A Siegel disc or Siegel disk is a connected component in the Fatou set where the dynamics is analytically conjugate to an irrational rotation.
Description
Given a holomorphic endomorphism
on a Riemann surface
we consider the dynamical system generated by the iterates of
denoted by
. We then call the orbit
of
as the set of forward iterates of
. We are interested in the asymptotic behavior of the orbits in
(which will usually be
, the complex plane or
, the Riemann sphere), and we call
the phase plane or dynamical plane.
One possible asymptotic behavior for a point
is to be a fixed point, or in general a periodic point. In this last case
where
is the period and
means
is a fixed point. We can then define the multiplier of the orbit as
and this enables us to classify periodic orbits as attracting if
superattracting if
), repelling if
and indifferent if
. Indifferent periodic orbits can be either rationally indifferent or irrationally indifferent, depending on whether
for some
or
for all
, 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
are analytically conjugate to an irrational rotation of the complex unit disc.
Name
The Siegel disc is named in honor of Carl Ludwig Siegel.
Gallery
Siegel disc for a polynomial-like mapping[1]
Julia set for
![{\displaystyle B(z)=\lambda a(e^{z/a}(z+1-a)+a-1)}](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
, where
![{\displaystyle a=15-15i}](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
and
![{\displaystyle \lambda }](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
is the
golden ratio. Orbits of some points inside the
Siegel disc emphasized
Julia set for
![{\displaystyle B(z)=\lambda a(e^{z/a}(z+1-a)+a-1)}](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
, where
![{\displaystyle a=-0.33258+0.10324i}](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
and
![{\displaystyle \lambda }](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
is the
golden ratio. Orbits of some points inside the
Siegel disc emphasized. The Siegel disc is either
unbounded or its boundary is an
indecomposable continuum.
[2] Filled Julia set for
![{\displaystyle f_{c}(z)=z*z+c}](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
for
Golden Mean rotation number with interior colored proportional to the average discrete velocity on the orbit = abs( z_(n+1) - z_n ). Note that there is only one Siegel disc and many preimages of the orbits within the Siegel disk
Infolding Siegel disc near 1/2
Infolding Siegel disc near 1/3. One can see virtual Siegel disc
Infolding Siegel disc near 2/7
Julia set for fc(z) = z*z+c where c = -0.749998153581339 +0.001569040474910*I. Internal angle in turns is t = 0.49975027919634618290
Julia set of quadratic polynomial with Siegel disk for rotation number [3,2,1000,1...]
Formal definition
Let
be a holomorphic endomorphism where
is a Riemann surface, and let U be a connected component of the Fatou set
. We say U is a Siegel disc of f around the point
if there exists a biholomorphism
where
is the unit disc and such that
for some
and
.
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.[3]
Later Alexander D. Brjuno improved this condition on the irrationality, enlarging it to the Brjuno numbers.[4]
This is part of the result from the Classification of Fatou components.
See also
Wikibooks has a book on the topic of: Fractals/Iterations in the complex plane/siegel
References
- ^ Polynomial-like maps by Nuria Fagella in The Mandelbrot and Julia sets Anatomy
- ^ Rubén Berenguel and Núria Fagella An entire transcendental family with a persistent Siegel disc, 2009 preprint: arXiV:0907.0116
- ^ Lennart Carleson and Theodore W. Gamelin, Complex Dynamics, Springer 1993
- ^ Milnor, John W. (2006), Dynamics in One Complex Variable, Annals of Mathematics Studies, vol. 160 (Third ed.), Princeton University Press (First appeared in 1990 as a Stony Brook IMS Preprint Archived 2006-04-24 at the Wayback Machine, available as arXiV:math.DS/9201272.)
- Siegel disks at Scholarpedia