Wolf Jung   jung@mndynamics.com
Gesamtschule Aachen-Brand, Rombachstraße 99, 52078 Aachen.

Forschung zur Mandelbrotmenge:

When a complex quadratic polynomial f_c(z)=z²+c is iterated, all points z with large modulus are escaping to ∞. The non-escaping points form the filled Julia set K_c. The Mandelbrot set M consists of those parameters c, such that K_c is connected. Its research was pioneered in the 1980s by A. Douady and J.H. Hubbard. They showed that M is connected, described it combinatorially by the landing properties of external rays, and explained the appearance of little Mandelbrot sets within M by renormalization. The mathematical field of complex dynamics became vivid by the use of quasi-conformal mappings and by the inspiration from computer graphics. The fascination was shared by physicists, programmers, artists ...

In 1996, I started to program the Mandelbrot set M in Qbasic, then C++, and recently in Java. I was interested in drawing external rays and in explicit formulas for periodic points. The latter had been found by E. Netto in 1900 and were rediscovered a few times. See the references in [0]. Soon I was fascinated by the principal branch point a of the 1/3-limb of M, a Misiurewicz point with preperiod 3 and period 1. The left and right branches can be described as a union of “frames”: on the left edge there is one of period 4, two of period 7, four of period 10 ... These are structured by two branch points with three branches each, but they are looking like stars with six branches when they are close to a. In 1999 I noted that the techniques of quasi-conformal surgery by Branner-Douady and Branner-Fagella could be adapted to construct a homeomorphism between frames. This formed the basis of my thesis [1], written under the direction of V. Enss, G. Jank and W. Bergweiler, in research collaboration with J. Riedl and D. Schleicher.

In the image, strips are marked with external rays. Each strip is containing a prominent frame of period 4, 7, ..., which are looking like stars close to the Misiurewicz point a at the center. The homeomorphism h in the parameter plane is expanding at a, mapping each strip to the next one. Consider a center parameter c of period 7: the critical point z=0 is 7-periodic under the iteration of the quadratic polynomial f_c(z)=z²+c. A new mapping g_c(z) is defined by cutting the dynamic plane into strips and sectors with external rays and choosing some iterate of f_c(z) on each piece. This quasi-regular quadratic-like mapping is conjugated to a new quadratic polynomial f_d(z)=z²+d by the Straightening Theorem. In this example, z=0 is 4-periodic under g_c(z) and thus under f_d(z) as well, since the dynamics are conjugate. So the new parameter d is the center of period 4. Now for any parameter c on the left edge, a new mapping g_c(z) is constructed from f_c(z) and straightened to a new polynomial f_d(z). The new parameter d depends on the old parameter c, and the mapping in parameter space is defined by h(c)=d. It is shown to be continuous and bijective, i.e., a homeomorphism. See [3] for details, and demo 8 of Mandel. This construction is possible under general combinatorial assumptions. Results on the homeomorphism group of M are obtained in [2]. The homeomorphisms constructed here are approximately linear at the Misiurewicz points bounding their domains. In [4], homeomorphisms are constructed at given Misiurewicz points.

Denote the multiplier at the Misiurewicz point a by ρ and the center of period n+7 by c_n. Then c_n-a and the diameter of the corresponding frame are of the order ρ^-n by Tan Lei's asymptotic similarity. Noting that the branch points of the frame have a distance of the order ρ^{-3/2 n}, I compared images for different values of n and found asymptotic models for M-c_n on the scales ρ^{-3/2 n}, ρ^{-7/4 n}... See the proof in [4], demo 6 of Mandel, and the interactive exploration with a Java applet here.

A local similarity between the decorations of little Mandelbrot sets and little Julia sets has been observed by H.-O. Peitgen. See his 1988 book. In 2005 I came to believe that this phenomenon was related to an approximately affine renormalization, which I checked with the program, and later found a proof [4]. See also demo 7 of Mandel, the interactive exploration with a Java applet here, and the presentation from the workshop in complex dynamics, Søminestationen in Holbæk, October 2007.

Recent results on embedded Julia sets [4] are illustrated in demo 5 of Mandel and here with a Java applet. I am also interested in homeomorphisms and similarity phenomena for other families of rational functions. See the announcements in [2,4] and the presentation from the workshop in complex dynamics, Søminestationen in Holbæk, October 2009.

Veröffentlichungen und Preprints:

[0] Some Explicit Formulas for the Iteration of Rational Functions
Unveröffentlichtes Manuskript von 1997. Download pdf.

[1] Homeomorphisms on Edges of the Mandelbrot Set
Doktorarbeit von 2002, erhältlich von der RWTH Bibliothek, dem IMS thesis server,
und hier als pdf (1.8 Mb).

[2] Homeomorphisms of the Mandelbrot Set
arXiv:math/0312279. Erschienen in Dynamics on the Riemann Sphere,
A Bodil Branner Festschrift, Poul G. Hjorth and Carsten Lunde Petersen Editors,
EMS, January 2006, pp 139-159. ISBN 3-03719-011-6. Download pdf.

[3] Quasi-conformal and Combinatorial Surgery
Preprint in Vorbereitung (2010).

[4] Self-Similarity and Homeomorphisms of the Mandelbrot Set
Preprint in Vorbereitung (2010).