#jsDisabledContent { display:none; } My Account | Register | Help

# Siegel disc

Article Id: WHEBN0012247909
Reproduction Date:

 Title: Siegel disc Author: World Heritage Encyclopedia Language: English Subject: Collection: Publisher: World Heritage Encyclopedia Publication Date:

### Siegel disc

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

## Description

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 $f^n=f\circ\stackrel\left\{\left\left(n\right\right)\right\}\left\{\cdots\right\}\circ f$. We then call the orbit $\mathcal\left\{O\right\}^+\left(z_0\right)$ 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 $\mathbb\left\{C\right\}$, the complex plane or $\mathbb\left\{\hat C\right\}=\mathbb\left\{C\right\}\cup\\left\{\infty\\right\}$, 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 $f^p\left(z_0\right)=z_0$ 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 $\rho=\left(f^p\right)\text{'}\left(z_0\right)$ 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 $n\in\mathbb\left\{Z\right\}$ or $\rho^n\neq1$ for all $n\in\mathbb\left\{Z\right\}$, 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.

## Name

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 $\mathcal\left\{F\right\}\left(f\right)$. We say U is a Siegel disc of f around the point z_0 if there exists an analytic homeomorphism $\phi:U\to\mathbb\left\{D\right\}$ where $\mathbb\left\{D\right\}$ is the unit disc and such that $\phi\left(f^n\left(\phi^\left\{-1\right\}\left(z\right)\right)\right)=e^\left\{2\pi i\alpha\right\}z$ for some $\alpha\in\mathbb\left\{R\right\}\backslash\mathbb\left\{Q\right\}$ and $\phi\left(z_0\right)=0$.

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.

## References

• Siegel disks ar Scholarpedia
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.

Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.