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Siegel disc

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Title: Siegel disc  
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Subject: Periodic points of complex quadratic mappings, Irrational rotation, Classification of Fatou components
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Siegel disc

Siegel disc is a connected component in the Fatou set where the dynamics is analytically conjugated to an irrational rotation.


Given a holomorphic endomorphism f:S\to S on a Riemann surface S we consider the dynamical system generated by the iterates of f denoted by f^n=f\circ\stackrel{\left(n\right)}{\cdots}\circ f. We then call the orbit \mathcal{O}^+(z_0) of z_0 as the set of forward iterates of z_0. We are interested in the asymptotic behavior of the orbits in S (which will usually be \mathbb{C}, the complex plane or \mathbb{\hat C}=\mathbb{C}\cup\{\infty\}, the Riemann sphere), and we call S the phase plane or dynamical plane.

One possible asymptotic behavior for a point z_0 is to be a fixed point, or in general a periodic point. In this last case f^p(z_0)=z_0 where p is the period and p=1 means z_0 is a fixed point. We can then define the multiplier of the orbit as \rho=(f^p)'(z_0) and this enables us to classify periodic orbits as attracting if |\rho|<1 superattracting if |\rho|=0), repelling if |\rho|>1 and indifferent if \rho=1. Indifferent periodic orbits split in rationally indifferent and irrationally indifferent, depending on whether \rho^n=1 for some n\in\mathbb{Z} or \rho^n\neq1 for all n\in\mathbb{Z}, respectively.

Siegel discs are one of the possible cases of connected components in the Fatou set (the complementary set of the Julia set), according to Classification of Fatou components, and can occur around irrationally indifferent periodic points. The Fatou set is, roughly, the set of points where the iterates behave similarly to their neighbours (they form a normal family). Siegel discs correspond to points where the dynamics of f is analytically conjugated to an irrational rotation of the complex disc.


The disk is named in honor of Carl Ludwig Siegel.


Formal definition

Let f:S\to S be a holomorphic endomorphism where S is a Riemann surface, and let U be a connected component of the Fatou set \mathcal{F}(f). We say U is a Siegel disc of f around the point z_0 if there exists an analytic homeomorphism \phi:U\to\mathbb{D} where \mathbb{D} is the unit disc and such that \phi(f^n(\phi^{-1}(z)))=e^{2\pi i\alpha}z for some \alpha\in\mathbb{R}\backslash\mathbb{Q} and \phi(z_0)=0.

Siegel's theorem proves the existence of Siegel discs for irrational numbers satisfying a strong irrationality condition (a Diophantine condition), thus solving an open problem since Fatou conjectured his theorem on the Classification of Fatou components.[2]

Later A. D. Brjuno improved this condition on the irrationality, enlarging it to the Brjuno numbers.[3]

This is part of the result from the Classification of Fatou components.

See also


  • Siegel disks ar Scholarpedia
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