Expansive homeomorphism

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 $(X,d)$ is a metric space, a homeomorphism $f\colon X\to X$ is said to be expansive if there is a constant $\varepsilon _{0}>0$, called the expansivity constant, such that for any two points of $X$, their n-th iterates are at least $\varepsilon _{0}$ apart for some integer $n$; i.e. if for any pair of points $x\neq y$ in $X$ there is $n\in \mathbb {Z}$ such that $d(f^{n}(x),f^{n}(y))\geq \varepsilon _{0}$. Note that in this definition, $n$ can be positive or negative, and so $f$ may be expansive in the forward or backward directions.

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

If $f\colon X\to X$ is a continuous map, we say that $X$ is positively expansive (or forward expansive) if there is a $\varepsilon _{0}$ such that, for any $x\neq y$ in $X$, there is an $n\in \mathbb {N}$ such that $d(f^{n}(x),f^{n}(y))\geq \varepsilon _{0}$.

Discussion

Positive expansivity is much stronger than expansivity. In fact, one can prove that if $X$ is compact and $f$ is a positively expansive homeomorphism, then $X$ is finite.

See also

• Schwarz-Ahlfors-Pick theorem

expansive at PlanetMath.