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Fractals/Iterations of real numbers/r iterations

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Contents

Diagram typesEdit

2D diagramsEdit

  • parameter is a variable on horizontal axis
    • bifurcation diagram : P-curves ( = periodic points) versus parameter[1]
    • orbit diagram : points of critical orbit versus parameter
    • skeleton diagram ( critical curves = q-curves)
    • Lyapunow diagram : Lyapunov exponent versus parameter[2]
    • multiplier diagrams : multiplier of periodic orbit versus parameter
  • constant parameter diagrams
    • cobweb diagram or a Verhulst diagram [3] = Graphical iteration
    • Iterates versus time diagram[4] = connected scatter graph = time series
    • invariant density diagram , histogram [5], the distribution of the orbit[6], frequency distribution [7] = power spectrum [8]


3D diagramsEdit

MapsEdit

  • due to numerical errors different ways of the same equation can give different trajectories. For example : rx(1-x) and rx - rxx
  • Sensitivity to initial conditions : "small difference in the initial condition will produce large differences in the long-term behaviour of the system. This property is sometimes called the “butterfly effect”."[9]

Logistic mapEdit

names :

  • logistic map :  
  • logistic equation  
  • logistic difference equation  
  • discrete dynamical system


The logistic map[10][11] is defined by a recurrence relation ( difference equation) :

 

where :

  •   is a given constant parameter
  •   is given the initial term
  •   is subsequent term determined by this relation


Do not confuse it with :

  • differential equation ( which gives continous version )


Bash code [12] Javascript code from Khan Academy [13] MATLAB code:

r_values = (2:0.0002:4)';
iterations_per_value = 10;
y = zeros(length(r_values), iterations_per_value);
y0 = 0.5;
y(:,1) = r_values.*y0*(1-y0);
for i = 1:iterations_per_value-1
    y(:,i+1) = r_values.*y(:,i).*(1-y(:,i));
end
plot(r_values, y, '.', 'MarkerSize', 1);
grid on;

See also Lasin [14]

Maxima CAS code [15]

/* Logistic diagram by Mario Rodriguez Riotorto using Maxima CAS draw packag  */
pts:[];
for r:2.5 while r <= 4.0 step 0.001 do /* min r = 1 */
                (x: 0.25,
                for k:1 thru 1000 do x: r * x * (1-x), /* to remove points from image compute and do not draw it */
                for k:1 thru 500  do (x: r * x * (1-x), /* compute and draw it */
                         pts: cons([r,x], pts))); /* save points to draw it later, re=r, im=x */
load(draw);
draw2d( terminal   = 'png,
        file_name = "v",
        dimensions = [1900,1300],
        title      = "Bifurcation diagram, x[i+1] = r*x[i]*(1 - x[i])",
        point_type = filled_circle,
        point_size = 0.2,
        color = black,
        points(pts));

explicit solutionsEdit

The system with r=4 has the explicit solution for the nth iteration :[16]

  

PrecisionEdit

Numerical Precision in the Chaotic Regime : " the number of digits of precision which must be specified is about 0.6 of the number of iterations. Hence, to determine is x10 000, we need about 6000 digits."

"Hence it is not possible to predict the value of xn for very large n in the chaotic regime." [17]

Better imageEdit

 
Bifurcation diagram of the logistic map

To show more detaile use tips by User:PAR:

"The horizontal axis is the r parameter, the vertical axis is the x variable. The image was created by forming a 1601 x 1001 array representing increments of 0.001 in r and x. A starting value of x=0.25 was used, and the map was iterated 1000 times in order to stabilize the values of x. 100,000 x -values were then calculated for each value of r and for each x value, the corresponding (x,r) pixel in the image was incremented by one. All values in a column (corresponding to a particular value of r) were then multiplied by the number of non-zero pixels in that column, in order to even out the intensities. Values above 250,000 were set to 250,000, and then the entire image was normalized to 0-255. Finally, pixels for values of r below 3.57 were darkened to increase visibility."

See also :

  • tips from learner.org [18]

Lyapunov exponentEdit

Invariant MeasureEdit

An invariant measure or probability density in state space [20]

Video on Youtube[21]

Real quadratic mapEdit

 
Lyapunov exponent - image and Maxima CAS code

Great images by Chip Ross[22]

For  , the code in MATLAB can be written as:

c = (0:0.001:2)';
iterations_per_value = 100;
y = zeros(length(c), iterations_per_value);
y0 = 0;
y(:,1) = y0.^2 - c;
for i = 1:iterations_per_value-1
    y(:,i+1) = y(:,i).^2 - c;
end
plot(c, y, '.', 'MarkerSize', 1, 'MarkerEdgeColor', 'black');

Maxima CAS code for drawing real quadratic map :   :

/* based on the code by by Mario Rodriguez Riotorto */
pts:[];
for c:-2.0 while c <= 0.25 step 0.001 do 
                (x: 0.0,
                for k:1 thru 1000 do x: x * x+c, /* to remove points from image compute and do not draw it */
                for k:1 thru 500  do (x:  x * x+c, /* compute and draw it */
                         pts: cons([c,x], pts))); /* save points to draw it later, re=r, im=x */
load(draw);
draw2d( terminal   = 'svg,
        file_name = "b",
        dimensions = [1900,1300],
        title      = "Bifurcation diagram, x[i+1] = x[i]*x[i] +c",
        point_type = filled_circle,
        point_size = 0.2,
        color = black,
        points(pts));

Lyapunov exponentEdit

program lapunow;

  { program draws bifurcation diagram y[n+1]=y[n]*y[n]+x,} { blue}
  {  x: -2 < x < 0.25 }
  {  y: -2 < y < 2    }
  {  and Lyapunov exponet for each x { white}

  uses crt,graph,
        { modul niestandardowy }
       bmpM, {screenCopy}
       Grafm;
  var xe,xemax,xe0,yemax,i1,i2:integer;
      yer,y,x,w,dx,lap:real;

  const xmin=-2;         { wspolczynnik   funkcji fx(y) }
        xmax=0.25;
        ymax=2;
        ymin=-2;
        i1max=100;            { liczba iteracji }
        i2max=20;
        lapmax=10;
        lapmin=-10;


   function wielomian2st(y,x:real) :real;
     begin
       wielomian2st:=y*y+x;
     end;  { wielomian2st }


  procedure wstep;
     begin
       opengraf;
       randomize;            { przygotowanie generatora liczb losowych }
       xemax:=getmaxx;              { liczba pixeli }
       yemax:=getmaxy;
       w:=(yemax+1)/(ymax-ymin);
       dx:=(xmax-xmin)/(xemax+1);
     end;

  begin {cialo}
    wstep;
    for xe:=xemax downTo 0 do
      begin {xe}
        x:=xmin+xe*dx;     { liniowe skalowanie x=a*xe+b }
        i1:=0;
        i2:=0;
        lap:=0;
        y:=random;        { losowy wybor    y0 : 0<y0<1 }

        while (abs(y)<ymax) and (i1<i1max)
        do
          begin {while i1}
            y:=wielomian2st(y,x);
            i1:=i1+1;
            lap:=lap+ln(abs(2*y)+0.1);
            if keypressed then halt;
          end; {while i1}

        while (i2<i2max) and (abs(y)<ymax)
         do
          begin   {while i2}
            y:=wielomian2st(y,x);
            yer:=(y-ymin)*w;         { skalowanie }
            putpixel(xe,yemax-round(yer),blue); { diagram bifurkacyjny }
            i2:=i2+1;
            lap:=lap+ln(abs(2*y)+0.1);
            if keypressed then halt;
          end; {while i2}

          lap:=lap/(i1max+i2max);
          yer:=(lap-lapmin)*(yemax+1)/(lapmax-lapmin);
          putpixel(xe,yemax-round(yer),white);         { wsp Lapunowa }
          putpixel(xe,yemax-round(-ymin*w),red);       { y=0 }
          putpixel(xe,yemax-round((1-ymin)*w),green);  { y=1}

      end; {xe}

    {..... os 0Y .......................................................}
    setcolor(red);
    xe0:=round((0-Xmin)/dx);     {xe0= xe : x=0 }
    line(xe0,0,xe0,yemax);
     SetColor(red);
    OutTextXY(XeMax-50,yemax-round((0-ymin)*w)+10,'y=0 ');
    SetColor(blue);
    OutTextXY(XeMax-50,yemax-round((1-ymin)*w)+10,'y=1');
    {....................................................................}
    screenCopy('screen',640,480);
    {}
    repeat until keypressed;
    closegraph;

 end.

{ Adam Majewski
Turbo Pascal 7.0  Borland
 MS-Dos / Microsoft}

ReferencesEdit

See alsoEdit