# Abel equation

(Redirected from Abel function)

The Abel equation, named after Niels Henrik Abel, is a type of functional equation which can be written in the form

${\displaystyle f(h(x))=h(x+1)}$

or, equivalently,

${\displaystyle \alpha (f(x))=\alpha (x)+1}$

and controls the iteration of   f.

## Equivalence

These equations are equivalent. Assuming that α is an invertible function, the second equation can be written as

${\displaystyle \alpha ^{-1}(\alpha (f(x)))=\alpha ^{-1}(\alpha (x)+1)\,.}$

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

${\displaystyle f(\alpha ^{-1}(y))=\alpha ^{-1}(y+1)\,.}$

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

The change of variables sα(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(sα(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,[1]

${\displaystyle \omega (\omega (x,u),v)=\omega (x,u+v)~,}$

e.g., for ${\displaystyle \omega (x,1)=f(x)}$,

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

## History

Initially, the equation in the more general form [2] [3] was reported. Even in the case of a single variable, the equation is non-trivial, and admits special analysis.[4] [5][6]

In the case of a linear transfer function, the solution is expressible compactly. [7]

## 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.,

${\displaystyle \alpha (f(f(x)))=\alpha (x)+2~,}$

and so on,

${\displaystyle \alpha (f_{n}(x))=\alpha (x)+n~.}$

## Solutions

Fatou coordinates describe local dynamics of discrete dynamical system near a parabolic fixed point.