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Equivalence of metrics
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Equivalence of metrics : ウィキペディア英語版
Equivalence of metrics
In the study of metric spaces in mathematics, there are various notions of two metrics on the same underlying space being "the same", or equivalent.
In the following, X will denote a non-empty set and d_ and d_ will denote two metrics on X.
==Topological equivalence==

The two metrics d_ and d_ are said to be topologically equivalent if they generate the same topology on X. The adjective "topological" is often dropped.〔Bishop and Goldberg, p. 10.〕 There are multiple ways of expressing this condition:
* a subset A \subseteq X is d_-open if and only if it is d_-open;
* the open balls "nest": for any point x \in X and any radius r > 0, there exist radii r', r'' > 0 such that
:B_ (x; d_) \subseteq B_ (x; d_) and B_ (x; d_) \subseteq B_ (x; d_).
* the identity function I : X \to X is both (d_, d_)-continuous and (d_, d_)-continuous.
The following are sufficient but not necessary conditions for topological equivalence:
* there exists a strictly increasing, continuous, and subadditive f:\mathbb_ \to \mathbb such that d_ = f \circ d_.〔Ok, p. 127, footnote 12.〕
* for each x \in X, there exist positive constants \alpha and \beta such that, for every point y \in X,
:\alpha d_ (x, y) \leq d_ (x, y) \leq \beta d_ (x, y).

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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