Fractals/Iterations in the complex plane/parabolic

"Most programs for computing Julia sets work well when the underlying dynamics is hyperbolic but experience an exponential slowdown in the parabolic case." (Mark Braverman)[1]

In other words it means that one can need days for making a good picture of parabolic Julia set with standard / naive algorithms.

There are 2 problems here:

• slow (= lazy) local dynamics (in the neighbourhood of a parabolic fixed point)
• some parts are very thin (hard to find using standard plane scanning)

Planes

Dynamic plane

Dynamic plane = complex z-plane ${\displaystyle \mathbb {C} =J_{f}\cup F_{f}}$ :

• Julia set ${\displaystyle J_{f}\subset \mathbb {C} }$  is a common boundary: ${\displaystyle J_{f}\,=\partial A_{f}(p)=\partial A_{f}(\infty )}$
• Fatou set ${\displaystyle F_{f}\subset \mathbb {C} }$
• exterior of Julia set = basin of attraction to infinity: ${\displaystyle A_{f}(\infty )\ {\overset {\underset {\mathrm {def} }{}}{=}}\ \{z\in \mathbb {C} :f^{(k)}(z)\to \infty \ as\ k\to \infty \}.}$
• interior of Julia set = basin of attraction of finite, parabolic fixed point p: ${\displaystyle A_{f}(p)\ {\overset {\underset {\mathrm {def} }{}}{=}}\ \{z\in \mathbb {C} :f^{(k)}(z)\to p\ as\ k\to \infty \}.}$
• immediate basin = sum of components which have parabolic fixed point p on it's boundary  ; the immediate parabolic basin of p is the union of periodic connected components of the parabolic basin.
• attracting Lea-Fatou flower = sum of n attracting petals = sum of 2*n attracting sepals
• petal = part of the flower. Each petal contains part of 2 sepals,
• sepals (Let 1 be an invariant curve in the first quadrant and L 1 the region enclosed by 1 ∪ {0}, called a sepal.)[2]

${\displaystyle \mathbb {C} \supset F(f)\supset A_{f}(p)}$

• Filled Julia set[3] ${\displaystyle K_{f}}$

Key words

• parabolic chessboard or checkerboard
• parabolic implosion
• Fatou coordinate
• Hawaiian earring
• in wikipedia = "The plane figure formed by a sequence of circles that are all tangent to each other at the same point and such that the sequence of radii converges to zero." (Barile, Margherita in MathWorld)[4]
• commons:Category:Hawaiian earrings
• Gevrey symptotic expansions
• the Écalle-Voronin invariants of (7.1) at the origin which have Gevrey- 1/2 asymptotic expansions[5]
• germ[6][7][8]
• germ of the function: Taylor expansion of the function
• multiplicity[9]
• Julia-Lavaurs sets
• The Leau-Fatou flower theorem:[10] repelling or attracting flower. Flower consist of petals
• Leau-Fatou flower
• Parabolic Linearization theorem
• The horn map
• Blaschke product
• Inou and Shishikura's near parabolic renormalization
• complex polynomial vector field[11]
• numbers
• "a positive integer ν, the parabolic degeneracy with the following property: there are νq attracting petals and νq repelling petals, which alternate cyclically around the fixed point."[12]
• combinatorial rotation number
• a Poincaré linearizer of function f at parabolic fixed point[13]
• "the parabolic pencil. This is the family of circles which all have one common point, and thus are all tangent to each other, either internally or externally."[14]

Leau-Fatou flower theorem

The Leau-Fatou flower theorem states that, if function ${\displaystyle f^{n}}$  has the Taylor expansion[15]

${\displaystyle f^{n}(z)=z+az^{n+1}+O(z^{n+2})}$

then the complex number ${\displaystyle v}$  on the unit circle ${\displaystyle v\in S^{1}}$  describes unit vector ( direction):

• an repulsion vector (from origin to v) for f at the origin if ${\displaystyle nav^{n}=1}$
• an attraction vector ( from v to origin ) if ${\displaystyle nav^{n}=-1}$

There are n equally spaced attracting directions, separated by n equally spaced repelling directions. The integer n+1 is called the multiplicity of fixed point

Examples

Function expansion: z+z^5

/* Maxima CAS */
display2d:false;
taylor(z+z^5, z,0,5);

z + z^5


So :

• a = 1
• n = 4

Compute attracting directions:

/* Maxima CAS */
solve (4*v^4 = 1, v);

[v = %i/sqrt(2),v = -1/sqrt(2),v = -%i/sqrt(2),v = 1/sqrt(2)]


Function m*z+z^2

(%i7) taylor(m*z+z^2, z,0,5);
(%o7) m*z+z^2



So :

• a = 1
• n = 1
(%i9) solve (v = 1, v);
(%o9) [v = 1]


Ecalle cylinder

Ecalle cylinders[16] or Ecalle-Voronin cylinders (by Jean Ecalle[17][18])[19]

"... the quotient of a petal P under the equivalence relation identifying z and f (z) if both z and f (z) belong to P. This quotient manifold is called the Ecalle cilinder, and it is conformally isomorphic to the infinite cylinder C/Z"[20]

eggbeater dynamics

Physical model: the behaviour of cake when one uses eggbeater.

The mathematical model: a 2D vector field with 2 centers (second-order degenerate points)[21][22]

The field is spinning about the centers, but does not appear to be diverging.

Maybe better description of parabolic dynamics will be Hawaiian earrings

parabolic germ

Germ:[23][24][25]

• z+z^2
• z+z^3
• z+z^{k+1}
• z+a_{k+1}z^{k+1}
• z+a_{k+1}z^{k+1}
• "a germ ${\displaystyle f(x)=a_{1}x+a_{2}x^{2}+\ldots }$  with ${\displaystyle \vert a_{1}\vert \neq 0,1}$  is holomorphically conjugated to its linear part ${\displaystyle g(x)=a_{1}z}$ " (Sylvain Bonnot)[26]

germ of vector field

The horn map

"the horn map h = Φ ◦ Ψ, where Φ is a shorthand for Φattr and Ψ for Ψrep (extended Fatou coordinate and parameterizations)."[27]

Petal

• "The petals are Jordan domains invariant by ${\displaystyle f^{q}}$ " R PEREZ-MARCO[28]
• Petal is trap which captures any orbit tending to parabolic point
• petal is a part of the flower

Definitions:

Flower

Sum of all petals creates a flower[30] with center at parabolic periodic point.[31]

Cauliflower

Cauliflower or broccoli:[32]

• empty (its interior is empty) for c outside Mandelbrot set. Julia set is a totally disconnected.
• filled cauliflower for c=1/4 on boundary of the Mandelbrot set. Julia set is a Jordan curve (quasi circle).

• size of image differs because of different z-planes.
• different algorithms are used so colours are hard to compare.

Bifurcation of the Cauliflower

How Julia set changes along real axis (going from c=0 thru c=1/4 and further):

Perturbation of a function ${\displaystyle f(z)}$  by complex ${\displaystyle \epsilon }$ :

${\displaystyle g(z)=f(z)+\epsilon }$

When one add epsilon > 0 (move along real axis toward + infinity) there is a bifurcation of parabolic fixed point:

• attracting fixed point (epsilon<0)
• one parabolic fixed point (epsilon = 0)
• one parabolic fixed point splits up into two conjugate repelling fixed points (epsilon > 0)

"If we slightly perturb with epsilon<0 then the parabolic fixed point splits up into two real fixed points on the real axis (one attracting, one repelling)."

See:

• demo 2 page 9 in program Mandel by Wolf Jung

parabolic implosion

Parabolic imposion

• on the parameter plane
• point c moves from interior of the component, through the boundary to the exterior of Mandelbrot set
• nucleus ( c=0), along internal ray 0, parabolic point ( c= 0.25), along external ray 0
• on the dynamic plane
• connected Julia set ( with interior) imploedes and comes disconnected ( without interior)
• fixed point moves from interior to Julia set ( parabolic)
• one basin ( interior) disappears
evolution of dynamics along escape route 0 ( parabolic implosion)
parameter c location Julia set interior type of dynamics critical point fixed points
c = 0 center, interior connected exist superattracting atracted to alfa fixed point fixed critical point equal to alfa fixed point, alfa is superattracting, beta is repelling
0<c<1/4 internal ray 0, interior connected exist attracting atracted to alfa fixed point alfa is attracting, beta is repelling
c = 1/4 cusp, boundary connected exist parabolic atracted to alfa fixed point alfa fixed point equal to beta fixed point, both are parabolic
c>1/4 external ray 0, exterior disconnected disappears - repelling to infinity both finite fixed points are repelling

Vector field

• 2D vector field and its

singularity

singularity types:

• center type: "In this case, one can find a neighborhood of the singular point where all integral curves are closed, inside one another, and contain the singular point in their interior"[34]
• non-center type: neighborhood of singularity is made of several curvilinear sectors:[35]

"A curvilinear sector is defined as the region bounded by a circle C with arbitrary small radius and two streamlines S and S! both converging towards singularity. One then considers the streamlines passing through the open sector g in order to distinguish between three possible types of curvilinear sectors."

dynamics

Dynamics:

• global
• local

Local dynamics:

• in the exterior of Julia set
• on the Julia set
• near parabolic fixed point (inside Julia set)

Near parabolic fixed point

Why analyze f^p not f ?

Forward orbit of f near parabolic fixed point is composite. It consist of 2 motions:

• around fixed point
• toward / away from fixed point

How to compute parabolic c values

Type of parabolic parameters:

• root points
• cusps
Parabolic points of period 1 component of Mandelbrot set (parameter plane)
n Internal angle (rotation number) t = 1/n The root point c = parabolic parameter Two external angles of parameter rays landing on the root point c (1/(2^n+1); 2/(2^n+1) fixed point ${\displaystyle z_{\alpha }}$  external angles of dynamic rays landing on fixed point ${\displaystyle z_{\alpha }}$
1 1/1 0.25 (0/1 ; 1/1) 0.5 (0/1 = 1/1)
2 1/2 -0.75 (1/3; 2/3) -0.5 (1/3; 2/3)
3 1/3 0.64951905283833*%i-0.125 (1/7; 2/7) 0.43301270189222*%i-0.25 (1/7; 2/7; 3/7)
4 1/4 0.5*%i+0.25 (1/15; 2/15) 0.5*%i (1/15; 2/15; 4/15; 8/15)
5 1/5 0.32858194507446*%i+0.35676274578121 (1/31; 2/31) 0.47552825814758*%i+0.15450849718747 (1/31; 2/31; 4/31; 8/31; 16/31)
6 1/6 0.21650635094611*%i+0.375 (1/63; 2/63) 0.43301270189222*%i+0.25 (1/63; 2/63; 4/63; 8/63; 16/63; 32/63)
7 1/7 0.14718376318856*%i+0.36737513441845 (1/127; 2/127) 0.39091574123401*%i+0.31174490092937 (1/127; 2/127, 4/127; 8/127; 16/127; 32/127, 64/127)
8 1/8 0.10355339059327*%i+0.35355339059327 0.35355339059327*%i+0.35355339059327
9 1/9 0.075191866590218*%i+0.33961017714276 0.32139380484327*%i+0.38302222155949
10 1/10 0.056128497072448*%i+0.32725424859374 0.29389262614624*%i+0.40450849718747

For internal angle n/p parabolic period p cycle consist of one z-point with multiplicity p[36] and multiplier = 1.0 . This point z is equal to fixed point ${\displaystyle z_{alfa}}$

Period 1

One can easily compute boundary point c

${\displaystyle c=c_{x}+c_{y}*i}$

of period 1 hyperbolic component (main cardioid) for given internal angle (rotation number) t using this cpp code by Wolf Jung[37]

 t *= (2*PI); // from turns to radians
cx = 0.5*cos(t) - 0.25*cos(2*t);
cy = 0.5*sin(t) - 0.25*sin(2*t);


or this Maxima CAS code:


/* conformal map from circle to cardioid ( boundary
of period 1 component of Mandelbrot set */
F(w):=w/2-w*w/4;

/*
circle D={w:abs(w)=1 } where w=l(t,r)
t is angle in turns ; 1 turn = 360 degree = 2*Pi radians
*/
ToCircle(t,r):=r*%e^(%i*t*2*%pi);

(
[w],
/* point of  unit circle   w:l(internalAngle,internalRadius); */
w:ToCircle(angle,radius),  /* point of circle */
float(rectform(F(w)))    /* point on boundary of period 1 component of Mandelbrot set */
)$compile(all)$

/* ---------- global constants & var ---------------------------*/
Numerator: 1;
DenominatorMax: 10;

/* --------- main -------------- */
for Denominator:1 thru DenominatorMax step 1 do
(
InternalAngle: Numerator/Denominator,
display(Denominator),
display(c),
/* compute fixed point */
alfa:float(rectform((1-sqrt(1-4*c))/2)), /* alfa fixed point */
display(alfa)
)$ Period 2 // cpp code by W Jung http://www.mndynamics.com t *= (2*PI); // from turns to radians cx = 0.25*cos(t) - 1.0; cy = 0.25*sin(t);  Periods 1-6 /* batch file for Maxima CAS computing bifurcation points for period 1-6 Formulae for cycles in the Mandelbrot set II Stephenson, John; Ridgway, Douglas T. Physica A, Volume 190, Issue 1-2, p. 104-116. */ kill(all); remvalue(all); start:elapsed_run_time (); /* ------------ functions ----------------------*/ /* exponential for of complex number with angle in turns */ /* "exponential form prevents allroots from working", code by Robert P. Munafo */ GivePoint(Radius,t):=rectform(ev(Radius*%e^(%i*t*2*%pi), numer))$ /* gives point of unit circle for angle t in turns */

GiveCirclePoint(t):=rectform(ev(%e^(%i*t*2*%pi), numer))$/* gives point of unit circle for angle t in turns Radius = 1 */ /* gives a list of iMax points of unit circle */ GiveCirclePoints(iMax):=block( [circle_angles,CirclePoints], CirclePoints:[], circle_angles:makelist(i/iMax,i,0,iMax), for t in circle_angles do CirclePoints:cons(GivePoint(1,t),CirclePoints), return(CirclePoints) /* multipliers */ )$

/* http://commons.wikimedia.org/wiki/File:Mandelbrot_set_Components.jpg
Boundary equation  b_n(c,P)=0
defines relations between hyperbolic components and unit circle for given period n ,
allows computation of exact coordinates of hyperbolic componenets.

b_n(w,c), is boundary polynomial (implicit function of 2 variables).

*/

GiveBoundaryEq(P,n):=
block(
if n=1 then return(c + P^2 - P),
if n=2 then return(- c + P - 1),
if n=3 then return(c^3 + 2*c^2 - (P-1)*c + (P-1)^2),
if n=4 then return(c^6 + 3*c^5 + (P+3)* c^4 + (P+3)* c^3 - (P+2)*(P-1)*c^2 - (P-1)^3),
if n=5 then return(c^15 + 8*c^14 + 28*c^13 + (P + 60)*c^12 + (7*P + 94)*c^11 +
(3*P^2 + 20*P + 116)*c^10 + (11*P^2 + 33*P + 114)*c^9 + (6*P^2 + 40*P + 94)*c^8 +
(2*P^3 - 20*P^2 + 37*P + 69)*c^7 + (3*P - 11)*(3*P^2 - 3*P - 4)*c^6 + (P - 1)*(3*P^3 + 20*P^2 - 33*P - 26)*c^5 +
(3*P^2 + 27*P + 14)*(P - 1)^2*c^4 - (6*P + 5)*(P - 1)^3*c^3 + (P + 2)*(P - 1)^4*c^2 - c*(P - 1)^5  + (P - 1)^6),
if n=6 then return (c^27+
13*c^26+
78*c^25+
(293 - P)*c^24+
(792 - 10*P)*c^23+
(1672 - 41*P)*c^22+
(2892 - 84*P - 4*P^2)*c^21+
(4219 - 60*P - 30*P^2)*c^20+
(5313 + 155*P - 80*P^2)*c^19+
(5892 + 642*P - 57*P^2 + 4*P^3)*c^18+
(5843 + 1347*P + 195*P^2 + 22*P^3)*c^17+
(5258 + 2036*P + 734*P^2 + 22*P^3)*c^16+
(4346 + 2455*P + 1441*P^2 - 112*P^3 + 6*P^4)*c^15 +
(3310 + 2522*P + 1941*P^2 - 441*P^3 + 20*P^4)*c^14 +
(2331 + 2272*P + 1881*P^2 - 853*P^3 - 15*P^4)*c^13 +
(1525 + 1842*P + 1344*P^2 - 1157*P^3 - 124*P^4 - 6*P^5)*c^12 +
(927 + 1385*P + 570*P^2 - 1143*P^3 - 189*P^4 - 14*P^5)*c^11 +
(536 + 923*P - 126*P^2 - 774*P^3 - 186*P^4 + 11*P^5)*c^10 +
(298 + 834*P + 367*P^2 + 45*P^3 - 4*P^4 + 4*P^5)*(1-P)*c^9 +
(155 + 445*P - 148*P^2 - 109*P^3 + 103*P^4 + 2*P^5)*(1-P)*c^8 +
2*(38 + 142*P - 37*P^2 - 62*P^3 + 17*P^4)*(1-P)^2*c^7 +
(35 + 166*P + 18*P^2 - 75*P^3 - 4*P^4)*((1-P)^3)*c^6 +
(17 + 94*P + 62*P^2 + 2*P^3)*((1-P)^4)*c^5 +
(7 + 34*P + 8*P^2)*((1-P)^5)*c^4 +
(3 + 10*P + P^2)*((1-P)^6)*c^3 +
(1 + P)*((1-P)^7)*c^2 +
-c*((1-P)^8) + (1-P)^9)
)$/* gives a list of points c on boundaries on all components for give period */ GiveBoundaryPoints(period,Circle_Points):=block( [Boundary,P,eq,roots], Boundary:[], for m in Circle_Points do (/* map from reference plane to parameter plane */ P:m/2^period, eq:GiveBoundaryEq(P,period), /* Boundary equation b_n(c,P)=0 */ roots:bfallroots(%i*eq), roots:map(rhs,roots), for root in roots do Boundary:cons(root,Boundary)), return(Boundary) )$

/* divide llist of roots to 3 sublists = 3  components */
/* ---- boundaries of period 3 components
period:3$Boundary3Left:[]$
Boundary3Up:[]$Boundary3Down:[]$

for m in CirclePoints do (
P:m/2^period,
eq:GiveBoundaryEq(P,period),
roots:bfallroots(%i*eq),
roots:map(rhs,roots),
for root in roots do
(
if realpart(root)<-1  then Boundary3Left:cons(root,Boundary3Left),
if (realpart(root)>-1 and imagpart(root)>0.5)
then Boundary3Up:cons(root,Boundary3Up),
if (realpart(root)>-1 and imagpart(root)<0.5)
then Boundary3Down:cons(root,Boundary3Down)

)

)$--------- */ /* gives a list of parabolic points for given: period and internal angle */ GiveParabolicPoints(period,t):=block ( [m,ParabolicPoints,P,eq,roots], m: GiveCirclePoint(t), /* root of unit circle, Radius=1, angle t=0 */ ParabolicPoints:[], /* map from reference plane to parameter plane */ P:m/2^period, eq:GiveBoundaryEq(P,period), /* Boundary equation b_n(c,P)=0 */ roots:bfallroots(%i*eq), roots:map(rhs,roots), for root in roots do ParabolicPoints:cons(float(root),ParabolicPoints), return(ParabolicPoints) )$

compile(all)$/* ------------- constant values ----------------------*/ fpprec:16; /* ------------unit circle on a w-plane -----------------------------------------*/ a:GiveParabolicPoints(6,1/3); a$



period d

/*
gcc c.c -lm -Wall
./a.out
Root point between period 1 component and period 987 component  = c = 0.2500101310666710+0.0000000644946597
Internal angle (c) = 1/987

*/

#include <stdio.h>
#include <math.h>		// M_PI; needs -lm also
#include <complex.h>

/*
c functions using complex type numbers
computes c from  component  of Mandelbrot set */
complex double Give_c( int Period,  int n, int d , double InternalRadius )
{
complex double c;
complex double w;  // point of reference plane  where image of the component is a unit disk
// alfa = ax +ay*i = (1-sqrt(d))/2 ; // result
double t; // InternalAngleInTurns

t  = (double) n/d;
t = t * M_PI * 2.0; // from turns to radians

w = InternalRadius*cexp(I*t); // map to the unit disk

switch ( Period ) // of component
{
case 1: // main cardioid = only one period 1 component
c = w/2 - w*w/4; // https://en.wikibooks.org/wiki/Fractals/Iterations_in_the_complex_plane/Mandelbrot_set/boundary#Solving_system_of_equation_for_period_1
break;
case 2: // only one period 2 component
c = (w-4)/4 ; // https://en.wikibooks.org/wiki/Fractals/Iterations_in_the_complex_plane/Mandelbrot_set/boundary#Solving_system_of_equation_for_period_2
break;
// period > 2
default:
printf("higher periods : to do, use newton method \n");
printf("for each q = Period of the Child component  there are 2^(q-1) roots \n");
c = 10000.0; // bad value

break; }

return c;
}

void PrintAndDescribe_c( int period,  int n, int d , double InternalRadius ){

complex double c = Give_c(period, n, d, InternalRadius);

printf("Root point between period %d component and period %d component  = c = %.16f%+.16f*I\t",period, d, creal(c), cimag(c));
printf("Internal angle (c) = %d/%d\n",n, d);

}

/*
https://stackoverflow.com/questions/19738919/gcd-function-for-c
The GCD function uses Euclid's Algorithm.
It computes A mod B, then swaps A and B with an XOR swap.
*/

int gcd(int a, int b)
{
int temp;
while (b != 0)
{
temp = a % b;

a = b;
b = temp;
}
return a;
}

int main (){

int period = 1;
// internal angle in turns as a ratio = p/q
int n = 1;
int d = 987;

// n/d = local angle in turns

for (n = 1; n < d; ++n ){
if (gcd(n,d)==1 )// irreducible fraction
}

return 0;

}


How to draw parabolic Julia set

All points of interior of filled Julia set tend to one periodic orbit (or fixed point). This point is in Julia set and is weakly attracting.[38] One can analyse only behavior near parabolic fixed point. It can be done using critical orbits.

There are two cases here: easy and hard.

If the Julia set near parabolic fixed point is like n-th arm star (not twisted) then one can simply check argument of zn, relative to the fixed point. See for example z+z^5. This is an easy case.

In the hard case Julia set is twisted around fixed.

topology

Julia set of root point is topologically the same as the Julia set of the child period center, but

• the center ( nucleus) Julia set is very easy to draw ( superattracting basin = very fast dynamics because critical point is also periodic point)
• while the root Julia set ( parabolic) is hard to draw ( parabolic basin and lasy dynamics)

Examples:

• t = 1/2
• Julia set of the root point = fat Basilica Julia set: c = -3/4 = - 0.75
• Julia set of period 2 center = (slim) Basicica Julia set: c = -1
• t = 1/3
• Julia set of the root point = fat Douady Rabbit: c = -0.125000000000000 +0.649519052838329i
• Julia set of period 3 center = (slim) Doudy Rabbit Julia set: c = -0.122561166876654 +0.744861766619744i period = 3

Estimation from exterior

Long iteration or big step method

Description

• Long iteration method[39]
• The big steps algorithm [40]

Steps

• choose fuction ${\displaystyle f}$ which has fixed parabolic point at origin ( z=0)
• choose internal angle ${\displaystyle \theta ={\frac {m}{k}}}$
• compute ${\displaystyle F_{k}}$  which is an aproximation of higher iterates of function ${\displaystyle f}$  for z close to zero using power series centered at zero ( Taylor series = Maclaurin series )
• find how many terms of power series to use and on which annuli to use specific ${\displaystyle F_{k}}$  experimentally
${\displaystyle F_{k}\approx f^{2^{k}}}$



each ${\displaystyle F_{k}}$  will be used on an annulus

${\displaystyle {\frac {1}{2^{k+K+1}}}<|z|<{\frac {1}{2^{k+K}}}}$

where K is fixed

example

Lambda form of complex quadratic polynomial which has an indifferent fixed point with multiplier ${\displaystyle \lambda }$  at the origin[41]

${\displaystyle f_{\lambda }(z)=\lambda z+z^{2}}$

where:

• multiplier of fixed point ${\displaystyle \lambda =e^{2\pi \theta i}}$
• internal angle is an rational number and proper fraction ${\displaystyle \theta ={\frac {m}{k}}}$

Choose

• ${\displaystyle \theta ={\frac {1}{7}}}$  so ${\displaystyle \lambda =e^{(2\pi i)/7}}$
• 30 terms of power series
• approximated function for annuli k=4,5,...,10 and default function f^n for larger values of z ( outside annuli)
• delta for function equal to 10^-5

(* code by Professor: Mark McClure from https://marksmath.org/classes/Spring2019ComplexDynamics/ *)
n = 7;
f[z_] = Exp[2 Pi*I/n] z + z^2;

Remove[F];
F[0][z_]    = N[Normal[Series[f[z], {z, 0, 30}]]];
Do[F[0][z_] = Chop[N[Normal[Series[F[0][f[z]], {z, 0, 30}]]], 10^-5],  {n - 1}];
Do[F[k][z_] = Chop[N[Normal[Series[F[k - 1][F[k - 1][z]], {z, 0, 30}]]], 10^-5], {k, 1, 10}]

(* define and compile function FF *)
FF = With[{
n = n,
f4 = Function[z, Evaluate[F[4][z]]],
f5 = Function[z, Evaluate[F[5][z]]],
f6 = Function[z, Evaluate[F[6][z]]],
f7 = Function[z, Evaluate[F[7][z]]],
f8 = Function[z, Evaluate[F[8][z]]],
f9 = Function[z, Evaluate[F[9][z]]],
f10 = Function[z, Evaluate[F[10][z]]]
},
Compile[{{z, _Complex}},
Which[
Abs[z] > 1/2^3,
Nest[Function[zz, N[Exp[2 Pi*I/n]] zz + zz^2], z, n],
Abs[z] <= 1/2^9, f10[z],
Abs[z] <= 1/2^8, f9[z],
Abs[z] <= 1/2^7, f8[z],
Abs[z] <= 1/2^6, f7[z],
Abs[z] <= 1/2^5, f6[z],
Abs[z] <= 1/2^4, f5[z],
Abs[z] <= 1/2^3, f4[z],
True, 0]]];

(* iterate 1000 times and then see what happens *)
iterate = With[{FF = FF, n = n},
Compile[{{z0, _Complex}},
Module[{z, i},
z = z0;
i = 0;
While[1/2^9 < Abs[z] <= 2 && i++ < 1000 n,
z = FF[z]];
z],
RuntimeOptions -> "Speed", CompilationTarget -> "C"]];

(* now compute some iteration data *)
data = Monitor[
Table[iterate[x + I*y], {y, Im[center] + 1.2, Im[center], -0.0025},
{x, Re[center] - 1.2, Re[center] + 1.2, 0.0025}],
y];

(* use some symmetry to cut computation time in half *)
center = First[Select[z /. NSolve[f[z] == 0, z], Im[#] < 0 &]]/2 (* center = -0.311745 - 0.390916*I *)
data = Join[data, Reverse[Rest[Reverse /@ data]]];

(* plot it  *)
kernel = {
{1, 1, 1},
{1, -8, 1},
{1, 1, 1}
};

(*
classifyArg = Compile[
{{z, _Complex}, {z0, _Complex}, {v, _Complex}, {n, _Integer}},
Module[{check, check2},
check = n (Arg[(z0 - z)/v] + Pi)/(2 Pi);
check2 = Ceiling[check];
If[check == check2, 0, check2]]];

classified = Map[classify, data, {2}];

convolvedData = ListConvolve[kernel, classified];

ArrayPlot[Sign[Abs[convolvedData]]]


Mathematical Functions of Wolfram language (  :

• Series[f,{x,x0,n}] generates a power series expansion for f about the point ${\displaystyle x=x_{0}}$  to order ${\displaystyle (x-x_{0})^{n}}$ , where n is an explicit integer
• N[expr] gives the numerical value of expr
• Chop[expr,delta] replaces numbers smaller in absolute magnitude than delta by 0
• Normal[expr] converts expr to a normal expression from a variety of special forms.
• Do[expr,n] evaluates expr n times
• Do[expr,{i,imin,imax}] evaluates expr and starts with i=imin
• With[{x=x0,y=y0,…},expr] specifies that all occurrences of the symbols x, y, … in expr should be replaced by x0, y0, ….
• Compile[{{x1,t1},…},expr] assumes that xi is of a type that matches ti.
• Which[test1,value1,test2,value2,…] evaluates each of the testi in turn, returning the value of the valuei corresponding to the first one that yields True
• Nest[f,expr,n] gives an expression with f applied n times to expr.

Dynamic rays

One can use periodic dynamic rays landing on parabolic fixed point to find narrow parts of exterior.

Let's check how many backward iterations needs point on periodic ray with external radius = 4 to reach distance 0.003 from parabolic fixed point:

period Inverse iterations time
1 340 0m0.021s
2 55 573 0m5.517s
3 8 084 815 13m13.800s
4 1 059 839 105 1724m28.990s

One can use only argument of point z of external rays and its distance to alfa fixed point (see code from image). It works for periods up to 15 (maybe more ...).

Estimation from interior

Julia set is a boundary of filled-in Julia set Kc.

• find points of interior of Kc
• find boundary of interior of Kc using edge detection

If components of interior are lying very close to each other then find components using:[42]

color = LastIteration % period


For parabolic components between parent and child component:[43]

periodOfChild = denominator*periodOfParent
color = iLastIteration % periodOfChild


where denominator is a denominator of internal angle of parent component of Mandelbrot set.

Angle

"if the iterate zn of tends to a fixed parabolic point, then the initial seed z0 is classified according to the argument of zn−z0, the classification being provided by the flower theorem" (Mark McClure[44])

Attraction time

Interior of filled Julia set consist of components. All comonents are preperiodic, some of them are periodic (immediate basin of attraction).

In other words:

• one iteration moves z to another component (and whole component to another component)
• all point of components have the same attraction time (number of iteration needed to reach target set around attractor)

It is possible to use it to color components. Because in the parabolic case the attractor is weak (weakly attracting) it needs a lot of iterations for some points to reach it.

 // i = number of iteration
// iPeriodChild = period of child component of Mandelbrot set ( parabolic c value is a root point between parant and child component
/* distance from z to Alpha  */
Zxt=Zx-dAlfaX;
Zyt=Zy-dAlfaY;
d2=Zxt*Zxt +Zyt*Zyt;
// interior: check if fall into internal target set (circle around alfa fixed point)
if (d2<dMaxDistance2Alfa2) return  iColorsOfInterior[i % iPeriodChild];


Here are some example values:

 iWidth  = 1001 // width of image in pixels
PixelWidth  = 0.003996
AR  = 0.003996 // Radius around attractor
denominator  = 1 ; Cx  = 0.250000000000000; Cy  = 0.000000000000000 ax  = 0.500000000000000; ay  = 0.000000000000000
denominator  = 2 ; Cx  = -0.750000000000000; Cy  = 0.000000000000000 ax  = -0.500000000000000; ay  = 0.000000000000000
denominator  = 3 ; Cx  = -0.125000000000000; Cy  = 0.649519052838329 ax  = -0.250000000000000; ay  = 0.433012701892219
denominator  = 4 ; Cx  = 0.250000000000000; Cy  = 0.500000000000000 ax  = 0.000000000000000; ay  = 0.500000000000000
denominator  = 5 ; Cx  = 0.356762745781211; Cy  = 0.328581945074458 ax  = 0.154508497187474; ay  = 0.475528258147577
denominator  = 6 ; Cx  = 0.375000000000000; Cy  = 0.216506350946110 ax  = 0.250000000000000; ay  = 0.433012701892219

denominator  = 1 ;   i =               243.000000
denominator  = 2 ;   i =            31 171.000000
denominator  = 3 ;   i =         3 400 099.000000
denominator  = 4 ;   i =       333 293 206.000000
denominator  = 5 ;   i =    29 519 565 177.000000
denominator  = 6 ;   i = 2 384 557 783 634.000000


where:

C = Cx + Cy*i
a = ax + ay*i // fixed point alpha
i // number of iterations after which critical point z=0.0 reaches disc around fixed point alpha with radius AR
denominator of internal angle (in turns)
internal angle = 1/denominator


Note that attraction time i is proportional to denominator.

Now you see what means weakly attracting.

One can:

• use brutal force method (Attracting radius < pixelSize; iteration Max big enough to let all points from interior reach target set; long time or fast computer)
• find better method (:-)) if time is to long for you

Estimation from interior and exterior

Julia set is a common boundary of filled-in Julia set and basin of attraction of infinity.

• find points of interior/components of Kc
• find escaping points
• find boundary points using Sobel filter

It works for denominator up to 4.

Inverse iteration of repelling points

Inverse iteration of alfa fixed point. It works good only for cuting point (where external rays land). Other points still are not hitten.

Gallery

External examples: