Fractals/Iterations in the complex plane/Fatou set

The Fatou set is called:

  • the domain of normality
  • the domain of equicontinuity

Fatou set, domains and componentsEdit

Then there is a finite number of open sets  , that are left invariant by   and are such that:

  1. the union of the sets   is dense in the plane and
  2.   behaves in a regular and equal way on each of the sets  

The last statement means that the termini of the sequences of iterations generated by the points of   are

  • either precisely the same set, which is then a finite cycle
  • or they are finite cycles of circular or annular shaped sets that are lying concentrically.

In the first case the cycle is attracting, in the second it is neutral.

These sets   are the Fatou domains of   and their union is the Fatou set   of  

Each domain of the Fatou set of a rational map can be classified into one of four different classes.[1]

Each of the Fatou domains contains at least one critical point of   that is:

  • a (finite) point z satisfying  
    • if the degree of the numerator   is at least two larger than the degree of the denominator  
    • if   for some c and a rational function   satisfying this condition.


The complement of   is the Julia set   of  

If all the critical points are preperiodic, that is they are not periodic but eventually land on a periodic cycle, then   is all the sphere; otherwise,   is a nowhere dense set (it is without interior points) and an uncountable set (of the same cardinality as the real numbers). Like     is left invariant by   and on this set the iteration is repelling, meaning that   for all w in a neighbourhood of z [within  ]. This means that   behaves chaotically on the Julia set. Although there are points in the Julia set whose sequence of iterations is finite, there are only a countable number of such points (and they make up an infinitesimal part of the Julia set). The sequences generated by points outside this set behave chaotically, a phenomenon called deterministic chaos.


Number of Fatou set's components in case of rational map:[2]

  • 0 ( Fatou set is empty, the whole Riemann sphere is a Julia set )[3]
  • 1 ( example  , here is only one Fatou domains which consist of one component = full Fatou set)
  • 2 ( example  , here are 2 Fatou domains, both have one component )
  • infinitely many ( example  , here are 2 Fatou domains, one ( the exterior) has one component, the other ( interior) has infinitely many componnets)

the Julia set for the The Samuel Lattes function   consists of the whole complex sphere = Fatou set is empty[4][5]


In case of discrete dynamical system based on complex quadratic polynomial Fatou set can consist of domains ( basins) :

  • attracting ( basin of attraction of fixed point / cycle )
    • superattracting ( Boettcher coordinate )
      • basin of infinity [6]
    • attracting but not superattracting (
  • parabolic (Leau-Fatou) basin ( Fatou coordinate ) Local dynamics near rationally indifferent fixed point/cycle ;
  • elliptic basin = Siegel disc ( Local dynamics near irrationally indifferent fixed point/cycle )

Local discrete dynamicsEdit

Types of dynamics
  • attracting : hyperbolic dynamics
    • superattracting : the very fast ( = exponential) convergence to periodic cycle ( fixed point )
  • parabolic component = slow ( lazy ) dynamics = slow ( exponential slowdown) convergence to parabolic fixed point ( periodic cycle)
  • Siegel disc component = rotation around fixed point and never reach the fixed point


Analysis of local dynamics :

  • drawing critical orbit(s)
  • finding periodic points
  • dividing complex move into simple paths
  • topological graph,[7]
  • drawing grid ( polar or rectangular )
method test description resulting sets true sets
binary escape time bailout abs(zn)>ER escaping and not escaping Escaping set contains fast escaping pixels and is a true exterior.

Not escaping set is treated as a filled Julia set ( interior and boundary) but it contains :

  • slow escaping points from exterior,
  • Julia sets
  • interior points
discrete escape time = Level Set Method = LSM bailout Last iteration or final_n = n : abs(zn)>ER escaping set is divided into subsets with the same n ( last iteration). This subsets are called Level Sets and create bands surrounding and approximating Julia set. Boundaries of level sets are called dwell-bands
continous escape time Example Example Example

Bailout testEdit


  1. Beardon, Iteration of Rational Functions, Theorem 7.1.1.
  2. Beardon, Iteration of Rational Functions, Theorem 5.6.2.
  3. Campbell, J.T., Collins, J.T. Blowup Points and Baby Mandelbrot Sets for a Family of Singularly Perturbed Rational Maps. Qual. Theory Dyn. Syst. 16, 31–52 (2017).
  4. Maru Sarazola : On the cardinalities of the Fatou and Julia sets
  5. Complex Dynamics by Fionn´an Howard
  7. A Topology Simplification Method For 2D Vector Fields by Xavier Tricoche, Gerik Scheuermann and Hans Hagen