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The Fatou set F(f)\, of f\, is the Complement of the Julia set: that is, the set of points which exhibit 'stable' behavior.

Thus on F(f)\,, the behavior of f\, is 'regular', while on J(f)\,, it is ' Chaotic '.

These sets are named in honor of the French mathematicians Gaston Julia and Pierre Fatou , who initiated the theory of Complex Dynamics in the early 20th century.


FORMAL DEFINITION

Let
:f:X o X\,
be an analytic self-map of a Riemann Surface X\,. We will assume that X\, is either the Riemann sphere, the Complex Plane , or the once-punctured complex plane, as the other cases do not give rise to interesting dynamics. (Such maps are Completely Classified .)

We will be considering f\, as a Discrete Dynamical System on the Phase Space
X\,, so we are interested in the behavior of the Iterates f^n\, of f\, (that is, the n\,-fold compositions of f\, with itself).

The Fatou set of f\, consists of all points z\in X\, such that the family of iterates
:(f^n)_{n\in\mathbb{N}}
forms a Normal Family in the sense of Montel when restricted to some open neighborhood of z\,.

The Julia set of f\, is the complement of the Fatou set in X\,.


EQUIVALENT DESCRIPTIONS OF THE JULIA SET


  • J(f)\, is the smallest closed set containing at least three points which is completely invariant under f\,.
  • J(f)\, is the Closure of the set of repelling Periodic Point s.
  • For all but at most two points z\in X\,, the Julia set is the set of limit points of the full backwards orbit \bigcup_n f^{-n}(z). (This suggests a simple algorithm for plotting Julia sets, see below.)
  • If f\, is an Entire Function - in particular, when f\, is a Polynomial , then J(f)\, is the Boundary of the set of points which converge to infinity under iteration.
  • If f\, is a polynomial, then J(f)\, is the boundary of the Filled Julia Set ; that is, those points whose orbits under f\, remain bounded.



PROPERTIES OF THE JULIA SET AND FATOU SET


The Julia set and the Fatou set of f are both Completely Invariant under f, i.e.

\ f^{-1}(J(f)) = f(J(f)) = J(f)

and

\ f^{-1}(F(f)) = f(F(f)) = F(f).
Beardon, ''Iteration of Rational Functions'', Theorem 3.2.4


RATIONAL MAPS

There has been extensive research on the Fatou set and Julia set of iterated Rational Functions , known as rational maps. For example, it is known that the Fatou set of a rational map has either 0,1,2 or infinitely many Components .Beardon, ''Iteration of Rational Functions'', Theorem 5.6.2 Each component of the Fatou set of a rational map can be classified into one of Four Different Classes .Beardon, Theorem 7.1.1


QUADRATIC POLYNOMIALS

A very popular complex dynamical system is given by the family of Quadratic Polynomials , a special case of Rational Map s. The Quadratic Polynomials can be expressed as
:f_c(z) = z^2 + c\,
(where c\, is a complex parameter).


  Image:Time Escape Julia Set From Coordinate (phi-2, Phi-1)jpgJulia Set For F<sub>c</sub>, C (φ&minus2)+(φ&minus1)i =-04+06i
  Image:Time Escape Julia Set From Coordinate (0285, 0)jpgJulia Set For F<sub>c</sub>, C 0285+0i
  Image:Julia Set (highres 01)jpgJulia Set For F<sub>c</sub>, C 0285+001i
  Image:Julia Set Camp3jpgJulia Set For F<sub>c</sub>, C 045+01428i
  Image:Julia Set Camp1jpgJulia Set For F<sub>c</sub>, C -070176-03842i
  Image:Julia Set Camp2jpgJulia Set For F<sub>c</sub>, C -0835-02321i
  Image:Julia Set Camp4 Hi RezpngJulia Set For F<sub>c</sub>, C -08+0156i