Newton's method, Julia and Mandelbrot sets, and complex coloring

Martin Pergler, 1999. Mildly revised in 2004

This page is a mildly annotated catalog of some cool images I've made when poking around with visualization of complex functions by coloration of the complex plane. It includes applications to iterated functions, more specifically Newton's method, Julia sets, and the Mandelbrot set. Both when I did it (1999) and now when I reread it and add a few lost images (2004), I'm persuaded that there is something interesting in here--but now, just as then, I don't have time to figure it out. I'd love it if it inspired someone to pursue it further and uncover whatever the magic is.

Some pages by others dealing with related topics (2004): Hans Lundmark, Frank Farris, Francois Labelle, and Tristan Needham (additional links). It seems the now accepted term for this sort of coloration is "domain coloring".

Coloration of the complex plane

The basic idea is that we color the complex plane with argument given by hue, modulus given by luminosity, so that 0=white, infinite=black.
All diagrams on this web page cover the region from 2-2i to 2+2i.

For some applications we need to adjust the rate at which colors collapse into black or white, so that the picture doesn't go all white or all black. For this we use an adjustment factor. In the above picture, the adjustment was 1.
 

adjustment=0.5
adjustment=2

This coloring is particularly useful for showing roots and poles of functions. One can see the multiplicity by counting the number of repeats of the hue pattern around a point--see how the colors cycle once around 0, twice around -1, and once in the reverse direction around +1.

Plot of (z+1)2z/(z-1). adjustment=0.65
 

Exploration 1: Newton's method for z3-1.

f(z)=z3-1. adjustment=.65

Here are iterations of the standard Newton mapping defined by this function (see here or here for more information of Newton mapping). They converge to to the familiar Newton basin attractor diagram, but the coloured iterations make it easy to see how the fractal self-similarity of the attractor arises. adjustment=0.4.
 

iteration 1
iteration 2
iteration 5
iteration 20

Exploration 2: Julia set of the iterated function g(z)=z2+0.3i.

Here are iterations of the above function g(z), leading to the Julia set attractor (not Newton's method, straight iterations of the function). Once again, the coloring of the 1st iteration graphically describes the twist/turn self-similarity of the attractor. Since the constant 0.3i lies in Mandelbrot set, the limiting Julia set is connected. adjustment=0.1.
iteration 1
iteration 3
iteration 6
iteration 9
 

Exploration 3: Julia set for z2+i

Adjustment=0.1. The constant i is on the border of the Mandelbrot set, so limiting Julia set is a curve (infinite tree).
iteration 1
iteration 3
iteration 6
iteration 9
iteration 12. (numeric loss of precision outside black region--it should be black)

Exploration 4: Creating the Mandelbrot set

We now iterate the Mandelbrot mapping z -> z2+c, starting with z1=c, and plot zn as a function of c. The Mandelbrot set consists of those c for which zn stays bounded. The first set of pictures, for n=2..9 and n=20, shows the twists and turns as the Mandelbrot set is created.
iteration 2
iteration 3
iteration 4
iteration 5
iteration 6
iteration 7
iteration 8
iteration 9
iteration 20 (white should be black, again)

Finally, here are a bunch of iterations starting with n=30. The inside and outside of the Mandelbrot set are firmly established (outside should be black, numeric loss of precision again). However, it's interesting to see how the interior of the set sloshes around. The different bulbs have different periods, I think related to their numbering in the proof of the connectedness of the Mandelbrot set, but I haven't looked it up.

iteration 30
iteration 31
iteration 32
iteration 33
iteration 34
iteration 35
iteration 36
iteration 37
iteration 38

MATLAB source files

Here are the Matlab source files I used to make these images back in 1999, if they're helpful to anyone.
Text and images copyright (c) 1999 and 2004, Martin Pergler. Feel free to quote or adapt, with appropriate credit.