complex dynamics
{{Use American English|date = March 2019}}
{{Short description|Branch of mathematics}}
Complex dynamics, or holomorphic dynamics, is the study of dynamical systems obtained by iterating a complex analytic mapping. This article focuses on the case of algebraic dynamics, where a polynomial or rational function is iterated. In geometric terms, that amounts to iterating a mapping from some algebraic variety to itself. The related theory of arithmetic dynamics studies iteration over the rational numbers or the p-adic numbers instead of the complex numbers.
Dynamics in complex dimension 1
{{Main|Julia set}}
A simple example that shows some of the main issues in complex dynamics is the mapping from the complex numbers C to itself. It is helpful to view this as a map from the complex projective line to itself, by adding a point to the complex numbers. ( has the advantage of being compact.) The basic question is: given a point in , how does its orbit (or forward orbit)
:
behave, qualitatively? The answer is: if the absolute value |z| is less than 1, then the orbit converges to 0, in fact more than exponentially fast. If |z| is greater than 1, then the orbit converges to the point in , again more than exponentially fast. (Here 0 and are superattracting fixed points of f, meaning that the derivative of f is zero at those points. An attracting fixed point means one where the derivative of f has absolute value less than 1.)
On the other hand, suppose that , meaning that z is on the unit circle in C. At these points, the dynamics of f is chaotic, in various ways. For example, for almost all points z on the circle in terms of measure theory, the forward orbit of z is dense in the circle, and in fact uniformly distributed on the circle. There are also infinitely many periodic points on the circle, meaning points with for some positive integer r. (Here means the result of applying f to z r times, .) Even at periodic points z on the circle, the dynamics of f can be considered chaotic, since points near z diverge exponentially fast from z upon iterating f. (The periodic points of f on the unit circle are repelling: if , the derivative of at z has absolute value greater than 1.)
Pierre Fatou and Gaston Julia showed in the late 1910s that much of this story extends to any complex algebraic map from to itself of degree greater than 1. (Such a mapping may be given by a polynomial with complex coefficients, or more generally by a rational function.) Namely, there is always a compact subset of , the Julia set, on which the dynamics of f is chaotic. For the mapping , the Julia set is the unit circle. For other polynomial mappings, the Julia set is often highly irregular, for example a fractal in the sense that its Hausdorff dimension is not an integer. This occurs even for mappings as simple as for a constant . The Mandelbrot set is the set of complex numbers c such that the Julia set of is connected.
File:Parabolic Julia set for internal angle 1 over 3.png
File:Julia set (Rev formula 02).jpg.]]
There is a rather complete classification of the possible dynamics of a rational function in the Fatou set, the complement of the Julia set, where the dynamics is "tame". Namely, Dennis Sullivan showed that each connected component U of the Fatou set is pre-periodic, meaning that there are natural numbers
The equilibrium measure of an endomorphism
Complex dynamics has been effectively developed in any dimension. This section focuses on the mappings from complex projective space
Let f be an endomorphism of
:
for some homogeneous polynomials
Then there is a unique probability measure
=Examples=
- For the mapping
f(z)=z^2 on\mathbf{CP}^1 , the equilibrium measure\mu_f is the Haar measure (the standard measure, scaled to have total measure 1) on the unit circle|z|=1 . - More generally, for an integer
d>1 , letf\colon \mathbf{CP}^n\to\mathbf{CP}^n be the mapping
::
:Then the equilibrium measure
=Characterizations of the equilibrium measure=
A basic property of the equilibrium measure is that it is invariant under f, in the sense that the pushforward measure
One striking characterization of the equilibrium measure is that it describes the asymptotics of almost every point in
Another characterization of the equilibrium measure (due to Briend and Duval) is as follows. For each positive integer r, the number of periodic points of period r (meaning that
The equilibrium measure gives zero mass to any closed complex subspace of
The support
For any continuous endomorphism f of a compact metric space X, the topological entropy of f is equal to the maximum of the measure-theoretic entropy (or "metric entropy") of all f-invariant measures on X. For a holomorphic endomorphism f of
Finally, one can say more about the dynamics of f on the support of the equilibrium measure: f is ergodic and, more strongly, mixing with respect to that measure, by Fornaess and Sibony.Dinh & Sibony (2010), "Dynamics ...", Theorem 1.6.3. It follows, for example, that for almost every point with respect to
=Lattès maps=
A Lattès map is an endomorphism f of
File:Equilibrium measure for Lattes map.png
File:Equilibrium measure for rational function.png
In dimension 1, more is known about the "irregularity" of the equilibrium measure. Namely, define the Hausdorff dimension of a probability measure
:
where
Automorphisms of projective varieties
More generally, complex dynamics seeks to describe the behavior of rational maps under iteration. One case that has been studied with some success is that of automorphisms of a smooth complex projective variety X, meaning isomorphisms f from X to itself. The case of main interest is where f acts nontrivially on the singular cohomology
Gromov and Yosef Yomdin showed that the topological entropy of an endomorphism (for example, an automorphism) of a smooth complex projective variety is determined by its action on cohomology.Cantat (2000), Théorème 2.2. Explicitly, for X of complex dimension n and
:
(The topological entropy of f is also the logarithm of the spectral radius of f on the whole cohomology
Let X be a compact Kähler manifold, which includes the case of a smooth complex projective variety. Say that an automorphism f of X has simple action on cohomology if: there is only one number p such that
For an automorphism f with simple action on cohomology, some of the goals of complex dynamics have been achieved. Dinh, Sibony, and Henry de Thélin showed that there is a unique invariant probability measure
=Kummer automorphisms=
Some abelian varieties have an automorphism of positive entropy. For example, let E be a complex elliptic curve and let X be the abelian surface
The Kummer automorphisms are defined by taking the quotient space by a finite group of an abelian surface with automorphism, and then blowing up to make the surface smooth. The resulting surfaces include some special K3 surfaces and rational surfaces. For the Kummer automorphisms, the equilibrium measure has support equal to X and is smooth outside finitely many curves. Conversely, Cantat and Dupont showed that for all surface automorphisms of positive entropy except the Kummer examples, the equilibrium measure is not absolutely continuous with respect to Lebesgue measure.Cantat & Dupont (2020), Main Theorem. In this sense, it is usual for the equilibrium measure of an automorphism to be somewhat irregular.
=Saddle periodic points=
A periodic point z of f is called a saddle periodic point if, for a positive integer r such that
For an automorphism f with simple action on cohomology, f and its inverse map are ergodic and, more strongly, mixing with respect to the equilibrium measure
A notable difference with the case of endomorphisms of
At least in complex dimension 2, the equilibrium measure of f describes the distribution of the isolated periodic points of f. (There may also be complex curves fixed by f or an iterate, which are ignored here.) Namely, let f be an automorphism of a compact Kähler surface X with positive topological entropy
See also
- Dynamics in complex dimension 1
- Complex analysis
- Complex quadratic polynomial
- Infinite compositions of analytic functions
- Montel's theorem
- Poincaré metric
- Schwarz lemma
- Riemann mapping theorem
- Carathéodory's theorem (conformal mapping)
- Böttcher's equation
- Orbit portraits
- Yoccoz puzzles
- Related areas of dynamics
- Arithmetic dynamics
- Chaos theory
- Symbolic dynamics
Notes
{{reflist}}
References
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External links
- [https://people.math.harvard.edu/~ctm/gallery/ Gallery of dynamics (Curtis McMullen)]
- [http://www.math.sunysb.edu/surveys-dynamical-systems Surveys in Dynamical Systems]