# Periodic point

__: Point which a function/system returns to after some time or iterations__

**Short description**In mathematics, in the study of iterated functions and dynamical systems, a **periodic point** of a function is a point which the system returns to after a certain number of function iterations or a certain amount of time.

## Iterated functions

Given a mapping f from a set X into itself,

- [math]\displaystyle{ f: X \to X, }[/math]

a point x in X is called periodic point if there exists an n so that

- [math]\displaystyle{ \ f_n(x) = x }[/math]

where f_{n} is the nth iterate of f. The smallest positive integer n satisfying the above is called the *prime period* or *least period* of the point x. If every point in X is a periodic point with the same period n, then f is called *periodic* with period n (this is not to be confused with the notion of a periodic function).

If there exist distinct n and m such that

- [math]\displaystyle{ f_n(x) = f_m(x) }[/math]

then x is called a **preperiodic point**. All periodic points are preperiodic.

If f is a diffeomorphism of a differentiable manifold, so that the derivative [math]\displaystyle{ f_n^\prime }[/math] is defined, then one says that a periodic point is *hyperbolic* if

- [math]\displaystyle{ |f_n^\prime|\ne 1, }[/math]

that it is *attractive* if

- [math]\displaystyle{ |f_n^\prime|\lt 1, }[/math]

and it is *repelling* if

- [math]\displaystyle{ |f_n^\prime|\gt 1. }[/math]

If the dimension of the stable manifold of a periodic point or fixed point is zero, the point is called a *source*; if the dimension of its unstable manifold is zero, it is called a *sink*; and if both the stable and unstable manifold have nonzero dimension, it is called a *saddle* or saddle point.

### Examples

A period-one point is called a fixed point.

The logistic map

[math]\displaystyle{ x_{t+1}=rx_t(1-x_t), \qquad 0 \leq x_t \leq 1, \qquad 0 \leq r \leq 4 }[/math]

exhibits periodicity for various values of the parameter r. For r between 0 and 1, 0 is the sole periodic point, with period 1 (giving the sequence 0, 0, 0, …, which attracts all orbits). For r between 1 and 3, the value 0 is still periodic but is not attracting, while the value [math]\displaystyle{ \tfrac{r-1}{r} }[/math] is an attracting periodic point of period 1. With r greater than 3 but less than [math]\displaystyle{ 1 + \sqrt 6, }[/math] there are a pair of period-2 points which together form an attracting sequence, as well as the non-attracting period-1 points 0 and [math]\displaystyle{ \tfrac{r-1}{r}. }[/math] As the value of parameter r rises toward 4, there arise groups of periodic points with any positive integer for the period; for some values of r one of these repeating sequences is attracting while for others none of them are (with almost all orbits being chaotic).

## Dynamical system

Given a real global dynamical system [math]\displaystyle{ (\R, X, \Phi), }[/math] with X the phase space and Φ the evolution function,

- [math]\displaystyle{ \Phi: \R \times X \to X }[/math]

a point x in X is called *periodic* with *period* T if

- [math]\displaystyle{ \Phi(T, x) = x\, }[/math]

The smallest positive T with this property is called *prime period* of the point x.

### Properties

- Given a periodic point x with period T, then [math]\displaystyle{ \Phi(t,x) = \Phi(t+T,x) }[/math] for all t in [math]\displaystyle{ \R. }[/math]
- Given a periodic point x then all points on the orbit γ
_{x}through x are periodic with the same prime period.

## See also

- Limit cycle
- Limit set
- Stable set
- Sharkovsky's theorem
- Stationary point
- Periodic points of complex quadratic mappings

Original source: https://en.wikipedia.org/wiki/Periodic point.
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