**Iris of Salisbury, MD asks:**

*Can you explain the mathematics behind the Mandelbrot set?*

**The Nerd responds:**

I will try. I say try not because I don't think people can handle it, but because I think I only have a rudimentary grasp of the whole thing. I will explain what I know, but I can't guarantee that a good mathematician wouldn't look at this answer and proclaim that I got it all wrong.

The mathematics behind the Mandelbrot set always reminds me of Bismarck's
quote about government: "People who like government or sausage
shouldn't watch them being made." I say this because when you look at
the Mandelbrot set and see its chaotically intricate and beautiful design, you
can't help but feel that there is something larger than life going on here--that
you are staring right at some unexplainable cosmic mystery. Having said
that, there is no denying that in many ways, the Mandelbrot set is a purely
man-made object (mathematicians might be pulling their hair at this
statement). What I mean by this is that you aren't going to spot the
Mandelbrot set out in Nature the way you find the logarithmic spiral in the
nautilus shell or the Fibonacci spiral in galaxies, because the Mandelbrot set
lies in the *complex plane*, which isn't a part of the ordinary world we
live in (and maybe exists only as a mathematical construct). On the other
hand, one of the most striking features about the Mandelbrot set is its *self-similar*
geometry which is a geometry you * will* find throughout Nature.

First things first. If you have never seen the Mandelbrot set, take a look at an image from the Mandelbrot set below. If your browser supports Java, click in the lavender box below the image, and a Java applet will launch (written by Peter Alfeld) consisting of two windows. The window on the left is the control panel, and the one on the right is an image from the mandelbrot set. Use your mouse to select a region of the image. When you release the mouse button, the applet zooms in on the portion you selected.

If you figured out the zoom feature of the Applet, then you
probably noticed the *self-similar* feature of the Mandelbrot set I
mentioned earlier. When you zoom in on a piece of the Mandelbrot set, you
realize that that piece contains, and consists of, another Mandelbrot set.
Zoom in again, and you see that that piece also contains and consists of another
Mandelbrot set. Zoom in again. Same thing. In fact, you can
zoom in forever and you will always see more Mandelbrot sets! This feature
isn't unique to the Mandelbrot set. This *self-similar* property is
true for a class of geometric patterns known as *fractals.* Fractals
are termed *infinitely complex* because the more closely you look at the
object, the more complex it appears.

What makes the Mandelbrot set so interesting is that this infinitely complex patterning is derived from a very simple formula. The basic formula for the Mandelbrot set is:

Z = Z^{2} + C

The Mandelbrot set is determined by *iterating* with this
equation. By iterating, I mean that we start with a value for Z and
C. We plug these into the equation to get a new value for Z. We then
plug that value for Z in and get a new Z, and so on. Let's look at a
simple example that will help us understand iteration.

Let's start with Z = 0 and C = 1. Let's iterate:

Z |
C |
Z = Z^{2} + C |
New Z |

0 | 1 | Z = 0 + 1 | 1 |

1 | 1 | Z = 1 + 1 | 2 |

2 | 1 | Z = 4 + 1 | 5 |

5 | 1 | Z = 25 + 1 | 26 |

As you can see, in this case Z just keeps getting bigger and bigger. But if we choose different values for C this won't always be the case. Suppose we use Z = 0 and C = -1

Z |
C |
Z = Z^{2} + C |
New Z |

0 | -1 | Z = 0 -1 | -1 |

-1 | -1 | Z = 1 - 1 | 0 |

0 | -1 | Z = 0 - 1 | -1 |

-1 | -1 | Z = 1 - 1 | 0 |

As you can see, in this case, Z just flip flops between 0 and -1.

So far we have been using ordinary integers (0, 1, -1, 2, etc.) to iterate
our function. The Mandelbrot set doesn't iterate over these simple
numbers. Instead it iterates over *complex* numbers.

Okay, remember how I told you that the Mandelbrot set wasn't part of the
ordinary world? Well here is the reason. The Mandelbrot set you see
is drawn in the *complex plane*. The complex plane is used to
describe complex numbers. That's easily said. What's harder to
explain is what complex numbers are (and hardest of all is explaining what they
are good for! We won't do that...).

Complex numbers come in two parts: a real part and an *imaginary *part.
The real part is easy to grasp. They are regular numbers that you know and
love: 1, 0, -5, 4.534343, 232423432.4787865, -0.0000000000002, etc.
The imaginary part of a complex number is a real number (like above) multiplied
by a unique little number called **i**.

What is i? Plainly said, i is the square root of -1. Plainly
said, but that doesn't make any sense. Let's look at square roots.
The square root of 4 is 2 because 2 * 2 = 4. The square root of 1 is 1
because 1 * 1 = 1. The square root of 64 is 8 because 8 * 8 = 64.
Now then, (*what) * (that same what)* = -1 ???? Answer:
there is no such thing. The *what* can't be -1 because -1 * -1
does not equal -1; it equals positive 1. Therefore there is no such thing
as the square root of negative one.

Well, this sort of common sense doesn't stop mathematicians. They say,
"Oh, there is no such thing as a number that has the value of the square
root of -1? Well, in that case that number must be *imaginary*."
And that is just what they call it.

More than likely, you find this frustrating. But for the sake of
learning about the Mandelbrot set, we will just have to accept the rationale (or
irrationale) of imaginary numbers and move on. Since the beautiful
pictures we see from the Mandelbrot set is founded on this logic, we must accept
that there is *something* to it ("Beauty is truth, truth beauty,"—that is all
Ye know on earth, and all ye need to know).

As I was saying, complex numbers come in two parts: real and imaginary. Mathematicians write complex numbers in this way:

4 + 3i

With the real part first and the complex part second. When
they want to envision complex numbers, they use the *complex plane*.
You will remember from grade school the good old number line. It wasn't
too hard to grasp. The numbers were just put out in order, usually with
zero as the center, like so:

The same principle goes for complex numbers, but they can't just
use the real number line, because that doesn't include information about the
imaginary part. So to cover that, they insert another axis (the imaginary
axis), and then plot the complex number on this newly formed *complex plane*.
Let's look at an example:

This chart represents the following complex numbers: 2 + 1i, -1.5 + 0.5i, 2 - 2i, -0.5 - 0.5i, 0 + 1i, and 2 + 0i. Let's look a color coded table to make sense of this.

Complex Number |
Real Part |
Imaginary Part |
Description |

2 + 1i |
2 | 1i | move right 2 on the real axis (positive) and move up 1 (positive) on the imaginary axis |

-1.5 + 0.5i |
-1.5 | 0.5 | move left 1.5 on the real axis (negative) and move up 0.5 (positive) on the imaginary axis |

2 - 2i |
2 | -2 | move right 2 on the real axis (positive) and move down 2 (negative) on the imaginary axis |

-0.5 - 0.5i |
-0.5 | -0.5 | move left 0.5 on the real axis (negative) and move down 0.5 (negative) on the imaginary axis |

0 + 1i |
0 | 1 | stay at 0 on the real axis and move up 1 (positive) on the imaginary axis |

2 + 0i |
2 | 0 | move right 2 on the real axis (positive) and stay at zero on the imaginary axis |

(Note: Images and idea for these numbers come from Introduction to the Mandelbrot Set. By David Dewey.)

So, now that we see how to plot complex numbers on the complex plane, let's tie it back in with the Mandelbrot set. Recall our basic formula:

Z = Z^{2} + C

For the Mandelbrot set, we start Z at zero as we did in our
earlier iteration examples. But our choice for C will be a complex
number. Our exact choice for C is what determines the Mandelbrot
set. Recall in our earlier iteration examples that some choices for C
produced iterations that kept getting bigger and bigger (C = 1, for instance)
while with other choices, the iterations never grew much beyond a certain point
(when C = -1, for instance). Well, **the Mandelbrot set consists of all the
choices for C we can find (where Z starts at zero and C is a complex number) so that the
iterations never grow beyond the number 2.** That is the mathematical definition
of the Mandelbrot set.

So how do we go from this definition to the eleborate pictures we see in computer graphics? Well, each value for C can be plotted on the complex plane. In our above figure of the complex plane, we just chose some simple complex values (simple complex?) such as 2 + 0i and -0.5 -0.5i, and put a dot on the complex plane for each point these complex numbers represented. But, of course, there are countless other points in the complex plane, just as there are countless points on the real number line we are more familar with. For example, in between 0 and 1 on the real number line are the points: 0.5, 0.25, 0.33, 0.00232, 0.04575322, etc. And since, the real number line we all know is just the horizontal axis for the complex plane, then clearly, all of these points are also in the complex plane as well as similar points that include the imaginary components: 0.5 +i or 0.5 -4i or 0.33 -4.56334i or 1.4 - 3.3i, etc.

Using computers, we filter through a large sampling of
these infinite number of points on the complex plane, and determine
which of these points fit the Mandelbrot set criteria for
C. The computer does this by running iterations of Z =
Z^{2} +C until it can verify that for that given value of C,
Z *never* gets larger than 2. If the computer is assured of that,
then it knows that **that** C is in the Mandelbrot set and so it plots that
value of C on the complex plane by putting a dot of some color (black
in the Applet above, but the choice of color is entirely arbitrary) on
the computer screen.
The only other possibility is that for that value of C, Z eventually
*does* exceed 2. In that case, C is not part of the Mandelbrot
set, and the computer does not plot it.

By plotting the millions of C values that *are* part of the
Mandelbrot set, the computer creates the distinctive Mandelbrot
geometry that you see on the Applet above. Since Mandelbrot sets are
just a huge collection of points plotted on the complex plane, they
could easily be just one color. However, you will notice that most of
the Mandelbrot sets you see have many colors. The colors you see on
most Mandelbrot sets are added for effect. They show points in
the complex plain that are *not* a part of the Mandelbrot
set. The color chosen
is indicative of how many *iterations* of the above formula were required
before it was shown that that particular value of C was outside the
set. Studying how long (how many iterations) it takes to *prove*
a value of C is outside the Mandelbrot set, is interesting to
mathematicians, so they color code them to help clarify different regions.