Julia set
G. Julia [a1] and P. Fatou
studied the iteration of rational mappings . Let f ^ { n } denote the n - fold composite of the function f with itself. A point z \in C is an element of the so-called Fatou set F ( f ) of f if there exists a neighbourhood U of z in \mathbf C such that the family of iterates \{ f ^ { n } \mid _ {U} \} is a normal family. The Julia set J ( f ) is the complement of the Fatou set. J ( f ) has the following properties: 1) J ( f ) is non-empty and perfect (cf. Perfect set); 2) J ( f ) equals the closure of the set of repelling periodic points (cf. Periodic point); 3) J ( f ) is either totally disconnected (cf. Totally-disconnected space) or connected by Jordan arcs or coincides with \mathbf C ; 4) J ( f ) is invariant with respect to f and f ^ { - 1 } ; and 5) J ( f ) is an attractor (cf. Strange attractor) of the inverse iterated mapping f ^ { - n } . In almost-all cases J( f ) has a fractal dimension and may be termed a fractal (cf. Fractals). D. Sullivan has given an exhaustive classification of the elements of the Fatou set with respect to their dynamics. Every component of F ( f ) is either periodic or pre-periodic. Let D be such a periodic domain and let n be its period. Writing g = f ^ { n } one has the following five kinds of dynamics:
a) D is an attracting domain; D contains an attracting periodic point p with 0 < | g ^ \prime ( p) | < 1 .
b) D is a super-attractive domain; D contains a periodic point p which is also a critical point, i.e. g ^ \prime ( p) = 0 .
c) D is a parabolic domain; its boundary contains a periodic point p with g ^ \prime ( p) = 1 .
d) D is a Siegel disc (cf. Siegel domain); D is simply connected and g \mid _ {D} is analytically equivalent to a rotation.
e) D is a Herman ring: D is conformally equivalent to an annulus and g \mid _ {D} is analytically conjugate to a rigid rotation of an annulus.
Here a pre-periodic point is a point some iterate of which is periodic. A fixed point z _ {0} of f is super-attractive if f ^ { \prime } ( z _ {0} ) = 0 . (Recall that if z _ {0} is a fixed point, then z _ {0} is attractive if | f ^ { \prime } ( z _ {0} ) | < 1 and repelling if | f ^ { \prime } ( z _ {0} ) | > 1 .)
The existence of Herman rings has been proved, but they have never yet (1989) been observed.
The best studied case is the quadratic mapping f = z ^ {2} + c . All phenomena are present there, with the exception of a Herman ring. All c for which J ( f ) is connected form the Mandelbrot set, the bifurcation diagram of J ( f ) in the parameter space of c . See also Chaos; Routes to chaos.
References
[a1] | G. Julia, "Mémoire sur l'iteration des fonctions rationnelles" J. de Math. , 8 (1918) pp. 47–245 |
[a2a] | P. Fatou, "Sur les équations fonctionnelles" Bull. Soc. Math. France , 47 (1919) pp. 161–271 |
[a2b] | P. Fatou, "Sur les équations fonctionnelles" Bull. Soc. Math. France , 48 (1920) pp. 33–94 |
[a2c] | P. Fatou, "Sur les équations fonctionnelles" Bull. Soc. Math. France , 48 (1920) pp. 208–314 |
[a3] | H. Brolin, "Invariant sets under iteration of rational functions" Ark. Mat. , 6 (1965) pp. 103–144 |
[a4] | P. Blanchard, "Complex analytic dynamics" Bull. Amer. Math. Soc. , 11 (1984) pp. 84–141 |
[a5] | R.L. Devaney, "An introduction to chaotic dynamical systems" , Benjamin/Cummings (1986) |
[a6] | H.-O. Peitgen, P.H. Richter, "The beauty of fractals" , Springer (1986) |
Julia set. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Julia_set&oldid=47471