Herman ring
In the mathematical discipline known as complex dynamics, the Herman ring is a Fatou component[1] where the rational function is conformally conjugate to an irrational rotation of the standard annulus.
Formal definition
Namely if ƒ possesses a Herman ring U with period p, then there exists a conformal mapping
and an irrational number , such that
So the dynamics on the Herman ring is simple.
Name
It was introduced by, and later named after, Michael Herman (1979[2]) who first found and constructed this type of Fatou component.
Function
- Polynomials do not have Herman rings.
- Rational functions can have Herman rings
- Transcendental entire maps do not have them[3]
Examples
Here is an example of a rational function which possesses a Herman ring.[1]
where such that the rotation number of ƒ on the unit circle is .
The picture shown on the right is the Julia set of ƒ: the curves in the white annulus are the orbits of some points under the iterations of ƒ while the dashed line denotes the unit circle.
There is an example of rational function that possesses a Herman ring, and some periodic parabolic Fatou components at the same time.
Further, there is a rational function which possesses a Herman ring with period 2.
Here the expression of this rational function is
where
This example was constructed by quasiconformal surgery[4] from the quadratic polynomial
which possesses a Siegel disk with period 2. The parameters a, b, c are calculated by trial and error.
Letting
then the period of one of the Herman ring of ga,b,c is 3.
Shishikura also given an example:[5] a rational function which possesses a Herman ring with period 2, but the parameters showed above are different from his.
So there is a question: How to find the formulas of the rational functions which possess Herman rings with higher period?
According to the result of Shishikura, if a rational function ƒ possesses a Herman ring, then the degree of ƒ is at least 3. There also exist meromorphic functions that possess Herman rings.
Herman rings for transcendental meromorphic functions have been studied by T. Nayak. According to a result of Nayak, if there is an omitted value for such a function then Herman rings of period 1 or 2 do not exist. Also, it is proved that if there is only a single pole and at least an omitted value, the function has no Herman ring of any period.
See also
References
- John Milnor, Dynamics in one complex variable: Third Edition, Annals of Mathematics Studies, 160, Princeton Univ. Press, Princeton, NJ, 2006.
- Herman, Michael-Robert (1979), "Sur la conjugaison différentiable des difféomorphismes du cercle à des rotations", Publications Mathématiques de l'IHÉS (49): 5–233, ISSN 1618-1913, MR 0538680
- Omitted Values and Herman rings by Tarakanta Nayak.
- Mitsuhiro Shishikura, On the quasiconformal surgery of rational functions. Ann. Sci. Ecole Norm. Sup. (4) 20 (1987), no. 1, 1–29.
- Mitsuhiro Shishikura, Surgery of complex analytic dynamical systems, in "Dynamical Systems and Nonlinear Oscillations", Ed. by Giko Ikegami, World Scientific Advanced Series in Dynamical Systems, 1, World Scientific, 1986, 93–105.