Current research is done in association with Jacobs University Bremen.

### Papers on homeomorphisms and self-similarity:

[A1]**Homeomorphisms on Edges of the Mandelbrot Set**

Ph.D. thesis of 2002. Available from the RWTH library, the IMS thesis server,

and here as pdf (1.8 Mb).

[A2]

**Homeomorphisms of the Mandelbrot Set**

arXiv:math/0312279. Appeared 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.

Summary.

X
Sketch of a general result on quasiconformal surgery, which
turns combinatorial data into homeomorphisms. Examples.

Definition of non-trivial homeomorphism groups. These are totally disconnected and they have the cardinality of R. Generalization to other parameter spaces, e.g., cubic Newton maps.

Definition of non-trivial homeomorphism groups. These are totally disconnected and they have the cardinality of R. Generalization to other parameter spaces, e.g., cubic Newton maps.

[A3]

**Renormalization and embedded Julia sets in the Mandelbrot set**

Preprint in preparation (August 2017). Summary.

X
Review of holomorphic motions, transversal sections,
straightening of quadratic-like maps.
Self-contained presentation of primitive and
satellite renormalization.

Construction of embedded Julia sets.

Construction of embedded Julia sets.

[A4]

**Quasiconformal and combinatorial surgery**

Preprint in preparation (November 2017). Summary.

X
Straightening of quasiregular quadratic-like maps.
General construction of homeomorphisms of the Mandelbrot set
from combinatorial assumptions.
Description and alternative construction by mapping external angles.
Examples of homeomorphisms on generalized edges.

[A5]

**Self-similarity and homeomorphisms of the Mandelbrot set**

Preprint in preparation (2018). Summary.

X
Combinatorial description of fundamental domains at Misiurewicz
points. Construction of corresponding homeomorphisms.
Review of asymptotic self-similarity.
Generalization to multiple scales around small Mandelbrot sets.
Local similarity, asymptotics, and non-hairiness of decorations.
Generalization to other parameter spaces.

[A6]

**Local similarity of the Mandelbrot set and Julia sets**

Preprint in preparation (2018). Summary.

X
Local similarity between the decorations of
small Mandelbrot sets and Julia sets.

[A7]

**Surgery for Newton-like maps**

Preprint in preparation (2019). See [A2].

### Papers on the Thurston Algorithm:

[B1]**The Thurston Algorithm for quadratic matings**

*Dedicated to the memory of Tan Lei*

arXiv:1706.04177. Preprint of June 2017. Summary.

X
The Thurston Algorithm for the formal mating is implemented by pulling back
a path in moduli space; an alternative initialization by a
repelling-preperiodic capture is discussed as well. When the Thurston
Algorithm diverges in ordinary Teichmueller space due to postcritical
identifications, it still converges on the level of rational maps and colliding
marked points --- it is not necessary to implement the essential mating by
encoding ray-equivalence classes numerically. The proof is based on the
extended pullback map on augmented Teichmueller space constructed by Selinger.

[B2]

**Quadratic matings and ray connections**

arXiv:1706.. Preprint of June 2017. Summary.

X
For certain classes of geometrically finite
and infinite polynomials, the shape and diameter of ray-equivalence classes
is described explicitly, and the topological mating can be constructed
without employing the Rees--Shishikura--Tan Theorem to exclude cyclic ray
connections. On the other hand, unbounded cyclic ray connections are
found when P is primitive renormalizable and Q is chosen appropriately;
then the topological mating is not even defined on a pinched sphere, but
there is no Hausdorff space at all. This paper gives simple examples of
unboundedly shared matings and of mating discontinuity as well.

[B3]

**Quadratic matings and Lattès maps**

Preprint in preparation (July 2017). Summary.

X
Lattes maps of type (2, 2, 2, 2) or (2, 4, 4) are represented by matings in
basically nine, respectively three, different ways. This is proved from
combinatorics of polynomials and ray-equivalence classes. The Shishikura
Algorithm relates the topology of the formal mating to the multiplier of the
corresponding affine map on a torus. The slow mating algorithm diverges in
certain cases: while the expected collisions are happening, a neutral
eigenvalue from the one-dimensional Thurston Algorithm persists, producing an
attracting center manifold in moduli space. (Joint work with Arnaud Cheritat.)
Twisted Lattes maps are discussed as well, and the Hurwitz equivalence between
quadratic rational maps with the same ramification portrait is constructed
explicitly, complementing the approach related to the moduli space map by
Sarah Koch.

See this poster, or order a prelinary version by email.

[B4] Jointly with Arnaud Chéritat:

**Slow mating and equipotential gluing**

Preprint in preparation (July 2017). Summary.

X
Equipotential gluing is an alternative definition of mating, not based on the
Thurston Algorithm. Equipotential lines of the two polynomials are glued to
define maps between spheres, and the limit of potential 0 is considered.
The initialization of the slow mating algorithm depends on an initial radius
R; when R goes to infinity, slow mating is shown to approximate equipotential
gluing. The visualization in terms of holomorphically moving Julia sets and
their convergence is discussed as well, and related to the notion of conformal
mating.

[B5]

**Quadratic captures and anti-matings**

Preprint in preparation (2018). Summary.

X
The slow Thurston Algorithm is implemented for captures and for anti-matings
as well. The latter means that two planes or half-spheres are mapped to each
other by quadratic polynomials, and the filled Julia sets of two quartic
polynomials are glued together. There are results analogous to matings, but
a complete combinatorial description does not exists due to the complexity of
even quartic polynomials. For specific families of quadratic rational
maps, the loci of mating, anti-mating, and captures are obtained numerically.

[B6]

**The Thurston Algorithm for quadratic polynomials**

Preprint in preparation (2018). Summary.

X
The slow Thurston algorithm is implemented for several kinds of Thurston maps
giving quadratic polynomials. These include a spider algorithm with a path
instead of legs, Dehn twisted polynomials, moving the critical value by
recapture or precapture, and tuning. Using the Selinger results on removable
obstructions, the spider algorithm is shown to converge in the obstructed
case of satellite Misiurewicz points as well. Recapture surgery is related
to internal addresses, and used to discuss a specific example of twisted
polynomials.

### Papers on combinatorics and external rays:

[C0]**Some Explicit Formulas for the Iteration of Rational Functions**

Unpublished manuscript of 1997. Download pdf.

[C1,C2]

**Core entropy and biaccessibility of quadratic polynomials**

arXiv:1401.4792. Preprint of January 2014. Summary.

X
Markov matrices for postcritically finite Hubbard trees are
combined with continuity results to discuss core entropy and
biaccessibility dimension of quadratic polynomials.
Specifically, results on monotonicity, level sets, renormalization,
Hoelder asymptotics and self-similarity are obtained.

*Erratum: Lemma 4.1 needs to be modified in the Siegel case.*

See also the appendix [C3] of arXiv:1412.8760.

[C4]

**Edges and frames in the Mandelbrot set**

Preprint in preparation (2019). Summary.

X
Correspondence between puzzle pieces and para-puzzle pieces.
Stepwise construction of new para-puzzle pieces corresponding to
preimages of a puzzle piece that corresponds to a known para-puzzle piece.
Examples: limbs, edges, and frames.

[C5]

**Combinatorics and external rays of the Mandelbrot set**

Preprint in preparation (2019). Summary.

X
Review of combinatorial descriptions by external angles, Hubbard trees,
kneading sequences, and internal addresses.
Discussion and proof of the Stripping Algorithm, which finds
external angles by iterating strips or intervals backwards according
to a given kneading sequence.
Early returns to the characteristic wake. Implementation details.
Review of Thurston Algorithm, Spider Algorithm, twisted rabbits.
Recapture surgery. Example with Dehn twists and early returns.

### Research on the Mandelbrot set and complex dynamics:

When a complex quadratic polynomial*f*

_{c}(z)=z^{2}+c*z*with large modulus escape to ∞. The non-escaping points form the filled Julia set

*K*.

_{c}*M*consists of those parameters

*c*, such that

*K*

_{c}*M*is connected, described it combinatorially by the landing properties of external rays, and explained the appearance of small Mandelbrot sets within

*M*by renormalization. The mathematical field of complex dynamics became active by the use of quasiconformal maps 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 later in Java. I was interested in drawing external rays and in explicit formulas for periodic points. The latter had been found by Netto in 1900 and were rediscovered a few times. See the references in [C0]. 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 ... [C4]. These are structured by two branch points with three branches each, but they look like stars with six branches when they are close to

*a*. In 1999 I noted that the techniques of quasiconformal surgery by Branner-Douady and Branner-Fagella could be adapted to construct a homeomorphism between frames. This formed the basis of my thesis [A1], 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 contains a prominent frame of period 4, 7, ..., which look 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)=z^{2}+c*g*

_{c}(z)*f*

_{c}(z)*f*

_{d}(z)=z^{2}+d*z*=0 is 4-periodic under

*g*

_{c}(z)*f*

_{d}(z)*d*is the center of period 4. Now for any parameter

*c*on the left edge, a new map

*g*

_{c}(z)*f*

_{c}(z)*f*.

_{d}(z)*d*depends on the old parameter

*c*, and the map in parameter space is defined by

*h(c)=d*. It is shown to be continuous and bijective, i.e., a homeomorphism. See [A4] 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 [A2]. The homeomorphisms constructed here are approximately linear at the Misiurewicz points bounding their domains. In [A5], homeomorphisms are constructed at given Misiurewicz points. See the presentation from the conference on complex analysis, Bedlewo 2014.

Denote the multiplier at the Misiurewicz point

*a*by ρ and the center of period

*n*+7

*c*. Then

_{n}*c*

_{n}-a^{-n}

^{-3/2 n}

*n*and found asymptotic models for

*M-c*

_{n}^{-3/2 n}

^{-7/4 n}

A local similarity between the decorations of small Mandelbrot sets and small 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 [A6]. See also demo 7 of Mandel, the interactive exploration with a Java applet here, and the presentation from the workshop on complex dynamics, Søminestationen in Holbæk 2007.

More recent results on embedded Julia sets [A3] are illustrated in demo 5 of Mandel and here with a Java applet. The research on local similarity and embedded Julia sets was accompanied by working out folk results on renormalization. I am also interested in homeomorphisms and similarity phenomena for other families of rational functions. See [A2,A5,A7] and the presentation from the workshop on complex dynamics, Søminestationen in Holbæk 2009. The papers on similarity phenomena are still incomplete, although I produced large quantities of handwritten notes in 2010, 2012, and 2014.

In 2011 I worked on the Stripping Algorithm, which converts kneading sequences to external angles. It is described in [C5] and in the presentation from the workshop on complex dynamics, Göttingen 2011. In 2013 I worked mainly on core entropy and biaccessibility dimension of quadratic polynomials [C1, C2, C3], in research collaboration with Henk Bruin, Dzmitry Dudko, Dierk Schleicher, Tan Lei, and Giulio Tiozzo. In 2015 I started to implement the Thurston Algorithm for quadratic matings with a path in moduli space, and since 2016 I am working on the papers [B1, B2, B3, B4 B5, B6], in research collaboration with Arnaud Chéritat, Adam Epstein, Daniel Meyer, and Dierk Schleicher. The papers provide an initialization of the path for several applications, including anti-matings, and give results on the convergence of slow mating and equipotential gluing. There are examples of mating discontinuity, shared matings, bounded and unbounded ray connections, and a discussion of Lattès matings as well. See the videos of matings, anti-matings, and captures. Some results are announced on the poster from the conference in honor of John H. Hubbard, Bremen 2015, and on the poster from the conference celebrating John Milnor, Cancún 2016 (which I attended only virtually). Convergence results were presented at the workshop on complex dynamics, Søminestationen in Holbæk 2016, and in Toulouse, Warwick, and Bremen as well. See this presentation.

An interactive exposition of complex dynamics is contained as a demo in the program Mandel.

A short introduction is given here with the Java applet Juliette. Online courses at other sites:

Mandelbrot set anatomy by Evgeny Demidov.

Mu-Ency glossary and encyclopedia by Robert Munafo.

Mandelbrot set at Wikipedia.

Logistic equation by Arnaud Chéritat.

Order course at the M. Casco Learning Center.

Fractal geometry and the Mandelbrot set at Yale University.

Math 215, University of Rochester.

Mandelbrot set explorer and interactive papers by Bob Devaney.