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4 Julia Sets

There is a second, more dynamic way to calculate the periods of these primary bulbs in M. To explain this, we have to introduce the notion of a Julia set.

The Julia set for x2 + c is subtly different from the Mandelbrot set. For M, we calculated only the orbit of 0 for each c-value and then displayed the result. A c-value lies in M if the corresponding orbit of 0 does not escape to infinity. Thus, M is a picture in the c-plane, the parameter plane.

For the Julia sets, we fix a c-value and then consider the fate of all possible seeds for that fixed value of c. Those seeds whose orbits do not escape form the Julia set of x2 + c. Orbits that do escape do not lie in the Julia set. Thus we get a different Julia set of each different choice of c. That is, the Julia set is a picture in the dynamical plane, not the parameter plane. We denote the Julia set for x2 + c by Jc. In Figure 7 we have displayed the Julia sets for a variety of c-values.


c = -1.037 + 0.17i in a period 2 bulb


c = -0.52 + 0.57i in a period 5 bulb


c = 0.295 + 0.55i in a period 4 bulb


c = -0.624 + 0.435i in a period 7 bulb.

Figure 7. Julia sets for several c-values

There is one c-value for which the Julia set is easy to compute by hand, namely c=0. The Julia set for x2 + 0, J0, is easily seen to be the unit disk centered at the origin in the plane. The reason is simple. Suppose that z0 is a seed. In polar coordinates we may write where t is the polar angle and r is the magnitude of z0 But then the orbit of z0 under x2 is and so forth. The magnitude of zn is r2^n. Thus, the orbit of x0 tends to infinity if r > 1 and to 0 if r < 1. If r = 1, the orbit of the seed remains trapped for all iterations on the unit circle.

It follows that any seed on or inside the circle of radius 1 centered at the origin has an orbit that does not escape to infinity. It therefore follows that J0 consists of all those seeds whose orbits lie on or insider the unit circle centered at the origin.

Incidentally, the fate of orbits of x2 that lie on the unit circle is quite an interesting story. These are precisely the orbits that behave in a chaotic fashion. See [3] for more details.

Other Julia sets are much more difficult to compute. To see them, we must use a computer. The algorithm is, of course, a direct consequence of the definition of Jc. We simply consider a grid of points centered at the origin and compute the orbit of each of these points under x2 + c. Either this orbit tends to infinity (in which case the seed was not in Jc) or else the orbit does not (and so the seed lies in Jc).
 

Important Difference

The Mandelbrot set:

The Julia set
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