# Fractals/Iterations in the complex plane/MandelbrotSetExteriorComplex potential

Complex potential is a complex number, so it has 2 parts:

• a real part = real potential = absolute value
• an imaginary part = external angle
Uniformization of complement of Mandelbrot set

One can take also its:

• curl
• divergence

So on one image one can use more than one variable to color image.[1]

Implementations:

Names:

### Real potential = CPM/M

Names:

• eponimes:
• electric:
• The electric potential [5]
• the voltage [6]
• Green function = G(c)

${\displaystyle G(c)=\lim _{n\to \infty }{\frac {1}{2^{n}}}ln|z_{n}|}$

${\displaystyle V(c)\approx V_{n}(c)={\frac {log|z_{n}|}{2^{n}}}}$

In Fractint :

potential =  log(modulus)/2^iterations


One can use real potential to:

• smooth (continuous) coloring[7]
• discrete coloring ( level sets of potential)
• 3D view

Code:

Here is Delphi function which gives level of potential :

 Function GiveLevelOfPotential(potential:extended):integer;
var r:extended;
begin
r:= log2(abs(potential));
result:=ceil(r);
end;

/******************************************************************/
// /fractint/common/calcfrac.c
/*
CALCFRAC.C contains the high level ("engine") code for calculating the
fractal images (well, SOMEBODY had to do it!).
Original author Tim Wegner, but just about ALL the authors have contributed
SOME code to this routine at one time or another, or contributed to one of
the many massive restructurings.
The following modules work very closely with CALCFRAC.C:
FRACTALS.C    the fractal-specific code for escape-time fractals.
FRACSUBR.C    assorted subroutines belonging mainly to calcfrac.
CALCMAND.ASM  fast Mandelbrot/Julia integer implementation
Additional fractal-specific modules are also invoked from CALCFRAC:
LORENZ.C      engine level and fractal specific code for attractors.
JB.C          julibrot logic
PARSER.C      formula fractals
and more
-------------------------------------------------------------------- */
/* Continuous potential calculation for Mandelbrot and Julia      */
/* Reference: Science of Fractal Images p. 190.                   */
/* Special thanks to Mark Peterson for his "MtMand" program that  */
/* beautifully approximates plate 25 (same reference) and spurred */
/* on the inclusion of similar capabilities in FRACTINT.          */
/*                                                                */
/* The purpose of this function is to calculate a color value     */
/* for a fractal that varies continuously with the screen pixels  */
/* locations for better rendering in 3D.                          */
/*                                                                */
/* Here "magnitude" is the modulus of the orbit value at          */
/* "iterations". The potparms[] are user-entered paramters        */
/* controlling the level and slope of the continuous potential    */
/* surface. Returns color.  - Tim Wegner 6/25/89                  */
/*                                                                */
/*                     -- Change history --                       */
/*                                                                */
/* 09/12/89   - added floatflag support and fixed float underflow */
/*                                                                */
/******************************************************************/

static int _fastcall potential(double mag, long iterations)
{
float f_mag,f_tmp,pot;
double d_tmp;
int i_pot;
long l_pot;

if(iterations < maxit)
{
pot = (float)(l_pot = iterations+2);
if(l_pot <= 0 || mag <= 1.0)
pot = (float)0.0;
else /* pot = log(mag) / pow(2.0, (double)pot); */
{
if(l_pot < 120 && !floatflag) /* empirically determined limit of fShift */
{
f_mag = (float)mag;
fLog14(f_mag,f_tmp); /* this SHOULD be non-negative */
fShift(f_tmp,(char)-l_pot,pot);
}
else
{
d_tmp = log(mag)/(double)pow(2.0,(double)pot);
if(d_tmp > FLT_MIN) /* prevent float type underflow */
pot = (float)d_tmp;
else
pot = (float)0.0;
}
}
/* following transformation strictly for aesthetic reasons */
/* meaning of parameters:
potparam[0] -- zero potential level - highest color -
potparam[1] -- slope multiplier -- higher is steeper
potparam[2] -- rqlim value if changeable (bailout for modulus) */

if(pot > 0.0)
{
if(floatflag)
pot = (float)sqrt((double)pot);
else
{
fSqrt14(pot,f_tmp);
pot = f_tmp;
}
pot = (float)(potparam[0] - pot*potparam[1] - 1.0);
}
else
pot = (float)(potparam[0] - 1.0);
if(pot < 1.0)
pot = (float)1.0; /* avoid color 0 */
}
else if(inside >= 0)
pot = inside;
else /* inside < 0 implies inside=maxit, so use 1st pot param instead */
pot = (float)potparam[0];

i_pot = (int)((l_pot = (long)(pot * 256)) >> 8);
if(i_pot >= colors)
{
i_pot = colors - 1;
l_pot = 255;
}

if(pot16bit)
{
if (dotmode != 11) /* if putcolor won't be doing it for us */
writedisk(col+sxoffs,row+syoffs,i_pot);
writedisk(col+sxoffs,row+sydots+syoffs,(int)l_pot);
}

return(i_pot);
}


### External angle and external ( parameter) ray

Image[Table[If[ColorQ@#,#,Black]&@Hue[(Arg[MandelbrotSetBoettcher[x+I y]]+Pi)/(2Pi)],{y,-2,2,.005},{x,-2,2,.005}]]


#### Methods

First find angle of last iteration. It is easy to compute and shows some external rays as a borders of level sets.

Then one go further.

• automatically calculate external angles from nucleus and period by Claude Heiland-Allen [9]

Methods:

#### Tests

##### The Wolf Jung test
Part of parameter plane with external rays 1/7, 321685687669320/2251799813685247 and 321685687669322/2251799813685247 landing on the Mandelbrot set

The external parameter rays for angles (in turns)

• 321685687669320/2251799813685247 (period 51, lands on c1 = -0.088891642419446 +0.650955631292636i )
• 321685687669322/2251799813685247 ( period 51 lands on c2 = -0.090588078906990 +0.655983860334813i )
• 1/7 ( period 3, lands on c3 = -0.125000000000000 +0.649519052838329i )

Angles differ by about ${\displaystyle 10^{-15}}$, but the landing points of the corresponding parameter rays are about 0.035 apart.[16] It can be computed with Maxima CAS :

(%i1) c1: -0.088891642419446  +0.650955631292636*%i;
(%o1) 0.650955631292636*%i−0.088891642419446
(%i2) c2:-0.090588078906990  +0.655983860334813*%i;
(%o2) 0.655983860334813*%i−0.09058807890699
(%i3) abs(c2-c1);
(%o3) .005306692383854863
(%i4) c3: -0.125000000000000  +0.649519052838329*%i$(%i5) abs(c3-c1); (%o5) .03613692356607755 (%i6) a3:1/7$
(%i7) float(abs(a3-a1));
(%o7) 4.440892098500628*10^−16


Informations from W Jung program :

The angle  1/7  or  p001 has  preperiod = 0  and  period = 3.
The conjugate angle is  2/7  or  p010 .
The kneading sequence is  AA*  and the internal address is  1-3 .
The corresponding parameter rays are landing at the root of a satellite component of period 3.
It is bifurcating from period 1.

The angle  321685687669320/2251799813685247  or  p001001001001001001001001001001001001001001001001000 has  preperiod = 0  and  period = 51.
The conjugate angle is  321685687669319/2251799813685247  or  p001001001001001001001001001001001001001001001000111 .
The kneading sequence is  AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABB*  and the internal address is  1-49-50-51 .
The corresponding parameter rays are landing at the root of a primitive component of period 51.

The angle  321685687669322/2251799813685247  or  p001001001001001001001001001001001001001001001001010 has  preperiod = 0  and  period = 51.
The conjugate angle is  321685687669329/2251799813685247  or  p001001001001001001001001001001001001001001001010001 .
The kneading sequence is  AABAABAABAABAABAABAABAABAABAABAABAABAABAABAABAABAA*  and the internal address is  1-3-51 .
The corresponding parameter rays are landing at the root of a satellite component of period 51.
It is bifurcating from period 3.

##### The test by G. Pastor and Miguel Romera

The external parameter rays for angles (in turns)

• 6871947673/34359738367 ( period 35 )
• 9162596898/34359738367 ( period 35 )

the central babies Mandelbrot sets of the cauliflowers located at -0.153756141 + 1.030383223i

(not that 34359738367 = 2^35 - 1)

##### test by M. Romera,1 G. Pastor, A. B. Orue,1 A. Martin, M.-F. Danca,and F. Montoya
Part of parameter plane with external 5 rays landing on the Mandelbrot set.png

G Pastor gave an example of external rays for which the resolution of the IEEE 754 is not sufficient:[17]

• ${\displaystyle \theta _{267}^{-}=0.((001)^{88}010)_{2}={\frac {33877456965431938318210482471113262183356704085033125021829876006886584214655562}{237142198758023568227473377297792835283496928595231875152809132048206089502588927}}}$
• ${\displaystyle \theta _{267}^{+}=0.((001)^{87}010001)_{2}={\frac {33877456965431938318210482471113262183356704085033125021829876006886584214655569}{237142198758023568227473377297792835283496928595231875152809132048206089502588927}}}$
• ${\displaystyle \theta _{3}^{-}=0.(001)_{2}={\frac {1}{7}}=0.(142857)_{10}}$ ( period 3, lands on root point of period 3 component c3 = -0.125000000000000 +0.649519052838329i )
• ${\displaystyle \theta _{268}^{-}=0.((001)^{88}0001)_{2}={\frac {67754913930863876636420964942226524366713408170066250043659752013773168429311121}{474284397516047136454946754595585670566993857190463750305618264096412179005177855}}}$
• ${\displaystyle \theta _{268}^{+}=0.((001)^{88}0010)_{2}={\frac {67754913930863876636420964942226524366713408170066250043659752013773168429311122}{474284397516047136454946754595585670566993857190463750305618264096412179005177855}}}$

One can analyze these angles using program by Claude Heiland-Allen :

./bin/mandelbrot_describe_external_angle ".(001)"
binary: .(001)
decimal: 1/7
preperiod: 0
period: 3

./bin/mandelbrot_describe_external_angle
".(001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001010)"
binary:
.(001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001010)
decimal:
33877456965431938318210482471113262183356704085033125021829876006886584214655562/237142198758023568227473377297792835283496928595231875152809132048206089502588927
preperiod: 0
period: 267

./bin/mandelbrot_describe_external_angle
".(001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001010001)"
binary:
.(001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001001010001)
decimal:
33877456965431938318210482471113262183356704085033125021829876006886584214655569/237142198758023568227473377297792835283496928595231875152809132048206089502588927
preperiod: 0
period: 267

./bin/mandelbrot_describe_external_angle
".(0010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010001)"
binary:
.(0010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010001)
decimal:
67754913930863876636420964942226524366713408170066250043659752013773168429311121/474284397516047136454946754595585670566993857190463750305618264096412179005177855
preperiod: 0
period: 268

./bin/mandelbrot_describe_external_angle
".(0010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010)"
binary:
.(0010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010010)
decimal:
67754913930863876636420964942226524366713408170066250043659752013773168429311122/474284397516047136454946754595585670566993857190463750305618264096412179005177855
preperiod: 0
period: 268


Landing points of above rays are roots with angled internal addresses ( description by Claude Heiland-Allen) :

• the upper one will be 1 -> 1/3 -> 3 -> 1/(period/3) -> period because it's the nearest bulb to the lower root cusp of 1/3 bulb and child bulbs of 1/3 bulb have periods 3 * denominator(internal angle) ie, 1 -> 1/3 -> 3 -> 1/89 -> 267
• the lower one will be 1 -> floor(period/3)/period -> period because it's the nearest bulb below the 1/3 cusp ie, 1 -> 89/268 -> 268
• the middle ray .(001) lands at the root of 1 -> 1/3 -> 3, from the cusp on the lower side (which is on the right in a standard unrotated view)