# Mandelbrot Set

Mathematician Mandelbrot defined this set in order to study the iteration behavior of the family of quadratic complex functions `z ⟶ f(z) := z*z - c`. Here c is a complex constant, the so called family parameter. We explain the initial part of this program in the exhibit Julia Set.

But the Mandelbrot Set M became even more popular outside of mathematics than inside because it turned out to yield an unlimited collection of wonderful images. These images can be computed with an extraordinarily simple algorithm due to a theorem of Julia and Fatou from the early 20th century: The points of the Mandelbrot set are exactly those points c in the complex plane for which the iteration `c ⟶ f(c) = c*c - c ⟶ f(f(c)) = square(c*c - c) - c ⟶ f(f…f(c)…))` stays bounded, no matter how often f is applied. In practise we have only to check whether this sequence gets an absolute value > 2, because from then on the absolute value increases at every step: `|z| ≥ |c| > 2` implies `|z*z - c| ≥ |z|*|z| - |c| ≥ |z|*|z| - |z| = |z|*(|z| - 1) > |z|` .

Note that `c = 2 ` implies `f(c) = 4 - 2` so that 2 is in M. For `c = 1` we get `f(c) = 0`, `f(f(1)) = -1`, `f(f(f(1))) = 0 = f(1)`, so that 1 is in M.