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Expansive
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Expansive : ウィキペディア英語版
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 (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 pair of points x\neq y in X there is an integer n 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 any other 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 such that d(f^n(x),f^n(y))\geq \varepsilon_0.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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