine these Galois groups in Sect. 7 and show how this question leads very naturally to questions about the combinatorial structure of $ \M$ of exactly the kind that was answered in previous sections.

Many results in this paper were first announced in [Lau and Schleicher1994]. The Galois groups had been determined earlier in a more algebraic way by Bousch ([Bousch1992]) and more recently by Morton and Patel ([Morton and Patel1994]). Related questions (from a somewhat different point of view) have been investigated in the PhD thesis by Penrose ([1994]); in particular our internal address is called there the "principal non-periodicity function".

2. Background

Much of the combinatorial structure of the Mandelbrot set $ \M$ is described in terms of parameter rays; these are defined as follows. By [Douady and Hubbard1984], the Mandelbrot set is compact, connected and full, i.e., there is a conformal isomorphism $ \Phi\colon(\Caa{\sm}\M)\to(\Caa{\sm}\diskbar)$; it can be normalized so that $ \Phi(c)/c\to 1$ as $ c\to\infty$. The parameter ray of $ \M$ at angle $ \theta\in\R/\Z$ is then defined as $ R(\theta):=\Phi^{-1}(e^{2\pi i\theta}(1,\infty))$. The parameter ray $ R(\theta)$ is said to land at some point $ c\in\partial\M$ if $ \lim_{r\searrow 1} \Phi^{-1}(re^{2\pi i\theta})$ exists and equals $ c$.

If $ R(\theta)$ and $ R(\theta')$ land at a common point $ z$, we call $ R(\theta)\cup R(\theta')\cup\{z\}$ a parameter ray pair and denote it $ \RP(\theta,\theta')$. Then $ \Caa{\sm}\RP(\theta,\theta')$ has two components, and we say that $ \RP(\theta,\theta')$ separates two points or sets if they are in different components. If a polynomial $ p_c(z):=z^2+c$ has connected Julia set, then there are analogous definitions of dynamic rays $ R_c(\theta)$ and dynamic ray pairs $ \RP_c(\theta,\theta')$. General background can be found in Milnor ([Milnor2005]).

A hyperbolic component of $ \M$ is a connected component of the set of $ c\in\M$ for which the map $ p_c\colon z\mapsto z^2+c$ has an attracting periodic point of some period, say $ n$. This period is necessarily constant throughout the component, and we say that $ n$ is the period of the hyperbolic component. Every hyperbolic component $ W$ has a distinguished boundary point called the root of $ W$; this is the unique parameter on $ \partial W$ on which the attracting orbit becomes indifferent with multiplier $ 1$.

The most important properties of parameter rays at periodic angles are collected in the following well known theorem; see [Douady and Hubbard1984], [Milnor2000a] and [Schleicher2004].

Theorem 2.1.

(Properties of periodic parameter ray pairs).

For every $ n\ge 1$, every parameter ray $ R(\theta)$ of period $ n$ lands at the root of a hyperbolic component $ W$ of period $ n$, and the root of every hyperbolic component of period $ n$ is the landing point of exactly two parameter rays, both of period $ n$.

If $ R(\theta')$ is the other parameter ray landing at the same root as $ R(\theta)$, then the ray pair $ \RP(\theta,\theta')$ partitions $ \Caa$ into two open components; let $ \wake{W}$ be the component containing $ W$: this is the wake of $ W$, and it does not contain the origin. This wake is the locus of parameters $ c\in\Caa$ for which the dynamic rays $ R_c(\theta)$ and $ R_c(\theta')$ land together at a repelling periodic point; the ray pair $ \RP_c(\theta,\theta')$ is necessarily characteristic.

Here a dynamic ray pair is characteristic if it separates the critical value from the rays $ R_c(2^k\theta)$ and $ R_c(2^k\theta')$ for all $ k\ge 1$ (except of course from those on the ray pair $ \RP_c(\theta,\theta')$ itself).

For period $ n=1$, there is a single parameter ray $ R(0)=R(1)$; the statement of the theorem holds if we count $ 0$ and $ 1$ separately; we do the same for dynamic rays.

In particular, we have the following important result.

Corollary 2.2.

(The ray correspondence theorem).

For every parameter $ c\in\Caa$, there is an angle-preserving bijection between parameter ray pairs separating the parameter $ c$ from the origin and characteristic dynamic ray pairs in the dynamical plane of $ p_c$ landing at repelling periodic points and separating the critical value $ c$ from the origin.

Parameter ray pairs at periodic angles have the following important property.

Lemma 2.3.

[Lavaurs' lemma ([Lavaurs1986])].

If any parameter ray pair at periodic angles separates another parameter ray pair at equal period from the origin, then these ray pairs are separated by a parameter ray pair of strictly lower period.

The following combinatorial concept will play an important role in our discussion.

Definition 2.4.

(Kneading sequence).

Every angle $ \theta\in\Circle:=\R/\Z$ has an associated kneading sequence $ \nu(\theta)=\nu_1\nu_2\nu_3\dots$, defined as the itinerary of $ \theta$ (under angle doubling) on the unit circle $ \Circle$ with respect to the partition $ \Circle{\sm}\{\theta/2,(\theta+1)/2\}$, so that \begin{eqnarray*} \nu_k=\left\{ \begin{array}{ll} \ajaja & \quad \text{ if }2^{k-1}\theta \in(\theta/2,(\theta+1)/2); \\ \akaka & \quad \text{ if }2^{k-1}\theta \in ((\theta+1)/2,\theta/2)=\Circle{\sm} [\theta/2,(\theta+1)/2]; \\ \aaa & \quad \text{ if }2^{k-1}\theta \in\{\theta/2,(\theta+1)/2\}. \end{array}\right. \end{eqnarray*}

If $ \theta$ is non-periodic, then $ \nu(\theta)$ is a sequence over $ \{\akaka,\ajaja\}$. If $ \theta$ is periodic of exact period $ n$, then $ \nu(\theta)$ also has period $ n$ so that the first $ n-1$ entries are in $ \{\akaka,\ajaja\}$ and the $ n$-th entry is $ \aaa$; such $ \nu(\theta)$ are called $ \aaa$ -periodic . The partition is such that every $ \nu(\theta)$ starts with $ \ajaja$ (unless $ \theta=0$). Kneading sequences were originally introduced for real quadratic polynomials $ x\mapsto x^2+c$ with $ c\in\R$. The critical point $ 0$ separates the dynamical interval into two parts, say $ \akaka$ and $ \ajaja$ so that the critical value is in part $ \ajaja$. Then the kneading sequence is the sequence of labels $ \akaka$ and $ \ajaja$ that the critical orbit visits. The relation to our definition above is as follows: suppose the dynamic ray at angle $ R(\theta)$ lands at the critical value. Then the two rays at angles $ \theta/2$ and $ (\theta+1)/2$ land at the critical point and separate the complex plane into two parts, and the kneading sequence of the critical value along the dynamical interval clearly equals the kneading sequence of the angle $ \theta$ as defined above.

3. Internal Addresses of the Mandelbrot Set

The parameter rays at periodic angles of periods up to $ n$ partition $ \Caa$ into finitely many components. This partition has interesting symbolic dynamic properties, compare Fig. 1 and Lemma 3.1: if two parameter rays $ R(\theta_1)$ and $ R(\theta_2)$ are in the same component, then the kneading sequences $ \nu(\theta_1)$ and $ \nu(\theta_2)$ associated to $ \theta_1$ and $ \theta_2$ coincide at least for $ n$ entries. In particular, when these two parameter rays land together, then $ \theta_1$ and $ \theta_2$ must have the same kneading sequence.

The combinatorics of these partitions, and thus the combinatorial structure of the Mandelbrot set, can conveniently be described in terms of internal addresses , which are "human-readable" recodings of kneading sequences. In this section we define internal addresses and give their fundamental properties.

Figure 1 Left The ten parameter rays of period up to four in the Mandelbrot set partition $ \Caa$ into $ 11$ components. In each component, the first four entries of the kneading sequences $ \nu(\theta)$ for arbitrary parameter rays $ R(\theta)$ are constant. Right The same parameter ray pairs sketched symbolically, and the first four entries in the kneading sequences drawn in.

Lemma 3.1.

(Parameter ray pairs and kneading sequences).

If two parameter rays $ R(\theta)$ and $ R(\theta')$ are not separated by a ray pair of period at most $ n$, then $ \theta$ and $ \theta'$ have kneading sequences which coincide for at least $ n$ entries (provided neither $ \theta$ nor $ \theta'$ are periodic of period $ n$ or less).

If $ R(\theta)$ and $ R(\theta')$ with $ \theta< \theta'$ form a ray pair, then $ \nu(\theta)=\nu(\theta')=:\nu_\aaa$. If in addition both angles are periodic with exact period $ n$, then $ \nu_\aaa$ is $ \aaa$-periodic of period $ n$, we have \begin{eqnarray*} \lim_{\phi\nearrow\theta}\nu(\phi)= \lim_{\phi\searrow\theta'}\nu(\phi) \qquad\mbox{ and } \qquad \lim_{\phi\searrow\theta}\nu(\phi)= \lim_{\phi\nearrow\theta'}\nu(\phi) \,\,, \end{eqnarray*} and both limits are periodic with period $ n$ or dividing $ n$ so that their first difference is exactly at the $ n$-th position.

Proof.

As $ \theta$ varies in $ \Circle$, the $ n$-th entry in its kneading sequence changes exactly at angles which are periodic of period dividing $ n$.

Consider two external angles $ \theta< \theta'$ which are not separated by any ray pair of period at most $ n$, and which are not periodic of period up to $ n$ (it is allowed that $ (\theta,\theta')$ contains periodic angles of period up to $ n$, as long as the other angle of the same ray pair is also contained in $ (\theta,\theta')$). Then for every $ k\leq n$, the parameter rays of period $ k$ with angles $ \phi\in(\theta,\theta')$ must land in pairs (Theorem 2.1), so the number of such rays is even. Therefore, as the angle varies from $ \theta$ to $ \theta'$, the $ k$-th entry in the kneading sequence of $ \theta$ changes an even number of times, and the kneading sequences of $ \theta$ and $ \theta'$ have identical $ k$-th entries. This settles the first claim, and it shows that $ \nu(\theta)=\nu(\theta')$ if $ R(\theta)$ and $ R(\theta')$ land together and neither $ \theta$ nor $ \theta'$ are periodic.

However, if $ R(\theta)$ and $ R(\theta')$ land together and one of the two angles is periodic, then both are periodic with the same exact period, say $ n$ (Theorem 2.1). In this case, the kneading sequences $ \nu(\theta)$ and $ \nu(\theta')$ are $ \aaa$-periodic with exact period $ n$, so they coincide as soon as their first $ n-1$ entries coincide; this case is covered by the first claim.

It is quite easy to see that limits such as $ \lim_{\phi\searrow\theta}\nu(\phi)$ exist; moreover, if $ \theta$ is non-periodic, then the limit equals $ \nu(\theta)$. If $ \theta$ is periodic of exact period $ n$, then $ \lim_{\phi\searrow\theta}\nu(\phi)$ and $ \lim_{\phi\searrow\theta}\nu(\phi)$ are periodic of period $ n$ or dividing $ n$, they contain no $ \aaa$, and their first $ n-1$ entries coincide with those of $ \nu(\theta)$. Clearly, $ \lim_{\phi\searrow\theta}\nu(\phi)$ and $ \lim_{\phi\nearrow\theta}\nu(\phi)$ differ exactly at positions which are multiplies of $ n$. Finally, $ \lim_{\phi\searrow\theta}\nu(\phi)= \lim_{\phi\nearrow\theta'}\nu(\phi)$ because for sufficiently small $ \eps$, the parameter rays $ R(\theta+\eps)$ and $ R(\theta'-\eps)$ are not separated by a ray pair of period at most $ n$, so the limits must be equal. ⬜

The following should be taken as an algorithmic definition of internal addresses in the Mandelbrot set.

Algorithm 3.2.

(Internal address in parameter space)

Given a parameter $ c\in\Caa$, the internal address of $ c$ in the Mandelbrot set is a strictly increasing finite or infinite sequence of integers. It is defined as follows:

s the internal address starts with $ S_0=1$ and the ray pair $ \RP(0,1)$;
ie isft$ep S_0\IntAdr\dots\IntAdr S_k$ is an initial segment of the internal address of $ c$, where $ S_k$ corresponds to a ray pair $ \RP(\theta_k,\theta'_k)$ of period $ S_k$, then let $ \RP(\theta_{k+1},\theta'_{k+1})$ be the ray pair of least period which separates $ \RP(\theta_k,\theta'_k)$ from $ c$ or for which $ c\in\RP(\theta_{k+1},\theta'_{k+1})$; let $ S_{k+1}$ be the common period of $ \theta_{k+1}$ and $ \theta'_{k+1}$. The internal address of $ c$ then continues as $ S_0\IntAdr\dots\IntAdr S_k \IntAdr S_{k+1}$ (with the ray pair $ \RP(\theta_{k+1},\theta'_{k+1})$).

This continues for every $ k\ge 1$ unless there is a finite $ k$ so that $ \RP(\theta_k,\theta'_k)$ is not separated from $ c$ by any periodic ray pair (in particular if $ c\in\RP(\theta_k,\theta'_k)$).

Remark.

The internal address is only the sequence $ S_0\IntAdr\cdots\IntAdr S_k \IntAdr \cdots$ of periods; it does not contain the ray pairs used in the construction. The ray pair $ \RP(\theta_{k+1},\theta'_{k+1}$) of lowest period is always unique by Lavaurs' Lemma (see Theorem 2.1).

The internal address of $ c\in\M$ can be viewed as a road map description for the way from the origin to $ c$ in the Mandelbrot set: the way begins at the hyperbolic component of period $ 1$, and at any intermediate place, the internal address describes the most important landmark on the remaining way to $ c$; see Fig. 1. Landmarks are hyperbolic components (or equivalently the periodic parameter rays landing at their roots, see Theorem 2.1), and hyperbolic components are the more important the lower their periods are. The road description starts with the most important landmark: the component of period $ 1$, and inductively continues with the period of the component of lowest period on the remaining way.

Different hyperbolic components (or combinatorial classes) are not distinguished completely by their internal addresses; the remaining ambiguity has a combinatorial interpretation and will be removed by angled internal addresses : see Theorem 3.9.

We can give an analogous definition of internal addresses in dynamic planes.

Definition 3.3.

(Internal address in dynamical planes)

Consider a polynomial $ p_c$ for which all periodic dynamic rays land and let $ K_c$ be the filled-in Julia set. For a point $ z\in K_c$, the internal address of $ z$ is defined as follows, in analogy to Algorithm 3.2:

s the internal address starts with $ S_0=1$ and the ray pair $ \RP_c(0,1)$;
ie sifte$p S_0\IntAdr\cdots\IntAdr S_k$ is an initial segment of the internal address of $ z$, where $ S_k$ corresponds to a dynamic ray pair $ \RP_c(\theta_k,\theta'_k)$ of period $ S_k$, then let $ \RP_c(\theta_{k+1},\theta'_{k+1})$ be the dynamic ray pair of least period which separates $ \RP_c(\theta_k,\theta'_k)$ from $ z$ or for which $ z\in\RP_c(\theta_{k+1},\theta'_{k+1})$; let $ S_{k+1}$ be the common period of $ \theta_{k+1}$ and $ \theta'_{k+1}$. The internal address of $ z$ then continues as $ S_0\IntAdr\cdots\IntAdr S_k \IntAdr S_{k+1}$ (with the ray pair $ \RP_c(\theta_{k+1},\theta'_{k+1})$). ¹

This continues for every $ k\ge 1$ unless there is a finite $ k$ so that $ \RP_c(\theta_k,\theta'_k)$ is not separated from $ z$ by any periodic ray pair.

Every kneading sequence has an associated internal address as follows:

Definition 3.4.

(Internal address and kneading sequence)

Given a kneading sequence $ \nu$ with initial entry $ \ajaja$, it has the following associated internal address $ S_0\IntAddr S_1\IntAddr \cdots\IntAddr S_k\IntAddr\cdots$: we start with $ S_0=1$ and $ \nu_0=\ovl{\ajaja}$. Then define recursively $ S_{k+1}$ as the position of the first difference between $ \nu$ and $ \nu_k$, and let $ \nu_{S_{k+1}}$ be the periodic continuation of the first $ S_{k+1}$ entries in $ \nu$ (if $ \nu$ is periodic or period $ S_k$, then the internal address is finite and stops with entry $ S_k$).

Observe that this definition is algorithmic. It can be inverted in the obvious way so as to assign to any finite or infinite strictly increasing sequence starting with $ 1$ (viewed as internal address) a kneading sequence consisting of entries $ \akaka$ and $ \ajaja$ and starting with $ \ajaja$.

For a $ \aaa$-periodic kneading sequence $ \nu$ of exact period $ n$, we define $ \A(\nu)$ and $ \Abar(\nu)$ as the two sequences in which every $ \aaa$ is replaced consistently by $ \akaka$ or consistently by $ \ajaja$, chosen so that the internal address of $ \A(\nu)$ contains the entry $ n$, while the internal address of $ \Abar(\nu)$ does not. It turns out that $ \A(\nu)$ has exact period $ n$, while the exact period of $ \Abar(\nu)$ equals or divides $ n$(see the proof of Lemma 7.5, ([Bruin et al.2009, Lemma 4.3]) or ([Bruin et al.2016, Proposition 5.16 and independently Lemma 19.2])). The sequences $ \A(\nu)$ and $ \Abar(\nu)$ are called the upper and lower periodic kneading sequences associated to $ \nu$.

The point of the various algorithmic definitions of internal addresses is of course the following.

Proposition 3.5.

(Equal internal addresses)

(1) For every $ c\in\Caa$ so that all periodic dynamic rays of $ p_c$ land, the internal address in parameter space equals the internal address of the critical value of $ c$ in the dynamical plane of $ p_c$.
(2) Let $ \RP(\theta_k,\theta'_k)$ be the ray pairs from Algorithm 3.2, let $ S_k$ be their periods and let $ \nu^k$ be the common kneading sequence of $ \theta_k$ and $ \theta'_k$. Then each $ \nu^k$ is a $ \aaa$-periodic kneading sequence of period $ S_k$ so that \begin{eqnarray*} \lim_{\phi\searrow\theta_k}\nu(\phi)= \lim_{\phi\nearrow\theta'_k}\nu(\phi)=\A(\nu^k) \qquad \mbox{ and } \qquad \lim_{\phi\nearrow\theta_k}\nu(\phi)= \lim_{\phi\searrow\theta'_k}\nu(\phi)=\Abar(\nu^k) \,\,. \end{eqnarray*}
(3) The first $ S_k$ entries in $ \A(\nu^k)$ coincide with those of $ \nu^{k+1}$ and, if $ c\in R(\theta)$, with the first $ S_k$ entries of $ \nu(\theta)$.
(4) For every $ \theta\in\Circle$, the internal address of any parameter $ c\in R(\theta)$ in Algorithm 3.2 is the same as the internal address associated to the kneading sequence of $ \theta$ from Definition 3.4.

Proof.

The first statement is simply a reformulation of Corollary 2.2 about the correspondence of parameter ray pairs and characteristic dynamic ray pairs: all we need to observe is that the internal address of the critical value in the Julia set uses only characteristic dynamic ray pairs; this follows directly from the definition of "characteristic".

We prove the second statement by induction, starting with the ray pair $ \RP(\theta_0,\theta'_0)$ with $ \theta_0=0$, $ \theta'_0=1$ and $ S_0=1$; both angles $ \theta_0$ and $ \theta'_0$ have kneading sequence $ \nu^0=\ovl\aaa$ and $ \A(\nu^0)=\ovl\ajaja$.

For the inductive step, suppose that $ \RP(\theta_k,\theta'_k)$ is a ray pair of period $ S_k$ with $ \theta_k< \theta'_k$, $ \nu(\theta_k)=\nu(\theta'_k)=\nu^k$ and $ \lim_{\phi\searrow\theta_k}\nu(\phi)=\lim_{\phi\nearrow\theta'_k}\nu(\phi)=\A(\nu^k)$, and $ c$ is not separated from $ \RP(\theta_k,\theta'_k)$ by a ray pair of period $ S_k$ or less.

As in Algorithm 3.2, let $ \RP(\theta_{k+1}, \theta'_{k+1})$ be a ray pair of lowest period, say $ S_{k+1}$, which separates $ \RP(\theta_k,\theta'_k)$ from $ c$ (or which contains $ c$); then $ \theta_k< \theta_{k+1}< \theta'_{k+1}< \theta'_k$. We have $ S_{k+1}> S_k$ by construction, and the ray pair $ \RP(\theta_{k+1},\theta'_{k+1})$ is unique by Lavaurs' Lemma. Since $ R(\theta_k)$ and $ R(\theta_{k+1})$ are not separated by a ray pair of period $ S_{k+1}$ or less, it follows that the first $ S_{k+1}$ entries in $ \lim_{\phi\searrow\theta_k}\nu(\phi)=\A(\nu^k)$ and in $ \lim_{\phi\nearrow \theta_{k+1}}\nu(\phi)$ are equal (the same holds for $ \lim_{\phi\nearrow \theta'_k}\nu(\phi)=\A(\nu^k)$ and $ \lim_{\phi\searrow \theta'_{k+1}}\nu(\phi)$).

Now we show that $ \lim_{\phi\nearrow\theta_{k+1}}\nu(\phi) =\lim_{\phi\searrow\theta'_{k+1}}\nu(\phi)=\Abar(\nu^{k+1})$; the first equality is Lemma 3.1. The internal address associated to $ \nu(\theta_k+\eps)$ has no entries in $ \{S_k+1,\dots,S_{k+1}\}$ provided $ \eps> 0$ is sufficiently small (again Lemma 3.1). The internal address associated to $ \nu(\theta_{k+1}-\eps)$ can then have no entry in $ \{S_k+1,\dots,S_{k+1}\}$ for small $ \eps$ either (or $ R(\theta_k)$ and $ R(\theta_{k+1})$ would have to be separated by a ray pair of period at most $ S_{k+1}$). Thus $ \lim_{\phi\nearrow\theta_{k+1}}\nu(\phi)=\Abar(\nu^{k+1})$. The other limiting kneading sequences $ \lim_{\phi\searrow\theta_{k+1}}\nu(\phi)$ and $ \lim_{\phi\nearrow\theta'_{k+1}}\nu(\phi)$ must then be equal to $ \A(\nu^{k+1})$.

Claim (3) follows because neither $ \RP(\theta_{k+1},\theta'_{k+1})$ nor $ c$ are separated from $ \RP(\theta_k,\theta'_k)$ by a ray pair of period $ S_k$ or less.

We prove Claim (4) again by induction. If $ c\in R(\theta)$, assume by induction that the internal address of $ \nu(\theta)$ starts with $ 1\IntAddr\dots\IntAddr S_k$ and $ \theta_k< \theta< \theta'_k$; then $ \nu(\theta)$ coincides with $ \A(\nu^k)$ for at least $ S_k$ entries. The ray pair of least period separating $ R(\theta_k)$ and $ R(\theta)$ is $ \RP(\theta_{k+1},\theta'_{k+1})$, so the first difference between $ \A(\nu^k)$ and $ \nu(\theta)$ occurs at position $ S_{k+1}$. Hence the internal address of $ \nu(\theta)$ as in Definition 3.4 continues as $ 1\IntAddr\cdots\IntAddr S_k\IntAddr S_{k+1}$ and we have $ \theta_{k+1}< \theta< \theta'_{k+1}$ as required to maintain the induction. ⬜

The previous proof also shows the following useful observation.

Corollary 3.6.

(Intermediate ray pair of lowest period)

Let $ \RP(\theta_1,\theta'_1)$ and $ \RP(\theta_2,\theta'_2)$ be two parameter ray pairs (not necessarily at periodic angles) and suppose that $ \RP(\theta_1,\theta'_1)$ separates $ \RP(\theta_2,\theta'_2)$ from the origin. If the limits $ \lim_{\phi\nearrow\theta_2}\nu(\phi)$ and $ \lim_{\phi\searrow\theta_1}\nu(\phi)$ do not differ, then the two ray pairs $ \RP(\theta_1,\theta'_1)$ and $ \RP(\theta_2,\theta'_2)$ are not separated by any periodic ray pair. If the limits do differ, say at position $ n$ for the first time, then both ray pairs are separated by a unique periodic ray pair $ \RP(\theta,\theta')$ of period $ n$ but not by ray pairs of lower period; the first $ n-1$ entries in $ \nu(\theta)=\nu(\theta')$ coincide with those of $ \lim_{\phi\nearrow\theta_2}\nu(\phi)$ and of $ \lim_{\phi\searrow\theta_1}\nu(\phi)$ (while the $ n$-th entries are of course $ \aaa$).

Proof.

Most internal addresses are infinite; exceptions are related to hyperbolic components as follows.

Corollary 3.7.

(Finite internal address)

The internal address of $ c\in\M$ is finite if and only if there is a hyperbolic component $ W$ with $ c\in\ovl W$. More precisely, if $ c\in\ovl W$ but $ c$ is not the root of a hyperbolic component other than $ W$, then the internal address of $ c$ terminates with the period of $ W$; if $ c$ is the root of a hyperbolic component $ W'\neq W$, then the internal address of $ c$ terminates with the period of $ W'$.

For a non-periodic external angle $ \theta\in\Circle$, the internal address of $ R(\theta)$ is finite if and only if $ R(\theta)$ lands on the boundary of a hyperbolic component $ W$.

Proof.

Remark.

The previous result illustrates (through the topology of the Mandelbrot set) the fact that non-periodic external angles can generate periodic kneading sequences which have finite internal addresses. There are periodic kneading sequences which have infinite internal addresses (for instance, $ \ovl{\ajaja\akaka\ajaja}$); but these are never generated by external angles; this essentially follows from the previous result (see also ([Bruin et al.2016, Lemma 14.6]). For a hyperbolic component $ W$, we have many different ways of associating an internal address to it:

(1) Using Algorithm 3.2 within the complex plane of the Mandelbrot set;
(2) Taking a parameter ray $ R(\theta)$ at a periodic angle that lands at the root of $ W$ and then Algorithm 3.2 for this parameter ray;
(3) Taking a parameter ray $ R(\theta')$ at an irrational angle that lands at a point in $ \partial W$ (as in Corollary 3.7) and then again Algorithm 3.2 for this parameter ray;
(4) Taking a parameter ray $ R(\theta)$ or $ R(\theta')$ as in (2) or (3) and then using the internal address associated to the kneading sequence of $ \theta$ or $ \theta'$; see Definition 3.4. The periodic angle $ \theta$ generates a $ \aaa$-periodic kneading sequence, while the non-periodic angle $ \theta'$ generates a periodic kneading sequence without $ \aaa$;
(5) For every $ c\in W$, we have the internal address of the critical value in the Julia set from Definition 3.3;
(6) Specifically if $ c$ is the center of $ W$, then the critical orbit is periodic and $ p_c$ has a Hubbard tree in the original sense of Douady and Hubbard ([Douady and Hubbard1984]), so we can use all definitions from ([Bruin et al.2016, Proposition 6.8]) or equivalently ([Bruin and Schleicher2001, Proposition 6.6]).

All these internal addresses are of course the same: this is obvious for the first three; the next two definitions agree with the first ones by Proposition 3.5 (4) and (1), and the last two agree by ([Bruin et al.2016, Proposition 6.8]) or ([Bruin and Schleicher2001, Proposition 6.6]).

Definition 3.8.

(Angled internal address)

For a parameter $ c\in\Caa$, the angled internal address is the sequence \begin{eqnarray*} (S_0)_{p_0/q_0}\IntAddr (S_1)_{p_1/q_1}\IntAddr \cdots \IntAddr (S_k)_{p_k/q_k}\IntAddr\cdots \end{eqnarray*} where $ S_0\IntAddr S_1\IntAddr\dots\IntAddr S_k\IntAddr\dots$ is the internal address of $ c$ as in Algorithm 3.2 and the angles $ p_k/q_k$ are defined as follows: for $ k\ge 0$, let $ \RP(\theta_k,\theta'_k)$ be the parameter ray pair associated to the entry $ S_k$ in the internal address of $ c$; then the landing point of $ \RP(\theta_k,\theta_k')$ is the root of a hyperbolic component $ W_k$ of period $ S_k$. The angle $ p_k/q_k$ is defined such that $ c$ is contained in the closure of the $ p_k/q_k$-subwake of $ W_k$.

If the internal address of $ c$ is finite and terminates with an entry $ S_k$ (which happens if and only if $ c$ is not contained in the closure of any subwake of $ W_k$), then the angled internal address of $ c$ is also finite and terminates with the same entry $ S_k$ without angle: $ (S_0)_{p_0/q_0}\IntAddr (S_1)_{p_1/q_1}\IntAddr \cdots \IntAddr (S_{k-1})_{p_{k-1}/q_{k-1}}\IntAddr (S_k)$.

Figure 2 Angled internal addresses for various hyperbolic components in $ \M$ .

This definition is illustrated in Fig. 2. The main point in this definition is that it distinguishes different points in the Mandelbrot set. A precise statement is given in terms combinatorial classes: a combinatorial class is the equivalence class of parameters in $ \M$ so that two parameters $ c_1$ and $ c_2$ are equivalent if and only if for both polynomials, the same periodic and preperiodic dynamic rays land at common points. Equivalently, it is a maximal subset of $ \M{\sm}\{\mbox{parabolic parameters}\}$ so that no two of its points can be separated by a parameter ray pair at periodic angles; a parabolic parameter $ c_0$ belongs to the combinatorial class of the hyperbolic component of which $ c_0$ is the root; see for instance ([Schleicher2004, Sect. 8]). Local connectivity of $ \M$ is equivalent to the conjecture that all non-hyperbolic combinatorial classes are points.

We can now describe precisely which points in $ \M$ angled internal addresses distinguish.

Theorem 3.9.

(Completeness of angled internal addresses)

Two parameters in $ \M$ have the same angled internal address if an only if they belong to the same combinatorial class. In particular, two hyperbolic parameters in $ \M$ have the same angled internal address if and only if they belong to the same hyperbolic component.

We postpone the proof of this theorem and of subsequent results to Sect. 5 and first discuss some interesting consequences.

4. The Geometry of Internal Addresses

Internal addresses give an efficient and convenient way to locate and describe parameters in $ \M$. For instance, they make it possible to give an answer to the folklore question how to tell where in the Mandelbrot set a given parameter ray lands.

It turns out that the internal address without angles completely determines the denominators of any associated angled internal address.

Lemma 4.1.

Estimates on denominators in internal address

In any internal address $ (S_0)_{p_0/q_0}\to \dots\to (S_k)_{p_k/q_k}\to (S_{k+1})_{p_{k+1}/q_{q+1}}\to\dots$, the denominators $ q_k$ satisfy $ S_{k+1}/S_k\le q_k< S_{k+1}/S_k+2$; moreover, $ q_k=S_{k+1}/S_k$ if and only if the latter is an integer.

If $ S_{k+1}$ is a multiple of $ S_k$, then the component of period $ S_{k+1}$ is a bifurcation from that of period $ S_k$.

This lemma provides a simple estimate on the denominators: if $ S_{k+1}/S_k\in\Nplus$, then $ q_k$ is uniquely determined, and in the general case when $ S_{k+1}/S_k\not\in\Nplus$, this allows exactly two possibilities for $ q_k$; this inequality is best possible in general.

Here we give a precise combinatorial formula for $ q_k$. Recall that every internal address has an associated kneading sequence $ \nu=\nu_1\nu_2\nu_3\dots$ (see Definition 3.4; this does not depend on the angles). To this kneading sequence $ \nu$ we define an associated function $ \rho_\nu$ as follows: for $ r\ge 1$, let \begin{eqnarray*} \rho_\nu(r):=\min\{k> r\colon \nu_{k}\neq\nu_{k-r}\} \,\,. \end{eqnarray*} The $ \rho_\nu$-orbit of $ r$ is denoted $ \orb_{\rho_\nu}(r)=\{r,\rho_\nu(r),\rho_\nu(\rho_\nu(r)),\dots\}$. We often write $ \rho$ for $ \rho_\nu$.

Lemma 4.2.

(Denominators in angled internal address)

In an angled internal address $ (S_0)_{p_0/q_0}\IntAddr \cdots \IntAddr (S_k)_{p_k/q_k} \IntAddr (S_{k+1})_{p_{k+1}/q_{k+1}}\dots$, the denominator $ q_k$ in the bifurcation angle is uniquely determined by the internal address $ S_0\IntAddr\cdots\IntAddr S_k\IntAddr S_{k+1}\cdots$ as follows: let $ \nu$ be the kneading sequence associated to the internal address and let $ \rho$ be the associated function as just described. Let $ r\in\{1,2,\ldots,S_k\}$ be congruent to $ S_{k+1}$ modulo $ S_k$. Then \begin{eqnarray*} q_k := \left\{ \begin{array}{ll} \frac{S_{k+1}-r}{S_k} + 1 & \quad \hbox{ if } S_k \in \orb_{\rho}(r)\,\,, \\ \frac{S_{k+1}-r}{S_k} + 2 & \quad \hbox{ if } S_k \notin \orb_{\rho}(r)\,\,. \end{array} \right. \end{eqnarray*}

While the internal address completely specifies the denominators in any corresponding angled internal address, it says says nothing about the numerators: of course, not all internal addresses are realized in the Mandelbrot set [(not all are (complex) admissible ; see [Bruin and Schleicher2008] or ([Bruin et al.2016, Sections 5 and 14])], but this is independent of the numerators.

Theorem 4.3.

(Independence of numerators in angled internal address)

If an angled internal address describes a point in the Mandelbrot set, then the numerators $ p_k$ can be changed arbitrarily (coprime to $ q_k$) and the modified angled internal address still describes a point in the Mandelbrot set.

In other words, for every hyperbolic component there is a natural bijection between combinatorial classes of the $ p/q$-sublimb and $ p'/q$-sublimb, for every $ q\ge 2$ and all $ p,p'$ coprime to $ q$. We thus conjectured in 1994 that these two limbs were homeomorphic by a homeomorphism preserving periods of hyperbolic components (but not the embedding into the plane). This conjecture was established recently [Dudko and Schleicher2012], based on work of Dudko ([Dudko2010]). These limbs are known to be homeomorphic by work of Branner and Fagella ([Branner and Fagella1999]), but their homeomorphisms (like all constructed by quasiconformal surgery, for instance as in [Riedl2000]) preserve the embedding into the complex plane and not periods of hyperbolic components.

Here is a way to find the numerators $ p_k$ of the internal address of an external angle.

Lemma 4.4.

(Numerators in angled internal address)

Suppose the external angle $ \theta$ has angled internal address $ (S_0)_{p_0/q_0}\IntAddr \cdots \IntAddr (S_k)_{p_k/q_k}\IntAddr (S_{k+1})_{p_{k+1}/q_{k+1}}\dots$. In order to find the numerator $ p_k$, consider the $ q_k-1$ angles $ \theta$, $ 2^{S_k}\theta$, $ 2^{2S_k}\theta$, ..., $ 2^{(q_k-2)S_k}\theta$. Then $ p_k$ is the number of these angles that do not exceed $ \theta$.

If $ \theta$ is periodic, then it is the angle of one of the two parameter rays landing at the root of a hyperbolic component. Here is a way to tell which of the two rays it is.

Lemma 4.5.

(Left or right ray)

Let $ R(\theta)$ and $ R(\theta')$ be the two parameter rays landing at the root of a hyperbolic component $ W$ of period $ n\ge 2$, and suppose that $ \theta< \theta'$. Let $ b$ and $ b'$ be the $ n$-th entries in the binary expansions of $ \theta$ and $ \theta'$. Then

If the kneading sequence of $ W$ has $ n$-th entry $ \akaka$, then $ b=1$ and $ b'=0$;
If the kneading sequence of $ W$ has $ n$-th entry $ \ajaja$, then $ b=0$ and $ b'=1$.

For example, the hyperbolic component with internal address $ 1_{1/3}\IntAddr 3_{1/2}\IntAddr 4$ has kneading sequence $ \ovl{\ajaja\ajaja\akaka\akaka}$, and the parameter ray $ R(4/15)$ lands at its root. The binary expansion of $ 4/15$ is $ 0.\ovl{0100}$. The $ 4$-th entries in the kneading sequence and in the binary expansion of $ 4/15$ are $ \akaka$ and $ 0$, so the second ray landing together with $ R(4/15)$ has angle $ \theta< 4/15$ (indeed, the second ray is $ R(3/15)$).

We define the width of the wake of a hyperbolic component $ W$ as $ |W|:=|\theta'-\theta|$, where $ \RP(\theta,\theta')$ is the parameter ray pair landing at the root of $ W$. If $ W$ has period $ n$, then the width of the $ p/q$-subwake of $ W$ equals

\begin{eqnarray}\label{Eq:WidthWake} |W_{p/q}| = |W|(2^n-1)^2/(2^{qn}-1)\,\,; \end{eqnarray}

(4.1)

this is a folklore result, related to Douady's Tuning Formula ([Douady1986]); a proof can be found in ([Bruin et al.2016, Proposition 9.34]).

The following result complements the interpretation of internal addresses as road descriptions by saying that whenever the path from the origin to a parameter $ c\in\M$ branches off from the main road, an entry in the internal address is generated: the way to most parameters $ c\in\M$ traverses infinitely many hyperbolic components, but most of them are traversed "straight on" and left into the $ 1/2$-limb.

Proposition 4.6.

(Sublimbs other than $ 1/2$ in internal address)

If a parameter $ c\in\Caa$ is contained in the subwake of a hyperbolic component $ W$ at internal angle other than $ 1/2$, then $ W$ occurs in the internal address of $ c$. More precisely, the period of $ W$ occurs in the internal address, and the truncation of the angled internal address of $ c$ up to this period describes exactly the component $ W$.

Proof.

The internal address of a parameter $ c$ also tells whether or not this parameter is renormalizable:

Proposition 4.7.

(Internal address and renormalization)

Let $ c\in\M$ be a parameter with internal address $ (S_0)_{p_0/q_0}\IntAddr \cdots \IntAddr (S_k)_{p_k/q_k}\dots$.

$ c$ is simply renormalizable of period $ n$ if and only if there is a $ k$ with $ S_k=n$ and $ n|S_{k'}$ for $ k'\ge k$;
$ c$ is crossed renormalizable of period $ n$ if and only if there is an $ m$ strictly dividing $ n$ so that $ S_k=m$ for an appropriate $ k$ and all $ S_{k'}$ with $ k'> k$ are proper multiples of $ n$ (in particular, $ n$ does not occur in the internal address). In this case, the crossing point of the little Julia sets has period $ m$.

Remark.

We can also describe combinatorially whether any given parameter rays $ R(\theta)$ and $ R(\theta')$ can land at the same point in $ \M$: a necessary condition is that they have the same angled internal address. This condition is also sufficient from a combinatorial point of view: suppose $ R(\theta)$ and $ R(\theta')$ have the same angled internal address. Then both rays accumulate at the same combinatorial class. If the internal address is finite, then $ R(\theta)$ and $ R(\theta')$ land on the boundary of the same hyperbolic component. Otherwise, both rays accumulate at the same combinatorial class. As soon as it is known that this combinatorial class consists of a single point [which would imply local connectivity of $ \M$ at that point ([Schleicher2004])], then both rays land together. Here is one example: where in the Mandelbrot set does the parameter ray at angle $ \theta=22/(2^7-1)$ land? The angle has period $ 7$, so it must land at the root of a hyperbolic component of period $ 7$; but there are $ 63$ such components; which one is it? And which of the two rays landing at this root is it? The angle has associated kneading sequence $ \nu(\theta)=\ovl{\ajaja\ajaja\akaka\ajaja\akaka\ajaja\aaa}$, also of period $ 7$ (see Definition 2.4). This binary sequences has an associated internal address (given by the simple algorithm in Definition 3.4): it equals $ 1\IntAddr 3\IntAddr 5\IntAddr 7$, and it describes where the ray lands (see Fig. 2): start with the unique hyperbolic component at period $ 1$; the most important landmark from here to the destination point is the component of period $ 3$ that bifurcates from the period $ 1$ component. From here, pass to the unique period $ 5$ component without passing by a component of period $ 4$ (or the $ 4$ would appear in the internal address); and there is a unique such component. The wake of this period $ 5$ component contains a unique period $ 6$ component and two period $ 7$ components: one inside and one outside of the wake of a period $ 6$ component. Our component is the one outside of the period $ 6$ wake, and the parameter ray at angle $ \theta$ lands at its root. There are two components of period $ 3$ bifurcating from the period $ 1$ component, and we need the one at internal angle $ 1/3$: this is resolved by angled internal addresses. The denominators are determined by Lemma 4.2, and the numerators by Lemma 4.4: the angled internal address is $ 1_{1/3}\IntAddr 3_{1/2} \IntAddr 5_{1/2}\IntAddr7$.

Finally, there are two rays landing at the root of the period $ 7$ component, and our ray is the greater of the two (Lemma 4.5). Internal addresses also specify which components bifurcate from each other, and which ones are renormalizable (Proposition 4.7).

5. Proofs About Internal Addresses

Now we give the proofs of the results stated so far; most of them go back to [Lau and Schleicher1994]. Many of the proofs are based on the concept of long internal addresses , which show that even though internal addresses themselves are a compact road description, they encode refinements to arbitrary detail. Parameter ray pairs have a partial order as follows: $ \RP(\theta_1,\theta'_1)< \RP(\theta_2,\theta'_2)$ iff $ \RP(\theta_1,\theta'_1)$ separates $ \RP(\theta_2,\theta'_2)$ from the origin $ c=0$, or equivalently from $ c=1/4$, the landing point of the ray pair $ \RP(0,1)$. It will be convenient to say $ \RP(0,1)< \RP(\theta,\theta')$ for every ray pair $ \RP(\theta,\theta')\neq \RP(0,1)$. For every parameter $ c\in\Caa$, the set of parameter ray pairs separating $ c$ from the origin is totally ordered (by definition, this set always includes the pair $ \RP(0,1)$ as its minimum). A similar partial order can be defined for dynamic ray pairs for every $ c\in\M$.

Definition 5.1.

( Long internal address in the Mandelbrot set )

For a parameter ray $ c\in\Caa$, consider the set of periodic parameter ray pairs which separate $ c$ from the origin, totally ordered as described above; the long internal address of $ c$ is the collection of periods of these ray pairs with the same total order.

The long internal address always starts with the entry $ 1$, and it is usually infinite and not well-ordered. For most $ c\in\Caa$, many periods appear several times on the long internal address (which means that several ray pairs of equal period separate $ c$ from the origin; only the ray pair of least period is unique).

One useful feature of internal addresses is that they completely encode the associated long internal addresses. The following proposition is in fact algorithmic, as its proof shows.

Proposition 5.2.

(Long internal address encoded in internal address)

Any internal address completely encodes the associated long internal address.

Proof.

Remark.

Of course, it makes perfect sense to speak of an angled long internal address , which is a long internal address in which all entries (except the last, if there is a last entry) are decorated with the bifurcation angles of the sublimb containing the parameter associated with this address. All entries with angles different from $ 1/2$ are already contained in the "short" internal address by Proposition 4.6, so the angled internal address completely encodes the angled long internal address.

Proof of Theorem 3.9.

(Completeness of angled internal addresses) If two parameters belong to the same combinatorial class, then by definition they cannot be separated by a parameter ray pair at periodic angles, and then their angled internal address is by definition the same.

For the converse, suppose two parameters $ c$ and $ c'$ have the same angled internal address. If the two sequences of hyperbolic components in these two angled internal addresses coincide, then it easily follows that $ c$ and $ c'$ are in the same combinatorial class. It thus suffices to prove the claim for periodic ray pairs and thus for hyperbolic components. If the claim is false, then there is a least period $ S_k$ for which there are two different hyperbolic components $ W$ and $ W'$ of period $ S_k$ that share the same angled internal address $ (S_1)_{p_1/q_1}\IntAddr \cdots \IntAddr (S_{k-1})_{p_{k-1}/q_{k-1}}\IntAddr S_k$. By minimality of $ S_k$, the ray pair of period $ S_{k-1}$ is the same in both internal addresses.

By the Branch Theorem (see [Douady and Hubbard1984] or [Schleicher2004][Theorem 3.1]), there are three possibilities: either (1) $ W$ is contained in the wake of $ W'$ (or vice versa), or (2) there is a hyperbolic component $ W_*$ so that $ W$ and $ W'$ are in two different of its sublimbs, or (3) there is a Misiurewicz-Thurston point $ c_*$ so that $ W$ and $ W'$ are in two different of its sublimbs. (A Misiurewicz-Thurston point is a parameter $ c_*$ for which the critical orbit is strictly preperiodic; such a point is the landing point of a finite positive number of parameter rays at preperiodic angles ([Douady and Hubbard1984], [Schleicher2000]), and a sublimb of $ c_*$ is a component in the complement of these rays and their landing point, other than the component containing $ 0$.)

(1) In the first case, there must by a parameter ray pair of period less than $ S_k$ separating $ W$ and $ W'$ by Lavaurs' Lemma. This ray pair would have to occur in the internal address of $ W'$ (between entries $ S_{k-1}$ and $ S_k$ or perhaps before $ S_{k-1}$) but not of $ W$, and this is a contradiction.
(2) The second possibility is handled by angled internal addresses: at least one of $ W$ or $ W'$ must be contained in a sublimb of $ W_*$ other than the $ 1/2$-sublimb, so by Proposition 4.6, $ W_*$ must occur in the internal address, and the angles at $ W_*$ in the angled internal address distinguish $ W$ and $ W'$.
(3) The case of a Misiurewicz-Thurston $ c_*$ point needs more attention. If $ c_*$ has at least two sublimbs, then it is the landing point of $ k\ge 3$ parameter rays $ R(\theta_1)$ ...$ R(\theta_k)$ at preperiodic angles.

By Proposition 5.2, $ W$ and $ W'$ have the same long internal address, and by minimality of $ S_k$ there is no hyperbolic component $ W^*$ of period less than $ S_k$ in a sublimb of $ c^*$ so that $ W\subset \wake{W^*}$.

Let $ \RP(\theta,\theta')$ be the parameter ray pair landing at the root of $ W$ and let $ c$ be the center of $ W$. In the dynamics of $ p_c$, there is a characteristic preperiodic point $ w$ which is the landing point of the dynamic rays $ R_c(\theta_1)$, ..., $ R_c(\theta_K)$ (the definition of characteristic preperiodic points is in analogy to the definition of characteristic periodic points after Theorem 2.1; the existence of the characteristic preperiodic point $ w$ is well known ("The Correspondence Theorem"); see e.g., ([Schleicher2004, Theorem 2.1]) or ([Bruin et al.2016, Theorem 9.4]). We use the Hubbard tree $ T_c$ of $ p_c$ in the original sense of Douady and Hubbard ([Douady and Hubbard1984]). Let $ I\subset T_c$ be the arc connecting $ w$ to $ c$.

If the restriction $ p_c^{\circ S_k}|_I$ is not injective, then let $ n\le S_k$ be maximal so that $ p_c^{\circ (n-1)}|_I$ is injective. Then there is a sub-arc $ I'\subset I$ starting at $ w$ so that $ p_c^{\circ n}|_{I'}$ is injective and $ p_c^{\circ n}(I')$ connects $ p_c^{\circ n}(w)$ with $ c$. If $ n=S_k$ then $ I'=I$ because $ c$ is an endpoint of $ p_c^{\circ n}(I)$, a contradiction; thus $ n< S_k$. Since $ w$ is characteristic, this implies that $ p_c^{\circ n}(I')\supset I'$, so there is a fixed point $ z$ of $ p_c^{\circ n}$ on $ I'$. If $ z$ is not characteristic, then the characteristic point on the orbit of $ z$ is between $ z$ and $ c$. In any case, we have a hyperbolic component $ W^*$ of period $ n< S_k$ in a sublimb of $ c^*$ so that $ W\subset \wake{W^*}$ (again by Theorem 2.1), and this is a contradiction.

It follows that the restriction $ p_c^{\circ S_k}|_I$ is injective. There is a unique component of \begin{equation*}\Caa{\sm} \left(R_c(2^{S_k}\theta_1)\cup\dots\cup R_c(2^{S_k}\theta_k)\cup\{p_c^{\circ S_k}(w)\}\right)\end{equation*} that contains $ p_c^{\circ S_k}(I)$: it is the component containing $ c$ and the dynamic ray pair $ \RP_c(\theta,\theta')$, and thus also the dynamic rays $ R_c(\theta_1),\dots,R_c(\theta_k)$, so this component is uniquely specified by the external angles of $ c_*$ together with $ S_k$. But by injectivity of $ p_c^{\circ S_k}|_I$, this also uniquely specifies the component of $ \Caa{\sm} (R_c(\theta_1)\cup\dots\cup R_c(\theta_k)\cup\{z\})$ that must contain $ I$ and hence $ c$. In other words, the subwake of $ c_*$ containing $ W$ (and, by symmetry, $ W'$) is uniquely specified. ⬜

Proof of Lemma 4.1.

(Estimates on denominators) The angled internal address \begin{equation*}(S_0)_{p_0/q_0}\IntAddr (S_1)_{p_1/q_1}\IntAddr \cdots \IntAddr (S_k)_{p_k/q_k}\IntAddr (S_{k+1})_{p_{k+1}/q_{k+1}}\dots,\end{equation*} when truncated at periods $ S_k$, describes a sequence of hyperbolic components $ W_k$ of periods $ S_k$. If $ W_{k+1}$ is contained in the $ p_k/q_k$-sublimb of $ W_k$, then we have $ S_{k+1}\le q_kS_k$ (otherwise an entry $ q_kS_k$ would be generated in the internal address); thus $ q_k\ge S_{k+1}/S_k$. The other inequality follows from (4.1): the width of the wake of $ W_{k+1}$ cannot exceed the width of the $ p_k/q_k$-subwake of $ W_k$, so $ 1/(2^{S_{k+1}}-1)\le (2^{S_k}-1)^2/(2^{q_kS_k}-1)$ or \begin{eqnarray*} 2^{q_kS_k}-1 \le (2^{S_{k+1}}-1) (2^{S_k}-1)^2 < 2^{S_{k+1}+2S_k}-1 \,\,, \end{eqnarray*} hence $ S_{k+1}> (q_k-2)S_k$ or $ q_k< S_{k+1}/S_k+2$.

It remains to show that whenever $ S_{k+1}/S_k$ is an integer, it equals $ q_k$: there are associated parameter ray pairs $ \RP(\theta_k,\theta'_k)$ of period $ S_k$ and $ \RP(\theta_{k+1},\theta'_{k+1})$ of period $ S_{k+1}$, and the limiting kneading sequences $ \lim_{\phi\searrow\theta_k}\nu(\phi)$ and $ \lim_{\phi\nearrow\theta_{k+1}}\nu(\phi)$ are periodic of period $ S_k$ and $ S_{k+1}$; both can be viewed as being periodic of period $ S_{k+1}$. Since they correspond to adjacent entries in the internal address, these ray pairs cannot be separated by a ray pair of period up to $ S_{k+1}$, so both limiting kneading sequences are equal. Therefore, Corollary 3.6 implies that the two ray pairs are not separated by any periodic parameter ray pair. If $ W_{k+1}$ was not a bifurcation from $ W_k$, then it would have to be in some subwake of $ W_k$ whose boundary would be some ray pair separating $ \RP(\theta_k,\theta'_k)$ from $ \RP(\theta_{k+1},\theta'_{k+1})$; this is not the case. So let $ p_k/q_k$ be the bifurcation angle from $ W_k$ to $ W_{k+1}$; then the corresponding periods satisfy $ S_{k+1}=q_kS_k$ as claimed. ⬜

In the following lemma, we use the function $ \rho_\nu$ as defined before Lemma 4.2.

Lemma 5.3.

(Intermediate ray pair of lowest period)

Let $ \RP(\theta_k,\theta_k')$ and $ \RP(\theta_{k+1},\theta'_{k+1})$ be two periodic parameter ray pairs with periods $ S_k< S_{k+1}$ and suppose that $ \RP(\theta_k,\theta_k')$ separates $ \RP(\theta_{k+1},\theta'_{k+1})$ from the origin. Write $ S_{k+1}=aS_k+r$ with $ r\in\{1,\dots,S_k\}$. Let $ S$ be the least period of a ray pair separating $ \RP(\theta_k,\theta_k')$ from $ \RP(\theta_{k+1},\theta'_{k+1})$. If $ S_{k+1}< S\le(a+1)S_k$, then $ S=aS_k+\rho_\nu(r)$, where $ \nu$ is any kneading sequence that has the same initial $ S_k$ entries as $ \nu(\theta_{k+1})=\nu(\theta'_{k+1})$.

Proof.

Proof of Lemma 4.2.

Proof of Theorem 4.3.

(Numerators arbitrary) We prove a stronger statement: given a hyperbolic component $ W$ of period $ n$, then we can combinatorially determine for any $ s'> 1$ how many components of period up to $ s'$ are contained in the wake $ \wake{W}$, how the wakes of all these components are nested, and what the widths of their wakes are; all we need to know about $ W$ are the width of its wake and its internal address (without angles). In particular, these data encode how the internal address of the component $ W$ can be continued within $ \M$. This proves the theorem (and it also shows that the width of the wake of $ W$ is determined by the internal address of $ W$).

For any period $ s$, the number of periodic angles of period $ s$ or dividing $ s$ within any interval of $ \Circle$ of length $ \delta$ is either $ \lfloor\delta/(2^s-1)\rfloor$ or $ \lceil\delta/(2^s-1)\rceil$ (the closest integers above and below $ \delta/(2^s-1)$). If the interval of length $ \delta$ is the wake of a hyperbolic component, then the corresponding parameter rays land in pairs, and the correct number of angles is the unique even integer among $ \lfloor\delta/(2^s-1)\rfloor$ and $ \lceil\delta/(2^s-1)\rceil$. This argument uniquely determines the exact number of hyperbolic components of any period within any wake of given width.

There is a unique component $ W_s$ of lowest period $ s$, say, in $ \wake{W}$ (if there were two such components, then this would imply the existence of at least one parameter ray of period less than $ s$ and thus of a component of period less than $ s$). The width of $ \wake{W_{s}}$ is exactly $ 1/(2^s-1)$ (this is the minimal possible width, and greater widths would imply the existence of a component with period less than $ s$).

Now suppose we know the number of components of periods up to $ s'$ within the wake $ \wake{W}$, together with the widths of all their wakes and how these wakes are nested. The wake boundaries of period up to $ s'$ decompose $ \wake{W}$ into finitely many components. Some of these components are wakes; the others are complements of wakes within other wakes. We can uniquely determine the number of components of period $ s'+1$ within each of these wakes (using the widths of these wakes), and then also within each complementary component outside of some of the wakes (simply by calculating differences). Each wake and each complementary component can contain at most one component of period $ s'+1$ by Theorem 3.9. Note that the kneading sequences and hence the internal addresses of all wakes of period $ s'+1$ are uniquely determined by those of periods up to $ s'$. The long internal addresses tell us which wakes of period $ s'+1$ contain which other wakes, and from this we can determine the widths of the wakes of period $ s'+1$. This provides all information for period $ s'+1$, and this way we can continue inductively and prove the claimed statement.

Starting with the unique component of period $ 1$, it follows that the width of a wake $ \wake{W}$ is determined uniquely by the internal address of $ W$. ⬜

Proof of Lemma 4.4.

(Finding numerators) We only need to find the numerator $ p_k$ if $ q_k\ge 3$. Let $ W_k$ be the unique hyperbolic component with angled internal address $ (S_0)_{p_0/q_0}\IntAddr\cdots\IntAddr(S_k)$ and let $ \RP(\theta_k,\theta'_k)$ be the ray pair bounding its wake $ \wake{W_k}$. Let $ W'_k$ be the component with angled internal address $ (S_0)_{p_0/q_0}\IntAddr\cdots\IntAddr(S_k)_{p_k/q_k}\IntAddr q_kS_k$; it is an immediate bifurcation from $ W_k$.

By Theorem 2.1, every $ c\in \wake{W_k}$ has a repelling periodic point $ z_c$ which is the landing point of the characteristic dynamic ray pair $ \RP_c(\theta_k,\theta'_k)$; we find it convenient to describe our proof in such a dynamic plane, even though the result is purely combinatorial. Let $ \Theta$ be the set of angles of rays landing at $ z_c$; this is the same for all $ c\in\wake{W_k}$. Especially if $ c\in W'_k$, it is well known and easy to see that $ \Theta$ contains exactly $ q_k$ elements, the first return map of $ z_c$ permutes the corresponding rays transitively and their combinatorial rotation number is $ p_k/q_k$. These rays disconnect $ \Caa$ into $ q_k$ sectors which can be labelled $ V_0,V_1,\dots,V_{q_k-1}$ so that the first return map of $ z_c$ sends $ V_j$ homeomorphically onto $ V_{j+1}$ for $ j=1,2,\dots,q_{k}-2$, and so that $ V_1$ contains the critical value and $ V_0$ contains the critical point and the ray $ R_c(0)$. Finally, under the first return map of $ z_c$, points in $ V_0$ near $ z_c$ map into $ V_1$, and points in $ V_{q_k-1}$ near $ z_c$ map into $ V_0$. The number of sectors between $ V_0$ and $ V_1$ in the counterclockwise cyclic order at $ z_c$ is then $ p_k-1$, where $ p_k$ is the numerator in the combinatorial rotation number.

Now suppose the dynamic ray $ R_c(\theta)$ contains the critical value or lands at it. Then $ R_c(\theta)\in V_1$, and $ R_c(2^{(j-1)S_k}\theta)\in V_{j}$ for $ j=1,2,\dots,q_k-1$. Counting the sectors between $ V_0$ and $ V_1$ means counting the rays $ R_c(\theta),R_c(2^{S_k}\theta),\dots,R_c(2^{(q_k-2)S_k}\theta)$ in these sectors, and this means counting the angles $ \theta, 2^{S_k}\theta, \dots, 2^{(q_k-2)S_k}$ in $ (0,\theta)$. The numerator $ p_k$ exceeds this number by one, and this equals the number of angles $ \theta, 2^{S_k}\theta, \dots, 2^{(q_k-2)S_k}$ in $ (0,\theta]$. ⬜

Proof of Lemma 4.5.

Proof of Proposition 4.7.

(Internal address and renormalization) We only discuss the case of simple renormalization (the case of crossed renormalization is treated in [Riedl and Schleicher1998][Corollary 4.2]).

Fix a hyperbolic component $ W$ of period $ n$. Let $ \M_W$ be the component of $ n$-renormalizable parameters in $ \M$ containing $ W$ (all $ c\in W$ are $ n$-renormalizable), and let $ \Psi_W\colon\M\to\M_W$ be the tuning homeomorphism; see [Haissinsky2000] and [Milnor2000a], [Milnor2000b] or ([Bruin et al.2016, Section 10]). Let $ 1\IntAddr S_1\IntAddr\cdots \IntAddr S_k=n$ be the internal address of $ W$. Then the internal address of every $ c\in\M_W$ starts with $ 1\IntAddr S_1\IntAddr\cdots \IntAddr S_k=n$ because $ \M_W$ contains no hyperbolic component of period less than $ n$, so points in $ \M_W$ are not separated from each other by parameter ray pairs of period less than $ n$. All hyperbolic components within $ \M_W$, and thus all ray pairs separating points in $ \M_W$, have periods that are multiples of $ n$, so all internal addresses of parameters within $ \M_W$ have the form $ 1\IntAddr S_1\IntAddr\cdots \IntAddr S_k\IntAddr S_{k+1}\dots$ so that all $ S_m\ge n$ are divisible by $ n$. In fact, if $ c\in\M$ has internal address $ 1\IntAddr S'_1\IntAddr\cdots \IntAddr S'_{k'}\dots$, then it is not hard to see that the internal address of $ \Psi_W(c)$ is $ 1\IntAddr S_1\IntAddr\cdots \IntAddr n\IntAddr nS'_1\IntAddr\cdots \IntAddr nS'_k\dots$ (hyperbolic components of period $ S'$ in $ \M$ map to hyperbolic components of period $ nS'$ in $ \M_W$, and all ray pairs separating points in $ \M_W$ are associated to hyperbolic components in $ \M_W$ that are images under $ \Psi_W$).

For the converse, we need dyadic Misiurewicz-Thurston parameters : these are by definition the landing points of parameter rays $ R(\theta)$ with $ \theta=m/2^k$; dynamically, these are the parameters for which the singular orbit is strictly preperiodic and terminates at the $ \beta$ fixed point. If $ \theta=m/2^k$, then the kneading sequence $ \nu(\theta)$ has only entries $ \akaka$ from position $ k+1$, so the internal address of $ \nu(\theta)$ contains all integers that are at least $ 2k-1$. But the internal address of $ \nu(\theta)$ from Algorithm 3.4 equals the internal address of $ R(\theta)$ in parameter space (Proposition 3.5), and the landing point of $ R(\theta)$ has the same internal address. Therefore, the internal address of any dyadic Misiurewicz-Thurston parameter contains all sufficiently large positive integers.

Suppose the internal address of some $ c\in\M$ has the form $ 1\IntAddr S_1\IntAddr\cdots \IntAddr S_k\IntAddr S_{k+1}\dots$ with $ S_k=n$ and all $ S_m\ge n$ are divisible by $ n$. There is a component $ W$ with address $ 1\IntAddr S_1\IntAddr\cdots \IntAddr S_k$ so that $ c\in\wake{W}$. If $ c\not\in\M_W$, then $ c$ is separated from $ \M_W$ by a Misiurewicz-Thurston parameter $ c_*\in\M_W$ which is the tuning image of a dyadic Misiurewicz-Thurston parameter [see [Douady1986], [Sect. 8] in [Milnor2000a], or ([Bruin et al.2016, Corollary 9.27])]. Therefore, the internal address of $ c_*$ contains all integers that are divisible by $ n$ and sufficiently large, say at least $ Kn$. Let $ S$ be the first entry in the internal address of $ c$ that corresponds to a component "behind $ \M_W$" (so that it is separated from $ \M_W$ by $ c_*$). The long internal address of $ c$ contains "behind" $ W$ only hyperbolic components of periods divisible by $ n$, and this implies that before and after $ c_*$ there must be two components of equal period (greater than $ Kn$) that are not separated by a ray pair of lower period. This contradicts Lavaurs' Lemma. ⬜

Remark.

In Definition 3.8, we defined angled internal addresses in parameter space, but they can also be defined dynamically: the angles are combinatorial rotation numbers of rays landing at characteristic periodic points the periods of which occur in the internal address. This allows us to give more dynamic proofs of several theorems that we proved in parameter space. For instance, changing numerators has no impact on whether an angled internal address is realized in the complex plane (Theorem 4.3): for the Hubbard trees at centers of hyperbolic components, this simply changes the embedding of the tree, but not the issue whether such a tree can be embedded [compare the discussion in ([Bruin et al.2016], Corollary 4.11 and Lemma 16.9)]. (The denominators are determined already by the internal address without angles; see Lemma 4.2). Similarly, the angled internal address completely determines a Hubbard tree with its embedding and thus the dynamics at the center of a hyperbolic component; this is Theorem 3.9: the (finite) angled internal address completely specifies every hyperbolic component.

6. Narrow Components and Admissibility

Angled internal addresses describe hyperbolic components, or more generally combinatorial classes, in $ \M$ uniquely. But are all possible angled internal addresses indeed realized ("admissible")? Since the angles have no impact on admissibility (Theorem 4.3), this depends only on the internal address without angles. An infinite internal address is admissible if and only if all its finite truncations are admissible (because $ \M$ is closed), so it suffices to investigate admissibility of all finite internal addresses $ 1\IntAddr \dots \IntAddr S_k$. An equivalent question is whether all sequences $ \ovl{\ajaja \nu_2\nu_3\dots \nu_{n-1}\aaa}$ with $ \nu_i\in\{\akaka,\ajaja\}$ are kneading sequences of periodic external angles.

Milnor and Thurston ([Milnor and Thurston1988]) classified all real-admissible kneading sequences, i.e., those sequences that occur for real quadratic polynomials: an $ n$-periodic sequence in $ \{\akaka,\ajaja\}^\Nzero$ is real-admissible if and only if it is the largest in a certain (non-lexicographic) order among its $ n$ periodic shifts; so roughly a fraction of $ 1/n$ of all $ n$-periodic sequences is real-admissible (note the analogy to our Lemma 6.5: if any periodic sequence is the largest, among all its shifts, with respect to lexicographic order, then it corresponds to a certain kind of (usually non-real) hyperbolic components that we call "purely narrow").

It turns out that not all sequences are (complex-)admissible either. This can be seen already by a statistical argument: there are $ 2^{n-2}$ possible kneading sequences of period $ n$ (starting the period with $ \ajaja$ and ending with $ \aaa$), and there are $ 2^n$ external rays of period $ n$, landing in pairs at $ 2^{n-1}$ hyperbolic components of period $ n$ (both up to terms of order $ O(2^{n/2})$ corresponding to exact periods strictly dividing $ n$). So every possible kneading sequence of period $ n$ should be realized on average by two hyperbolic components. Those on the real axis are only a small fraction, and all the others appear in pairs corresponding to complex conjugation of $ \M$ (or equivalently replacing in the angled internal address the first angle $ p_k/q_k\neq 1/2$ with $ 1-p_k/q_k$). But a significant fraction of internal addresses exists more than twice: whenever there is a denominator $ q_k\not\in\{2,3,4,6\}$, then there are more than two numerators possible, or also when there are at least two denominators other than $ 1/2$. The average time an admissible internal address is realized is thus greater than $ 2$, so that the non complex-admissible internal addresses have positive measure (among all $ \akaka$-$ \ajaja$-sequences with the product measure). In [Bruin and Schleicher2008], a complete classification of all complex admissible kneading sequences was given, but the criterion is rather non-trivial; and in [Bruin and Schleicher2013] it was shown that complex admissible kneading sequences have positive measure among all $ \akaka$-$ \ajaja$-sequences.

The simplest candidate internal address that is not complex admissible is $ 1\IntAddr 2\IntAddr 4\IntAddr 5\IntAddr 6$ with kneading sequence $ \nu=\ovl{\ajaja\akaka\ajaja\ajaja\akaka\akaka}$. If it existed, the kneading sequence "just before" the component would be $ \Abar(\nu)=\ovl{\ajaja\akaka\ajaja\ajaja\akaka\ajaja}$ of period $ 3$, so the period $ 6$ component (the limiting kneading sequence outside the wake) would be a bifurcation from a period $ 3$ component, but the period $ 3$ component does not show up in the internal address. Moreover, the period $ 3$ component would have kneading sequence $ \ovl{\ajaja\akaka\ajaja}$ with an infinite internal address! There is a hyperbolic component of period $ 3$ with internal address $ \ajaja\IntAddr 2\IntAddr 3$ and kneading sequence $ \ovl{\ajaja\akaka\akaka}$, so our candidate period $ 6$ component would bifurcate from this component "outside of the wake". So this internal address does not exist, and neither does any that starts with $ 1\IntAddr 2\IntAddr 4\IntAddr 5\IntAddr 6\IntAddr\dots$ It turns out ([Bruin and Schleicher2008]) that any non-existing kneading sequence is generated by a similar process. How can one describe all admissible internal addresses? Given any finite internal address $ 1 \IntAddr S_1 \IntAddr \cdots \IntAddr S_k$, what are the possible continuations $ S_{k+1} > S_k$? In this section, we give a precise answer for a particular kind of components that we call "purely narrow": a "narrow" component is one for which we have good control, and purely narrow means that we construct the internal address along components that are all narrow.

Every component $ W$ of period $ n$ has an associated internal address (finite, ending with entry $ n$); compare the remark before Definition 3.8. The component $ W$ also has an associated periodic kneading sequence $ \nu(W)$ consisting only of entries $ \akaka$ and $ \ajaja$: one way of defining this is to take any parameter ray $ R(\phi)$ with irrational $ \ph$ landing at $ \partial W$; then $ \nu(W):=\nu(\phi)$. Equivalently, let $ \RP(\theta,\theta')$ be the parameter ray pair landing at the root of $ W$; then $ \theta$ and $ \theta'$ have period $ n$ and $ \nu(\theta)=\nu(\theta')$ are $ \aaa$-periodic of period $ n$, and $ \nu(W)=\A(\nu(\theta))=\A(\nu(\theta'))$ (see Lemma 3.1 and Proposition 3.5). Of course, the internal address of $ W$ is the same as the internal address of $ R(\phi)$, $ R(\theta)$, $ R(\theta')$ or of $ \phi$, $ \theta$ or $ \theta'$.

Definition 6.1.

(Narrow component)

A hyperbolic component of period $ n$ is narrow if its wake contains no component of lower period, or equivalently if the wake has width $ 1/(2^n-1)$.

Remark.

It follows directly from (4.1) that if $ W'$ is a bifurcation from another component $ W$, then $ W'$ is narrow if and only if $ W$ has period $ 1$.

If $ W,W'$ are two hyperbolic components of periods $ n$ and $ n'$ so that $ W'$ is contained in the wake $ \wake{W}$ of $ W$, then we say that $ W'$ is visible from $ W$ if there exists no parameter ray pair of period less than $ n'$ that separates $ W$ and $ W'$. If $ n< n'$, then this is equivalent to the condition that the internal address of $ W'$ be formed by the internal address of $ W$, directly extended by the entry $ n'$.

Lemma 6.2.

(Visible components from narrow component) For every narrow hyperbolic component of period $ n$, there are visible components of all periods greater than $ n$. More precisely, every $ p/q$ sublimb contains exactly $ n$ visible components: exactly one component each of period $ qn-(n-1)$, $ qn-(n-2)$,..., $ qn$.

Proof.

Let $ W$ be a narrow hyperbolic component of period $ n$ and consider its $ p/q$-subwake. The visible components in this wake have periods at most $ qn$. First we show that any two visible components within the $ p/q$-subwake of $ W$ have different internal addresses. By way of contradiction, suppose there are two components $ W_1$ and $ W_2$ of equal period $ m\le qn$ with the same internal address. By Theorem 3.9 there must be another hyperbolic component $ W'$ in the same subwake of $ W$ so that $ W_1$ and $ W_2$ are in different $ p_1/q'$- and $ p_2/q'$-subwakes of $ W'$ with $ q'\ge 3$ (different hyperbolic components with the same internal address must have different angled internal addresses). Let $ n'> n$ be the period of $ W'$. Then the width of these subwakes is, according to (4.1), \begin{eqnarray*} |W'|\frac{(2^{n'}-1)^2}{2^{q'n'}-1} &\le& |W| \frac{(2^n-1)^2}{2^{qn}-1}\cdot\frac{(2^{n'}-1)^2}{2^{q'n'}-1} = \frac{2^n-1}{2^{qn}-1}\cdot\frac{(2^{n'}-1)^2}{2^{q'n'}-1} \\ &< & \frac{2^n(2^{(2-q')n'})(1-2^{-q'n'})^{-1}} {2^{qn}-1} \le \frac{2^{(n-n')} \cdot 2} {2^{qn}-1} \le \frac{1}{2^{qn}-1} \end{eqnarray*} because $ W'$ is contained in the $ p/q$-wake of $ W$. But this is not large enough to contain a component of period at most $ qn$, a contradiction.

The next step is to prove that every subwake of $ W$ of denominator $ q$ contains exactly $ 2^k$ external rays with angles $ a/(2^{(q-1)n+k}-1)$ for $ 1\leq k\leq n$, including the two rays bounding the wake. In fact, the width of the wake is $ (2^n-1)/(2^{qn}-1)$, so the number of rays one expects by comparing widths of wakes is \begin{eqnarray*} \frac{(2^n-1)(2^{(q-1)n+k}-1)}{2^{qn}-1}= 2^k+\frac{2^k-2^n-2^{(q-1)n+k}+1}{2^{qn}-1} =: 2^k+\alpha, \end{eqnarray*} where an easy calculation shows that $ -1\leq\alpha< 1$ and that $ \alpha=-1$ can occur only for $ k=n$. The actual number of rays can differ from this expected value by no more than one and is even, hence equal to $ 2^k$. Moreover, no such ray of angle $ a/(2^{(q-1)n+k}-1)$ can have period smaller than $ (q-1)n+k$ because otherwise it would land at a hyperbolic component of some period dividing $ (q-1)n+k$; but in the considered wake there would not be room enough to contain a second ray of equal period. This shows that, for any $ k\leq n$, the number of hyperbolic components of period $ m=(q-1)n+k$ in any subwake of $ W$ of denominator $ q$ equals $ 2^{k-1}$. They must all have different internal addresses. The single component of period $ (q-1)n+1$ takes care of the case $ k=1$, and its wake subdivides the $ p/q$-subwake of $ W$ into two components. There are two components of period $ (q-1)n+2$, and since their internal addresses are different, exactly one of them must be in the wake of the component of period $ (q-1)n+1$, while the other is not; the latter one is visible from $ W$. (The non-visible component is necessarily narrow, while the visible component may or may not be narrow.)

So far we have taken are of $ 3$ components, and their wake boundaries subdivide the $ p/q$-subwake of $ W$ into $ 4$ pieces. Each piece most contain one component of period $ (q-1)n+3$, so exactly one of these components is visible from $ W$, and so on. The argument continues as long as we have uniqueness of components for given internal addresses, which is for $ k\le n$. ⬜

Lemma 6.3.

(Narrow visible components from narrow component)

Suppose $ W$ and $ W'$ are two hyperbolic components of periods $ n$ and $ n+s$ with $ s> 0$ so that $ W$ is narrow and $ W'$ is visible from $ W$. Let $ k\in\{1,\dots,n-1,n\}$ be so that $ s\equiv k$ modulo $ n$. Then the question whether or not $ W'$ is narrow depends only on the first $ k$ entries in the kneading sequence of $ W$ (but not otherwise on $ W$).

Proof.

Proposition 6.4.

(Combinatorics of purely narrow components)

Consider a hyperbolic component $ W$ with internal address $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_k$ and associated kneading sequence $ \nu$. Suppose that $ \nu_{S_i}=\akaka$ for all $ i=1=2,\dots,k$. Then $ W$ is narrow and for every $ S_{k+1}> S_k$, there exists a hyperbolic component with internal address $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_k\IntAddr S_{k+1}$; it is narrow if and only if $ \nu_{S_{k+1}}=\ajaja$.

Proof.

We prove the claim by induction on the length of internal addresses, starting with the address $ 1$ of length $ 0$. The associated component has period $ 1$, it is narrow and has $ \nu=\ovl{\ajaja}$, and there is no condition on $ \nu_{S_{i}}$ to check. For every $ S_1> 1$, there exists a hyperbolic component with internal address $ 1\IntAddr S_1$ (these are components of period $ S_1$ bifurcating immediately from the main cardioid). By the remark after Definition 6.1, these components are narrow, and indeed $ \nu_{S_1}=\ajaja$.

Now assume by induction that the claim is true for a narrow component $ W_{k-1}$ with internal addresses $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_{k-1}$ of length $ k-1$ and associated kneading sequence $ \mu$. We will prove the claim when $ W$ is a hyperbolic component with internal address $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_{k-1}\IntAddr S_k$ of length $ k$ and with associated internal address $ \nu$ so that $ \nu_{S_k}=\akaka$. First we show that $ W$ is narrow: by the inductive hypothesis, it is narrow if and only if $ \mu_{S_k}=\ajaja$, but this is equivalent to $ \nu_{S_k}=\akaka$ because $ \nu$ and $ \mu$ first differ at position $ S_k$.

Consider some integer $ S_{k+1}> S_k$. By Lemma 6.2, there exists another component $ W_{k+1}$ with internal address $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_k\IntAddr S_{k+1}$. We need to show that it is narrow if and only if $ \nu_{S_{k+1}}=\ajaja$. If $ S_{k+1}$ is a proper multiple of $ S_k$, then the assumption $ \nu_{S_k}=\akaka$ implies $ \nu_{S_{k+1}}=\akaka$, and we have to show that $ W_{k+1}$ is not narrow. Indeed, $ W_{k+1}$ is a bifurcation from $ W$ by Lemma 4.1, and by the remark after Definition 6.1 bifurcations are narrow if and only if they bifurcate from the period $ 1$ component. We can thus write $ S_{k+1}=qS_k+S'_{k+1}$ with $ S'_{k+1}\in\{S_k+1,S_k+2,\dots,2S_k-1\}$ and $ q\in\Nzero$. Again by Lemma 6.2, there exists another component $ W'_{k+1}$ with internal address $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_k\IntAddr S'_{k+1}$; by Lemma 6.3, it is narrow if and only if $ W_{k+1}$ is. But $ \nu$ has period $ S_k$, so $ \nu_{S_{k+1}}=\nu_{S'_{k+1}}$ and the claim holds for $ S_{k+1}$ if and only if it holds for $ S'_{k+1}$. It thus suffices to restrict attention to the case $ S_{k+1}< 2S_k$. Whether or not $ W_{k+1}$ is narrow is determined by the initial $ S_{k+1}-S_k< S_k$ entries in $ \nu$.

Now we use the inductive hypothesis for $ W_{k-1}$: there exists a component $ W'$ with address $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_{k-1}\IntAddr (S_{k-1}+S_{k+1}-S_k)$, and it is narrow if and only if $ \mu_{S_{k-1}+S_{k+1}-S_k}=\ajaja$. The kneading sequences $ \mu$ and $ \nu$ first differ at position $ S_k$, so their initial $ S_{k+1}-S_k$ entries coincide. Therefore, again by Lemma 6.3, the component $ W'$ is narrow if and only if $ W_{k+1}$ is. Therefore, $ W_{k+1}$ is narrow if and only if $ \mu_{S_{k-1}+S_{k+1}-S_k}=\ajaja$. Finally, we have \begin{eqnarray*} \nu_{S_{k+1}}=\nu_{S_{k+1}-S_k}=\mu_{S_{k+1}-S_k}=\mu_{S_{k-1}+S_{k+1}-S_k} \end{eqnarray*} by periodicity of $ \nu$ (period $ S_k$) and of $ \mu$ (period $ S_{k-1}$) and because the first difference between $ \nu$ and $ \mu$ occurs at position $ S_k> S_{k+1}-S_k$. This proves the proposition. ⬜

Remark.

We call a hyperbolic component $ W_{k+1}$ with internal address $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_k\IntAddr S_{k+1}$ and associated kneading sequence $ \nu$ purely narrow if $ \nu_{S_i}=\akaka$ for $ i=1,2,\dots,k+1$. Proposition 6.4 implies that this is equivalent to the condition that $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_{i-1}\IntAddr S_{i}$ describes a narrow component for $ i=1,\dots,k+1$ (hence the name). The asymmetry in the statement of the proposition (for narrow components, the last entry in $ \nu$ must be $ \ajaja$, rather than $ \akaka$ for all earlier components) is because the condition is with respect to the kneading sequence of period $ S_k$, not with respect to the sequence of period $ S_{k+1}$ associated to $ W_{k+1}$.

Remark.

For every narrow hyperbolic component, the previous results allow to construct combinatorially the trees of visible components within any sublimb; for purely narrow components, the global tree structure can thus be reconstructed by what we call "growing of trees": see Fig. 3. These issues have been explored further by Kauko ([Kauko2000]).

Figure 3 Left For a narrow hyperbolic component $ W$ (here of period $ 5$), the trees of visible components within any $ p/q$- and $ p'/q'$-subwake are the same when adding $ (q'-q)n$ to the periods of all components in the $ p/q$-subwake; this "translation principle" follows directly from Lemma 6.3, using just the combinatorics of internal addresses or kneading sequences (even the embeddings of the trees are the same; this follows from comparisons with dynamical planes). Right if $ W'$ (here of period $ 19$) is visible from $ W$ and both are narrow, then the tree of visible components within the $ 1/2$-subwake of $ W'$ can be reconstructed from the trees of visible components within various subwakes of $ W$: if $ n'$ and $ n$ are the periods of $ W'$ and $ W$, then the tree formed by the visible components in the $ 1/2$-subwake of $ W'$ of periods $ n'+1$, ..., $ 2n'-1$ (excluding the bifurcating component of period $ n'$) equals the tree formed by the visible components of periods $ n+1,\dots,n+n'-1$ in the $ 1/q$-subwakes of $ W$, adding $ n'-n$ to all periods .

Lemma 6.5.

(Maximal shift of kneading sequence)

For a kneading sequence $ \nu$ (without $ \aaa$) with associated internal address $ 1\IntAddr S_1\IntAddr \cdots \IntAddr S_k\IntAddr\cdots$ (finite or infinite), the following are equivalent:

(1) No shift $ \sigma^k(\nu)$ exceeds $ \nu$ with respect to lexicographic ordering;
(2) For every $ r\ge 1$, we have $ \nu_{\rho(r)}=\akaka$.
(3) For every $ k\ge 1$, we have $ \nu_{S_k}=\akaka$.

In all these cases, $ r< S_k$ implies $ \rho(r)\le S_k$.

Proof.

Proposition 6.6.

(Internal address of purely narrow hyp. component)

Every finite internal address of the type described in Lemma 6.5 is realized and corresponds to purely narrow hyperbolic components, and conversely each purely narrow hyperbolic component has an internal address of this type.

Proof.

Remark.

The existence of kneading sequences as described in this result can also be derived from the general admissibility condition on kneading sequences, see ([Bruin et al.2016, Corollary 5.20]) (this is a more abstract and difficult, but also more general result). Schmeling observed that sequences that are maximal shifts with respect to the lexicographic order are exactly the fixed points of the map $ \theta\mapsto\nu(\theta)$. Recently, Buff and Tan Lei ([Buff and Lei2014]) observed that this gives a simple combinatorial proof of the existence of kneading sequences as in Lemma 6.5, without interpretation in parameter space.

7. Symbolic Dynamics and Permutations

Acknowledgements.

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[Hubbard1993] Hubbard, J.: Local connectivity of Julia sets and bifurcation loci: three theorems of J.-C. Yoccoz. In: Goldberg, L.R., Philipps, A.V. (eds.) Topological Methods in Modern Mathematics, pp. 467–511. Publish or Perish, Houston (1993)
[Kauko2000] Kauko, V.: Trees of visible components in the Mandelbrot set. Fund. Math. 164 , 41–60 (2000)
[Lau and Schleicher1994] Lau, E., Schleicher, D.: Internal addresses in the Mandelbrot set and irreducibility of polynomials, Stony Brook Preprint 19 (1994)
[Lavaurs1986] Lavaurs, P.: Une description combinatoire de l'involution définie par M sur les rationnels à dénominateur impair, vol. 303, pp. 143–146. C. R. Acad. Sci. Paris, Série I Math. (1986)
[Milnor and Thurston1988] Milnor, J., Thurston, W.: On Iterated Maps of the Interval, vol. 1342, pp. 465–563. Springer, Berlin (1988)
[Milnor2000b] Milnor, J.: Local connectivity of Julia sets: expository lectures. In: Tan Lei (ed.) The Mandelbrot Set, Theme and Variations, London Math. Soc. Lecture Note Ser. 274, pp. 67–116. Cambridge Univ. Press, Cambridge (2000)
[Milnor2000a] Milnor, J.: Periodic orbits, external rays, and the Mandelbrot set: an expository account. Astérisque 261 , 277–333 (2000)
[Milnor2005] Milnor, J.: Dynamics in One Complex Variable, 3rd edn. Princeton University Press, Princeton (2005)
[Morton and Patel1994] Morton, P., Patel, P.: The Galois theory of periodic points of polynomial maps. Proc. Lond. Math. Soc. 68 (3), 225–263 (1994)
[Mündlein1997] Mündlein, H.: Permutationen vorperiodischer Punkte bei Iteration unikritischer Polynome (permutations of preperiodic points under iteration of unicritical polynomials). Diploma thesis, Technische Universität München (1997)
[1994] Penrose, C.: On quotients of shifts associated with dendrite Julia sets of quadratic polynomials. University of Coventry, Thesis (1994)
[Riedl2000] Riedl, J: Arcs in Multibrot sets, locally connected Julia sets and their construction by quasiconformal surgery. Ph.D. thesis, Technische Universität München (2000)
[Riedl and Schleicher1998] Riedl, J., Schleicher, D.: On the locus of crossed renormalization. Proc. Res. Inst. Math. Sci. Kyoto 4 , 21–54 (1998)
[Schleicher2004] Schleicher, D.: On fibers and local connectivity of Mandelbrot and Multibrot sets. In: Lapidus, M., van Frankenhuysen, M. (eds) Fractal Geometry and Applications: A Jubilee of Beno^[U+0131]t Mandelbrot. In: Proceedings of Symposia in Pure Mathematics 72, American Mathematical Society, pp. 477–507 (2004)
[Schleicher2000] Schleicher, D.: Rational external rays of the Mandelbrot set. Asterisque 261 , 405–443 (2000)
[Thurston2009] Thurston, W.: On the geometry and dynamics of iterated rational maps. In: Schleicher, D., Peters, A.K. (eds.) Complex Dynamics: Families and Friends, pp. 3–137. Wellesley (2009)

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× Hubbard, J.: Local connectivity of Julia sets and bifurcation loci: three theorems of J.-C. Yoccoz. In: Goldberg, L.R., Philipps, A.V. (eds.) Topological Methods in Modern Mathematics, pp. 467–511. Publish or Perish, Houston (1993)
× Kauko, V.: Trees of visible components in the Mandelbrot set. Fund. Math. 164 , 41–60 (2000)
× Lau, E., Schleicher, D.: Internal addresses in the Mandelbrot set and irreducibility of polynomials, Stony Brook Preprint 19 (1994)
× Lavaurs, P.: Une description combinatoire de l'involution définie par M sur les rationnels à dénominateur impair, vol. 303, pp. 143–146. C. R. Acad. Sci. Paris, Série I Math. (1986)
× Milnor, J., Thurston, W.: On Iterated Maps of the Interval, vol. 1342, pp. 465–563. Springer, Berlin (1988)
× Milnor, J.: Local connectivity of Julia sets: expository lectures. In: Tan Lei (ed.) The Mandelbrot Set, Theme and Variations, London Math. Soc. Lecture Note Ser. 274, pp. 67–116. Cambridge Univ. Press, Cambridge (2000)
× Milnor, J.: Periodic orbits, external rays, and the Mandelbrot set: an expository account. Astérisque 261 , 277–333 (2000)
× Milnor, J.: Dynamics in One Complex Variable, 3rd edn. Princeton University Press, Princeton (2005)
× Morton, P., Patel, P.: The Galois theory of periodic points of polynomial maps. Proc. Lond. Math. Soc. 68 (3), 225–263 (1994)
× Mündlein, H.: Permutationen vorperiodischer Punkte bei Iteration unikritischer Polynome (permutations of preperiodic points under iteration of unicritical polynomials). Diploma thesis, Technische Universität München (1997)
× Penrose, C.: On quotients of shifts associated with dendrite Julia sets of quadratic polynomials. University of Coventry, Thesis (1994)
× Riedl, J: Arcs in Multibrot sets, locally connected Julia sets and their construction by quasiconformal surgery. Ph.D. thesis, Technische Universität München (2000)
× Riedl, J., Schleicher, D.: On the locus of crossed renormalization. Proc. Res. Inst. Math. Sci. Kyoto 4 , 21–54 (1998)
× Schleicher, D.: On fibers and local connectivity of Mandelbrot and Multibrot sets. In: Lapidus, M., van Frankenhuysen, M. (eds) Fractal Geometry and Applications: A Jubilee of Beno^[U+0131]t Mandelbrot. In: Proceedings of Symposia in Pure Mathematics 72, American Mathematical Society, pp. 477–507 (2004)
× Schleicher, D.: Rational external rays of the Mandelbrot set. Asterisque 261 , 405–443 (2000)
× Thurston, W.: On the geometry and dynamics of iterated rational maps. In: Schleicher, D., Peters, A.K. (eds.) Complex Dynamics: Families and Friends, pp. 3–137. Wellesley (2009)

Abstract

Keywords

Mathematics Subject Classification

1. Introduction

2. Background

Theorem 2.1.

Corollary 2.2.

Lemma 2.3.

Definition 2.4.

3. Internal Addresses of the Mandelbrot Set

Lemma 3.1.

Proof.

Algorithm 3.2.

Remark.

Definition 3.3.

Definition 3.4.

Proposition 3.5.

Proof.

Corollary 3.6.

Proof.

Corollary 3.7.

Proof.

Remark.

Definition 3.8.

Theorem 3.9.

4. The Geometry of Internal Addresses

Lemma 4.1.

Lemma 4.2.

Theorem 4.3.

Lemma 4.4.

Lemma 4.5.

Proposition 4.6.

Proof.

Proposition 4.7.

Remark.

5. Proofs About Internal Addresses

Definition 5.1.

Proposition 5.2.

Proof.

Remark.

Proof of Theorem 3.9.

Proof of Lemma 4.1.

Lemma 5.3.

Proof.

Proof of Lemma 4.2.

Proof of Theorem 4.3.

Proof of Lemma 4.4.

Proof of Lemma 4.5.

Proof of Proposition 4.7.

Remark.

6. Narrow Components and Admissibility

Definition 6.1.

Remark.

Lemma 6.2.

Proof.

Lemma 6.3.

Proof.

Proposition 6.4.

Proof.

Remark.

Remark.

Lemma 6.5.

Proof.

Proposition 6.6.

Proof.

Remark.

7. Symbolic Dynamics and Permutations

Theorem 7.1.

Conjecture 7.2.

Theorem 7.3.

Lemma 7.4.

Proof.

Lemma 7.5.

Proof.

Proposition 7.6.

Proof.

Proof of Theorem 7.3.

Corollary 7.7.

Corollary 7.8.

Proof.