The Mandelbrot set and Julia Sets

The boundary of the Mandelbrot set

For a point c in an n-cycle component, the iterates of z0 = 0 converge to an n-cycle. In each n-cycle component, there is exactly one c value for which z0 = 0 belongs to an n-cycle.
That is, taking z0 = 0, we have zn = 0. This c is called the center of the n-cycle component.
For example, the center of the big cardioid (1-cycle component) is c = 0. Observe
z0 = 0,
z1 = z02 + c = 02 + 0 = 0,
and so all zn = 0.
For another example, the center of the 2-cycle component is c = -1. Observe
z0 = 0,
z1 = z02 + c = 02 - 1 = -1,
z2 = z12 + c = (-1)2 - 1 = 0,
so z3 = -1,
z4 = 0,
and so on.
A few centers (indicated by red crosses) are pictured above.

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