External ray

An external ray is a curve that runs from infinity toward a Julia or Mandelbrot set. This curve is only sometimes a half-line ( ray ) but is called ray because it is image of ray.

External rays are used in complex analysis, particularly in complex dynamics and geometric function theory,

History

External rays were introduced in Douady and Hubbard's study of the Mandelbrot set

Notation

External rays of (connected) Julia sets on dynamical plane are often called dynamic rays.

External rays of the Mandelbrot set (and similar one-dimensional connectedness loci) on parameter plane are called parameter rays.

Polynomials

Dynamical plane = z-plane

External rays are associated to a compact, full, connected subset $K\,$ of the complex plane as :

• the images of radial rays under the Riemann map of the complement of $K\,$
• the gradient lines of the Green's function of $K\,$
• field lines of Douady-Hubbard potential
• an integral curve of the gradient vector field of the Green's function on neighborhood of infinity

External rays together with equipotential lines of Douady-Hubbard potential ( level sets) form a new polar coordinate system for exterior ( complement ) of $K\,$.

In other words the external rays define vertical foliation which is orthogonal to horizontal foliation defined by the level sets of potential.

Uniformization

Let $\Psi_c\,$ be the mapping from the complement (exterior) of the closed unit disk $\overline{\mathbb{D}}$ to the complement of the filled Julia set $\ Kc$.

$\Psi_c:\mathbb{\hat{C}}\setminus \overline{\mathbb{D}}\to\mathbb{\hat{C}}\setminus Kc$

and Boettcher map (function) $\Phi_c\,$, which is uniformizing map of basin of attraction of infinity , because it conjugates complement of the filled Julia set $\ Kc$ and the complement (exterior) of the closed unit disk

$\Phi_c: \mathbb{\hat{C}}\setminus Kc \to \mathbb{\hat{C}}\setminus \overline{\mathbb{D}}$

where :

$\mathbb{\hat{C}}$ denotes the extended complex plane

Boettcher map $\Phi_c\,$ is an isomorphism :

$\Psi_c = \Phi_{c}^{-1} \,$

$w = \Phi_c(z) = \lim_{n\rightarrow \infty} (f_c^n(z))^{2^{-n}}$

where :

$z \in \mathbb{\hat{C}}\setminus K_c$

$w \in \mathbb{\hat{C}}\setminus \overline{\mathbb{D}}$

$w\,$ is a Boettcher coordinate

Formal definition of dynamic ray

polar coordinate system and Psi_c for c=-2

The external ray of angle $\theta\,$ is:

• the image under $\Psi_c\,$ of straight lines $\mathcal{R}_{\theta} = \{\left(r*e^{2\pi i \theta}\right) : \ r > 1 \}$

$\mathcal{R}^K _{\theta} = \Psi_c(\mathcal{R}_{\theta})$

• set of points of exterior of filled-in Julia set with the same external angle θ

$\mathcal{R}^K _{\theta} = \{ z\in \mathbb{\hat{C}}\setminus Kc  : \arg(\Phi_c(z)) = \theta \}$

Parameter plane = c-plane

Uniformization

Boundary of Mandelbrot set as an image of unit circle under $\Psi_M\,$

Uniformization of complement (exterior) of Mandelbrot set

Let $\Psi_M\,$ be the mapping from the complement (exterior) of the closed unit disk $\overline{\mathbb{D}}$ to the complement of the Mandelbrot set $\ M$.

$\Psi_M:\mathbb{\hat{C}}\setminus \overline{\mathbb{D}}\to\mathbb{\hat{C}}\setminus M$

and Boettcher map (function) $\Phi_M\,$, which is uniformizing map of complement of Mandelbrot set , because it conjugates complement of the Mandelbrot set $\ M$ and the complement (exterior) of the closed unit disk

$\Phi_M: \mathbb{\hat{C}}\setminus M \to \mathbb{\hat{C}}\setminus \overline{\mathbb{D}}$

it can be normalized so that :

$\frac{\Phi_M(c)}{c} \to 1 \ as\ c \to \infty \,$

where :

$\mathbb{\hat{C}}$ denotes the extended complex plane

Jungreis function $\Psi_M\,$ is the inverse of uniformizing map :

$\Psi_M = \Phi_{M}^{-1} \,$

In the case of complex quadratic polynomial one can compute this map using Laurent series about infinity

$c = \Psi_M (w) = w + \sum_{m=0}^{\infty} b_m w^{-m} = w -\frac{1}{2} + \frac{1}{8w} - \frac{1}{4w^2} + \frac{15}{128w^3} + ...\,$

where

$c \in \mathbb{\hat{C}}\setminus M$

$w \in \mathbb{\hat{C}}\setminus \overline{\mathbb{D}}$

Formal definition of parameter ray

The external ray of angle $\theta\,$ is:

• the image under $\Psi_c\,$ of straight lines $\mathcal{R}_{\theta} = \{\left(r*e^{2\pi i \theta}\right) : \ r > 1 \}$

$\mathcal{R}^M _{\theta} = \Psi_M(\mathcal{R}_{\theta})$

• set of points of exterior of Mandelbrot set with the same external angle θ

$\mathcal{R}^M _{\theta} = \{ c\in \mathbb{\hat{C}}\setminus M  : \arg(\Phi_M(c)) = \theta \}$

Definition of $\Phi_M \,$

Douady and Hubbard define:

$\Phi_M(c) \ \overset{\underset{\mathrm{def}}{}}{=} \ \Phi_c(z=c)\,$

so external angle of point $c\,$ of parameter plane is equal to external angle of point $z=c\,$ of dynamical plane

External angle

Angle $\theta\,$ is named external angle ( argument ).

Principal value of external angles are measured in turns modulo 1

1 turn = 360 degrees = 2 * Pi radians

Compare different types of angles :

• external ( point of set's exterior )
• internal ( point of component's interior )
• plain ( argument of complex number )

	external angle	internal angle	plain angle
parameter plane	$arg(\Phi_M(c)) \,$	$arg(\rho_n(c)) \,$	$arg(c) \,$
dynamic plane	$arg(\Phi_c(z)) \,$		$arg(z) \,$

Computation of external argument

• argument of Böttcher coordinate as an external argument
  • $arg_M(c) = arg(\Phi_M(c)) \,$
  • $arg_c(z) = arg(\Phi_c(z)) \,$
• kneading sequence as a binary expansion of external argument

Transcendental maps

For transcendental maps ( for example exponential ) infinity is not a fixed point but an essential singularity and there is no Boettcher isomorphism.

Here dynamic ray is defined as a curve :

• connecting a point in an escaping set and infinity
• lying in an escaping set

Images

Dynamic rays

• 

  Julia set for f_c(z) = z² − 1 with 2 external ray landing on repelling fixed point alpha

• 

  Julia set and 3 external rays landing on fixed point $\alpha_c\,$

• 

  Julia set with external rays landing on period 3 parabolic orbit

Mandelbrot set for complex quadratic polynomial with parameter rays of root points

• 

  External rays for angles of the form  : n / ( 2¹ - 1) (0/1; 1/1) landing on the point c= 1/4, which is cusp of main cardioid ( period 1 component)

• 

  External rays for angles of the form  : n / ( 2² - 1) (1/3, 2/3) landing on the point c= - 3/4, which is root point of period 2 component

• 

  External rays for angles of the form : n / ( 2³ - 1) (1/7,2/7) (3/7,4/7) landing on the point c= -1.75 = -7/4 (5/7,6/7) landing on the root points of period 3 components.

• 

  External rays for angles of form  : n / ( 2⁴ - 1) (1/15,2/15) (3/15, 4/15) (6/15, 9/15) landing on the root point c= -5/4 (7/15, 8/15) (11/15,12/15) (13/15, 14/15) landing on the root points of period 4 components.

• 

  External rays for angles of form  : n / ( 2⁵ - 1) landing on the root points of period 5 components

Parameter space of the complex exponential family f(z)=exp(z)+c. Eight parameter rays landing at this parameter are drawn in black.

Center, root, external and internal ray

internal ray of main cardioid of angle 1/3:
starts from center of main cardioid c=0
ends in the root point of period 3 component
which is the landing point of parameter (external) rays of angles 1/7 and 2/7

Programs that can draw external rays

• Mandel - program by Wolf Jung written in C++ using Qt with source code available under the GNU General Public License
• Java applets by Evgeny Demidov ( code of mndlbrot::turn function by Wolf Jung has been ported to Java ) with free source code
• OTIS by Tomoki KAWAHIRA - Java applet without source code
• Spider XView program by Yuval Fisher
• YABMP by Prof. Eugene Zaustinsky for DOS without source code
• DH_Drawer by Arnaud Chéritat written for Windows 95 without source code
• Linas Vepstas C programs for Linux console with source code
• Program Julia by Curtis T McMullen written in C and Linux commands for C shell console with source code
• mjwinq program by Matjaz Erat written in delphi/windows without source code ( For the external rays it uses the methods from quad.c in julia.tar by Curtis T McMullen)
• RatioField by Gert Buschmann, for windows with Pascal source code for Dev-Pascal 1.9.2 (with Free Pascal compiler )
• Mandelbrot program by Milan Va, written in Delphi with source code
• Power MANDELZOOM by Robert Munafo
• ruff by Claude Heiland-Allen

See also

• external rays of Misiurewicz point
• Orbit portrait
• Periodic points of complex quadratic mappings
• Prouhet-Thue-Morse constant
• Carathéodory's theorem
• Field lines of Julia sets

References

• Lennart Carleson and Theodore W. Gamelin, Complex Dynamics, Springer 1993
• Adrien Douady and John H. Hubbard, Etude dynamique des polynômes complexes, Prépublications mathémathiques d'Orsay 2/4 (1984 / 1985)
• John W. Milnor, Periodic Orbits, External Rays and the Mandelbrot Set: An Expository Account; Géométrie complexe et systèmes dynamiques (Orsay, 1995), Astérisque No. 261 (2000), 277–333. (First appeared as a Stony Brook IMS Preprint in 1999, available as arXiV:math.DS/9905169.)
• John Milnor, Dynamics in One Complex Variable, Third Edition, Princeton University Press, 2006, ISBN 0-691-12488-4
• Wolf Jung : Homeomorphisms on Edges of the Mandelbrot Set. Ph.D. thesis of 2002

External links

• Hubbard Douady Potential, Field Lines by Inigo Quilez
• Drawing Mc by Jungreis Algorithm
• Internal rays of components of Mandelbrot set
• John Hubbard's presentation, The Beauty and Complexity of the Mandelbrot Set, part 3.1
• videos by ImpoliteFruit
• Milan Va. "Mandelbrot set drawing". http://sweb.cz/milan_va/Mandelbrot/. Retrieved 2009-06-15.