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Journal of the London Mathematical Society
Article . 2004 . Peer-reviewed
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Explicit Rational Functions Whose Julia Sets are Jordan Arcs

Explicit rational functions whose Julia sets are Jordan arcs
Authors: Inninger, Clemens; Peherstorfer, Franz;

Explicit Rational Functions Whose Julia Sets are Jordan Arcs

Abstract

The paper studies a certain class of rational maps and the goal is to provide necessary and sufficient conditions for the Julia set of such maps to be a Jordan arc. The class under consideration is the conjugate under the Joukowski map \(J(z)=\frac{1}{2} (z+\frac{1}{z})\) of a product \(g_{n,\nu}^\pm=\pm z^{n-\nu} \prod_{k=1}^{\nu}\frac{z-z_k}{1-zz_k}\) for some positive integer \(n\) and it appears denoted as \(\mathcal{R}_n^\pm(y,\rho_\nu)=J\circ g_{n,\nu}^\pm\circ J^{-1}(y)\), where \(\rho_\nu(z)= \prod_{k=1}^{\nu}(z-z_k)\). The results are obtained for a slightly more general class, but in all cases, the dynamical behavior is given by the product \(g_{n,\nu}^\pm(z)\), which is a perturbation of a Blaschke product. The Julia set of a Blaschke product is the unit circle \(\mathbb{D}\). Under a very weak condition that the zeros of \(g_{n,\nu}^\pm\) stay in a disc \(\{|z|\leq r\}\), where \(r\) is dependent on \(\nu\) and \(n\), it follows that \(|g_{n,\nu}^\pm(z)|<|z|\) for \(|z|\leq r\). Thus, all orbits starting in the attracting basin at 0 converge to 0, which means that this basin is completely invariant and so is the basin of \(\infty\), by the \((1/z)\)-symmetry. With two completely invariant Fatou components, the Julia set is indeed a Jordan arc (Theorem 2.3). By contrast, when the zeros are somewhat scattered outside an annulus centered at 0, it may happen that the Julia set of \(g_{n,\nu}^\pm\) is not even connected. An example may be found among a sequence of such \(g_{n,\nu}^\pm\)'s, as proved in Theorem 2.5. A necessary condition for the Julia set to be a Jordan arc is given in Section 3, where it is proved that any rational map of degree \(\geq2\) with Julia set a Jordan arc has the form \(\mathcal(R)_n^\pm(y,\rho_\nu)\). In Section 4, a class of rational functions with a rationally indifferent fixed-point is given for which the Julia set is a Jordan arc. First, a transformed Blaschke product by the map \(M(z)=\frac{1+z}{1-z}\) yields a rational function with only one rationally indifferent fixed point at \(\infty\) (Lemma 4.3). This function is then perturbed by an odd polynomial to give a rational map with a rationally indifferent fixed point at \(\infty\) and with Julia set a Jordan arc (Theorems 4.5 and 4.6). These final results allow one to describe a more general class of rational functions whose Julia set is a Jordan arc, which are similar to \(\mathcal{R}_n^\pm(y,\rho_\nu)\), but which may have rationally indifferent fixed points. They are of the form \[ \mathcal{R}_n^\pm(y,\rho_\nu,a,b)= \frac{a+b}{2}+\frac{a-b}{2} \mathcal{R}_n^\pm\left(\frac{2y}{a-b}-\frac{a+b}{a-b},\rho_\nu\right), \] with \(a,b\in\mathbb{C},a\neq b\).

Keywords

rationally indifferent fixed point, Jordan arc, Julia set, Iteration theory, iterative and composite equations, Functional equations in the complex plane, iteration and composition of analytic functions of one complex variable, Small divisors, rotation domains and linearization in holomorphic dynamics, Dynamics of complex polynomials, rational maps, entire and meromorphic functions; Fatou and Julia sets, Real rational functions

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
1
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