Abel equation

Short description: Equation for function that computes iterated values

This article is about certain functional equations. For ordinary differential equations which are cubic in the unknown function, see Abel equation of the first kind.

The Abel equation, named after Niels Henrik Abel, is a type of functional equation of the form

[math]\displaystyle{ f(h(x)) = h(x + 1) }[/math]

or

[math]\displaystyle{ \alpha(f(x)) = \alpha(x)+1 }[/math].

The forms are equivalent when α is invertible. h or α control the iteration of f.

Equivalence

The second equation can be written

[math]\displaystyle{ \alpha^{-1}(\alpha(f(x))) = \alpha^{-1}(\alpha(x)+1)\, . }[/math]

Taking x = α⁻¹(y), the equation can be written

[math]\displaystyle{ f(\alpha^{-1}(y)) = \alpha^{-1}(y+1)\, . }[/math]

For a known function f(x) , a problem is to solve the functional equation for the function α⁻¹ ≡ h, possibly satisfying additional requirements, such as α⁻¹(0) = 1.

The change of variables s^α(x) = Ψ(x), for a real parameter s, brings Abel's equation into the celebrated Schröder's equation, Ψ(f(x)) = s Ψ(x) .

The further change F(x) = exp(s^α(x)) into Böttcher's equation, F(f(x)) = F(x)^s.

The Abel equation is a special case of (and easily generalizes to) the translation equation,

[math]\displaystyle{ \omega( \omega(x,u),v)=\omega(x,u+v) ~, }[/math]

e.g., for [math]\displaystyle{ \omega(x,1) = f(x) }[/math],

[math]\displaystyle{ \omega(x,u) = \alpha^{-1}(\alpha(x)+u) }[/math].     (Observe ω(x,0) = x.)

The Abel function α(x) further provides the canonical coordinate for Lie advective flows (one parameter Lie groups).

History

Initially, the equation in the more general form was reported. Even in the case of a single variable, the equation is non-trivial, and admits special analysis.

In the case of a linear transfer function, the solution is expressible compactly.

Special cases

The equation of tetration is a special case of Abel's equation, with f = exp.

In the case of an integer argument, the equation encodes a recurrent procedure, e.g.,

[math]\displaystyle{ \alpha(f(f(x)))=\alpha(x)+2 ~, }[/math]

and so on,

[math]\displaystyle{ \alpha(f_n(x))=\alpha(x)+n ~. }[/math]

Solutions

The Abel equation has at least one solution on [math]\displaystyle{ E }[/math] if and only if for all [math]\displaystyle{ x \in E }[/math] and all [math]\displaystyle{ n \in \mathbb{N} }[/math], [math]\displaystyle{ f^{n}(x) \neq x }[/math], where [math]\displaystyle{ f^{n} = f \circ f \circ ... \circ f }[/math], is the function f iterated n times.

Analytic solutions (Fatou coordinates) can be approximated by asymptotic expansion of a function defined by power series in the sectors around a parabolic fixed point. The analytic solution is unique up to a constant.

See also

• Functional equation
  • Schröder's equation
  • Böttcher's equation
• Infinite compositions of analytic functions
• Iterated function
• Shift operator
• Superfunction