The Mandelbrot set is the set of complex points *c* that do not escape to
infinity when repeatedly applying the mapping
*z* → *z*^{2} + *c*,
which admittedly doesn’t sound very interesting. (We might as well
define *f*(*z*) = *z*^{2} + *c*
to avoid having to type “the mapping” again and again; and this is as good
a time as any to mention that we always start the iteration with *f*(0).)

Suppose we start with *c* = 0. The first iteration of *f* gives
*f*(0) = 0^{2} + 0 = 0; the next iteration gives
*f*(*f*(0)) = *f*(0) = 0; next is
*f*(*f*(*f*(0))) = 0, and so forth: when *c* is
zero, all iterates of *f* are equal to zero.

Next, suppose we choose *c* = 1. This gives us

f(0) = 0^{2} + 1 = 1, |

f(1) = 1^{2} + 1 = 2, |

f(2) = 2^{2} + 1 = 5, |

f(5) = 5^{2} + 1 = 26, |

f(26) = 26^{2} + 1 = 677, |

and so forth: the values ultimately escape to infinity.

When we choose *c* = -1, we get

f(0) = 0^{2} - 1 = -1, |

f(-1) = (-1)^{2} - 1 = 0, |

f(0) = 0^{2} - 1 = -1, |

so the values alternate between -1 and 0, and the iteration does not escape.

For any *c*, the first few iterations of *f* are
{0, *c*, *c* + *c*^{2},
*c* + (*c* + *c*^{2})^{2},
*c* + (*c* + (*c* + *c*^{2})^{2})^{2}, ...},
and it turns out that the boundary of the set of complex values of *c* that
do not escape is very complicated.
It can be shown that if the absolute value of an iterate of *f* becomes larger
than 2, the iteration escapes to infinity; the following is a graph of the
level curves (|*f*^{n}(*z*)| = 2) of the successive iterates of *f*.

Show[ MapIndexed[ Graphics[ ContourPlot[Abs[#], {a, -2.0, 0.5}, {b, -1.2, 1.2}, ContourShading->False, Contours->{2.0}, ContourStyle :> Hue[#2[[1]]/11.0], DisplayFunction->Identity, PlotPoints->123]]&, NestList[#^2 + a + b I &, a + b I, 10]], DisplayFunction->$DisplayFunction, AspectRatio->Automatic]

The simplest way to draw filled Mandelbrot sets seems to be to compute the
iterates of *f*, using a more or less fine grid of complex values of *c*,
coloring each point according to the number of iterations it took for the
values to escape to infinity (exceed 2, that is).
(At least that‘s the way I did it here.)

Designed and rendered using *Mathematica* 3.0 for the Apple Macintosh and (much, much later)
7.0 for Microsoft Windows.

© 1996–2024 Robert Dickau

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