Metric map

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In the mathematical theory of metric spaces, a metric map is a function between metric spaces that does not increase any distance (such functions are always continuous). These maps are the morphisms in the category of metric spaces, Met (Isbell 1964). They are also called Lipschitz functions with Lipschitz constant 1, nonexpansive maps, nonexpanding maps, weak contractions, or short maps.

Specifically, suppose that X and Y are metric spaces and ƒ is a function from X to Y. Thus we have a metric map when, for any points x and y in X,

Here dX and dY denote the metrics on X and Y respectively.


Let us consider the metric space with the Euclidean metric. Then the function is a metric map, since for , .

Category of metric maps[edit]

The composite of metric maps is also metric map, and the identity map on a metric space is a metric map. Thus metric spaces together with metric maps form a category Met. Met is a subcategory of the category of metric spaces and Lipschitz functions. A map between metric spaces is an isometry if and only if it is a bijective metric map whose inverse is also a metric map. Thus the isomorphisms in Met are precisely the isometries.

Strictly metric maps[edit]

One can say that ƒ is strictly metric if the inequality is strict for every two different points. Thus a contraction mapping is strictly metric, but not necessarily the other way around. Note that an isometry is never strictly metric, except in the degenerate case of the empty space or a single-point space.

Multivalued version[edit]

A mapping from a metric space X to the family of nonempty subsets of X is said to be Lipschitz if there exists such that

for all , where H is the Hausdorff distance. When , T is called nonexpansive and when , T is called a contraction.

See also[edit]


  • Isbell, J. R. (1964). "Six theorems about injective metric spaces". Comment. Math. Helv. 39: 65–76. doi:10.1007/BF02566944.