# Expansive

In mathematics, the notion of **expansivity** formalizes the notion of points moving away from one another under the action of an iterated function. The idea of expansivity is fairly rigid, as the definition of positive expansivity, below, as well as the Schwarz-Ahlfors-Pick theorem demonstrate.

## Definition

If is a metric space, a homeomorphism is said to be **expansive** if there is a constant

called the **expansivity constant**, such that for any pair of points in there is an integer such that

Note that in this definition, can be positive or negative, and so may be expansive in the forward or backward directions.

The space is often assumed to be compact, since under that assumption expansivity is a topological property; i.e. if is any other metric generating the same topology as , and if is expansive in , then is expansive in (possibly with a different expansivity constant).

If

is a continuous map, we say that is **positively expansive** (or **forward expansive**) if there is a

such that, for any in , there is an such that .

## Theorem of uniform expansivity

Given *f* an expansive homeomorphism, the theorem of uniform expansivity states that for every and there is an such that for each pair of points of such that , there is an with such that

where is the expansivity constant of (proof).

## Discussion

Positive expansivity is much stronger than expansivity. In fact, one can prove that if is compact and is a positively expansive homeomorphism, then is finite (proof).

## External links

- Expansive dynamical systems on scholarpedia

*This article incorporates material from expansive on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.*
*This article incorporates material from uniform expansivity on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.*