Arithmetic dynamics

Template:Short description Arithmetic dynamics is a field that amalgamates two areas of mathematics, dynamical systems and number theory. Part of the inspiration comes from complex dynamics, the study of the iteration of self-maps of the complex plane or other complex algebraic varieties. Arithmetic dynamics is the study of the number-theoretic properties of integer, rational, [[p-adic number|Template:Mvar-adic]], or algebraic points under repeated application of a polynomial or rational function. A fundamental goal is to describe arithmetic properties in terms of underlying geometric structures.

Global arithmetic dynamics is the study of analogues of classical diophantine geometry in the setting of discrete dynamical systems, while local arithmetic dynamics, also called p-adic or nonarchimedean dynamics, is an analogue of complex dynamics in which one replaces the complex numbers Template:Math by a Template:Mvar-adic field such as Template:Math or Template:Math and studies chaotic behavior and the Fatou and Julia sets.

The following table describes a rough correspondence between Diophantine equations, especially abelian varieties, and dynamical systems:

Diophantine equations	Dynamical systems
Rational and integer points on a variety	Rational and integer points in an orbit
Points of finite order on an abelian variety	Preperiodic points of a rational function

Definitions and notation from discrete dynamics

Let Template:Mvar be a set and let Template:Math be a map from Template:Mvar to itself. The iterate of Template:Mvar with itself Template:Mvar times is denoted

$F^{(n)}=F\circ F\circ \cdots \circ F.$

A point Template:Math is periodic if Template:Math for some Template:Math.

The point is preperiodic if Template:Math is periodic for some Template:Math.

The (forward) orbit of Template:Mvar is the set

$O_F(P)=\{P,F(P),F^{(2)}(P),F^{(3)}(P),\cdots \}.$

Thus Template:Mvar is preperiodic if and only if its orbit Template:Math is finite.

Number theoretic properties of preperiodic points

Template:See also Let Template:Math be a rational function of degree at least two with coefficients in Template:Math. A theorem of Douglas Northcott says that Template:Mvar has only finitely many Template:Math-rational preperiodic points, i.e., Template:Mvar has only finitely many preperiodic points in Template:Math. The uniform boundedness conjecture for preperiodic points of Patrick Morton and Joseph Silverman says that the number of preperiodic points of Template:Mvar in Template:Math is bounded by a constant that depends only on the degree of Template:Mvar.

More generally, let Template:Math be a morphism of degree at least two defined over a number field Template:Mvar. Northcott's theorem says that Template:Mvar has only finitely many preperiodic points in Template:Math, and the general Uniform Boundedness Conjecture says that the number of preperiodic points in Template:Math may be bounded solely in terms of Template:Mvar, the degree of Template:Mvar, and the degree of Template:Mvar over Template:Math.

The Uniform Boundedness Conjecture is not known even for quadratic polynomials Template:Math over the rational numbers Template:Math. It is known in this case that Template:Math cannot have periodic points of period four, five, or six, although the result for period six is contingent on the validity of the conjecture of Birch and Swinnerton-Dyer. Bjorn Poonen has conjectured that Template:Math cannot have rational periodic points of any period strictly larger than three.

Integer points in orbits

The orbit of a rational map may contain infinitely many integers. For example, if Template:Math is a polynomial with integer coefficients and if Template:Mvar is an integer, then it is clear that the entire orbit Template:Math consists of integers. Similarly, if Template:Math is a rational map and some iterate Template:Math is a polynomial with integer coefficients, then every Template:Mvar-th entry in the orbit is an integer. An example of this phenomenon is the map Template:Math, whose second iterate is a polynomial. It turns out that this is the only way that an orbit can contain infinitely many integers.

Theorem. Let Template:Math be a rational function of degree at least two, and assume that no iterate of Template:Mvar is a polynomial. Let Template:Math. Then the orbit Template:Math contains only finitely many integers.

Dynamically defined points lying on subvarieties

There are general conjectures due to Shouwu Zhang and others concerning subvarieties that contain infinitely many periodic points or that intersect an orbit in infinitely many points. These are dynamical analogues of, respectively, the Manin–Mumford conjecture, proven by Michel Raynaud, and the Mordell–Lang conjecture, proven by Gerd Faltings. The following conjectures illustrate the general theory in the case that the subvariety is a curve.

Conjecture. Let Template:Math be a morphism and let Template:Math be an irreducible algebraic curve. Suppose that there is a point Template:Math such that Template:Mvar contains infinitely many points in the orbit Template:Math. Then Template:Mvar is periodic for Template:Mvar in the sense that there is some iterate Template:Math of Template:Mvar that maps Template:Mvar to itself.

p-adic dynamics

The field of [[p-adic dynamics|Template:Mvar-adic (or nonarchimedean) dynamics]] is the study of classical dynamical questions over a field Template:Mvar that is complete with respect to a nonarchimedean absolute value. Examples of such fields are the field of Template:Mvar-adic rationals Template:Math and the completion of its algebraic closure Template:Math. The metric on Template:Mvar and the standard definition of equicontinuity leads to the usual definition of the Fatou and Julia sets of a rational map Template:Math. There are many similarities between the complex and the nonarchimedean theories, but also many differences. A striking difference is that in the nonarchimedean setting, the Fatou set is always nonempty, but the Julia set may be empty. This is the reverse of what is true over the complex numbers. Nonarchimedean dynamics has been extended to Berkovich space, which is a compact connected space that contains the totally disconnected non-locally compact field Template:Math.

Generalizations

There are natural generalizations of arithmetic dynamics in which Template:Math and Template:Math are replaced by number fields and their Template:Mvar-adic completions. Another natural generalization is to replace self-maps of Template:Math or Template:Math with self-maps (morphisms) Template:Math of other affine or projective varieties.

Other areas in which number theory and dynamics interact

There are many other problems of a number theoretic nature that appear in the setting of dynamical systems, including:

• dynamics over finite fields.
• dynamics over function fields such as Template:Math.
• iteration of formal and Template:Mvar-adic power series.
• dynamics on Lie groups.
• arithmetic properties of dynamically defined moduli spaces.
• equidistribution and invariant measures, especially on Template:Mvar-adic spaces.
• dynamics on Drinfeld modules.
• number-theoretic iteration problems that are not described by rational maps on varieties, for example, the Collatz problem.
• symbolic codings of dynamical systems based on explicit arithmetic expansions of real numbers.

The Arithmetic Dynamics Reference List gives an extensive list of articles and books covering a wide range of arithmetical dynamical topics.

See also

• Arithmetic geometry
• Arithmetic topology
• Combinatorics and dynamical systems
• Arboreal Galois representation

Further reading

• Lecture Notes on Arithmetic Dynamics Arizona Winter School, March 13–17, 2010, Joseph H. Silverman
• Chapter 15 of A first course in dynamics: with a panorama of recent developments, Boris Hasselblatt, A. B. Katok, Cambridge University Press, 2003, Template:ISBN

External links

• The Arithmetic of Dynamical Systems home page
• Arithmetic dynamics bibliography
• Analysis and dynamics on the Berkovich projective line
• Book review of Joseph H. Silverman's "The Arithmetic of Dynamical Systems", reviewed by Robert L. Benedetto