# Isometry

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Isometry
For the mechanical engineering and architecture usage, see isometric projection. For isometry in differential geometry, see isometry (Riemannian geometry).

In mathematics, an isometry is a distance-preserving map between metric spaces. Geometric figures which can be related by an isometry are called congruent.

Isometries are often used in constructions where one space is embedded in another space. For instance, the completion of a metric space M involves an isometry from M into M', a quotient set of the space of Cauchy sequences on M. The original space M is thus isometrically isomorphic to a subspace of a complete metric space, and it is usually identified with this subspace. Other embedding constructions show that every metric space is isometrically isomorphic to a closed subset of some normed vector space and that every complete metric space is isometrically isomorphic to a closed subset of some Banach space.

An isometric surjective linear operator on a Hilbert space is called a unitary operator.

## Definitions

The notion of isometry comes in two main flavors: global isometry and a weaker notion path isometry or arcwise isometry. Both are often called just isometry and one should determine from context which one is intended.

Let X and Y be metric spaces with metrics dX and dY. A map ƒ : X → Y is called an isometry or distance preserving if for any a,b ∈ X one has $d_Y\left(f(a),f(b)\right)=d_X(a,b).$

An isometry is automatically injective. Clearly, every isometry between metric spaces is a topological embedding.

A global isometry, isometric isomorphism or congruence mapping is a bijective isometry.

Two metric spaces X and Y are called isometric if there is a bijective isometry from X to Y. The set of bijective isometries from a metric space to itself forms a group with respect to function composition, called the isometry group.

There is also the weaker notion of path isometry or arcwise isometry:

A path isometry or arcwise isometry is a map which preserves the lengths of curves (not necessarily bijective).

This is often called just isometry and one should determine from context which one is intended.

## Examples

• The map R $\to$R defined by $x\mapsto |x|$ is a path isometry but not an isometry.

## Linear isometry

Given two normed vector spaces V and W, a linear isometry is a linear map f : VW that preserves the norms: $\|f(v)\| = \|v\|$

for all v in V. Linear isometries are distance-preserving maps in the above sense. They are global isometries if and only if they are surjective.

By the Mazur-Ulam theorem, any isometry of normed vector spaces over R is affine.

## Generalizations

• Given a positive real number ε, an ε-isometry or almost isometry (also called a Hausdorff approximation) is a map $f:X\to Y$ between metric spaces such that
1. for x,x′ ∈ X one has |dY(ƒ(x),ƒ(x′))−dX(x,x′)| < ε, and
2. for any point y ∈ Y there exists a point x ∈ X with dY(y,ƒ(x)) < ε
That is, an ε-isometry preserves distances to within ε and leaves no element of the codomain further than ε away from the image of an element of the domain. Note that ε-isometries are not assumed to be continuous.
• Quasi-isometry is yet another useful generalization.

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### Look at other dictionaries:

• isometry — 1941, from Gk. isometria equality of measure, from iso (see ISO (Cf. iso )) + metria a measuring (see METRY (Cf. metry)) …   Etymology dictionary

• isometry — [ī säm′ə trē] n. [ ISO + METRY] 1. equality of measure 2. Geog. equality of height above sea level …   English World dictionary

• isometry — noun (plural tries) Date: 1881 a mapping of a metric space onto another or onto itself so that the distance between any two points in the original space is the same as the distance between their images in the second space < rotation and… …   New Collegiate Dictionary

• isometry — n. [Gr. isos, equal; metron, measure] Growth of two body parts remaining constant relative to each other as body size increases …   Dictionary of invertebrate zoology

• isometry — /uy som i tree/, n. 1. equality of measure. 2. Biol. equal growth rates in two parts of a developing organism. 3. Geog. equality with respect to height above sea level. 4. Math. a function from one metric space onto a second metric space having… …   Universalium

• isometry — noun A function between metric spaces (or on a single metric space) having the property that the distance between two images is equal to the distance between their pre images. See Also: isometric, isometer …   Wiktionary

• isometry — isom·e·try (i somґə tre) equality of dimension …   Medical dictionary

• isometry — isom·e·try …   English syllables

• isometry — i•som•e•try [[t]aɪˈsɒm ɪ tri[/t]] n. 1) equality of measure 2) geo equality with respect to height above sea level • Etymology: 1940–45; iso + metry …   From formal English to slang

• isometry — /aɪˈsɒmətri/ (say uy somuhtree) noun 1. equality of measure. 2. Geography equality with respect to height above sea level …   Australian English dictionary