Complex Numbers
The Mandelbrot set is a set of complex numbers, and a basic understanding of complex numbers is necessary to understand the definition of this set.
A complex number has the general format a+bi where a and b are real numbers while i is the imaginary unit which is defined as the square root of -1. A complex number thus has a real part a and an imaginary part bi. If z is a complex number such that z=a+bi, then a is sometimes referred to as Re(z) while b is referred to as Im(z) (Re - real part, Im - imaginary part).
Addition, subtraction, multiplication and division of complex numbers are defined as follows:
(a+bi) + (c+di) = (a+c) + (b+d)i
(a+bi) - (c+di) = (a-c) + (b-d)i
(a+bi) * (c+di) = (a*c-b*d) + (b*c+a*d)i
(a+bi) / (c+di) = (a*c + b*d) / (c*c + d*d) + ((b*c - a*d) / (c*c + d*d))i
The absolute value of a complex number is defined as:
| a+bi | = √(a2+b2)
The square of a complex number (according to the multiplication rule above) is:
a2-b2 + 2abi
Complex Plane
A complex number can be viewed as a point in a two-dimensional Cartesian coordinate system, referred to as the complex plane. By convention, a is then the horizontal (x-axis) component, while b is the vertical (y-axis) component. The complex number 2.5+1.2i will then correspond to the point (2.5, 1.2) in the coordinate system.
Mandelbrot Set
The Mandelbrot Set has the most well-known graphical visualization of mathematically derived fractal patterns. To understand the set, consider the complex quadratic polynomial
zn+1 = zn2 + c
where c is a complex parameter.
The following sequence can be defined from the polynomial above:
z0 = c
z1 = z02+c = c2+c
z2 = z12+c = (c2+c)2+c
:
For the complex parameter c = 1+2i the sequence develops as follows:
z0 = 1+2i
z1 = 12-22+2*1*2i+1+2i = -2+6i
z2 = (-2)2-62+2*(-2)*6i+1+2i = -31-22i
z3 = (-31)2-222+2*(-31)*(-22)i+1+2i = 478+1366i
:
It will easily been seen that this sequence will yield increasingly "big" numbers, that is, it is not bounded. A sequence which on the other hand is bounded, must be such that there exists a complex number s such that s is greater than zn for any n.
∀n∈{0,1,2,...}∃s∈R : |zn| < s
For the complex parameter c = 0.5+0.6i the sequence develops as follows:
z0 = 0.5+0.6i
z1 = 0.52-0.62+2*0.5*0.6i +0.5+0.6i = -0.39+1.2i
z2 = (-0.39)2-1.22+2*(-0.39)*1.2i +0.5+0.6i = -0.7879-0.336i
:
This sequence look like it is bounded, however, there is generally no way to know this for sure (unless the same number appears for a second time in the sequence, i.e. there exists a j and a k where j ≠ k and zj = zk).
The Mandelbrot set is defined as the set of all complex numbers c for which the sequence zn+1 = zn2 + c is bounded.