 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 distancepreserving 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.
Contents
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 d_{X} and d_{Y}. A map ƒ : X → Y is called an isometry or distance preserving if for any a,b ∈ X one has
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
 Any reflection, translation and rotation is a global isometry on Euclidean spaces. See also Euclidean group.
 The map RR defined by is a path isometry but not an isometry.
 The isometric linear maps from C^{n} to itself are the unitary matrices.
Linear isometry
Given two normed vector spaces V and W, a linear isometry is a linear map f : V → W that preserves the norms:
for all v in V. Linear isometries are distancepreserving maps in the above sense. They are global isometries if and only if they are surjective.
By the MazurUlam theorem, any isometry of normed vector spaces over R is affine.
Generalizations
 Given a positive real number ε, an εisometry or almost isometry (also called a Hausdorff approximation) is a map between metric spaces such that
 for x,x′ ∈ X one has d_{Y}(ƒ(x),ƒ(x′))−d_{X}(x,x′) < ε, and
 for any point y ∈ Y there exists a point x ∈ X with d_{Y}(y,ƒ(x)) < ε
 That is, an εisometry 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 εisometries are not assumed to be continuous.
 Quasiisometry is yet another useful generalization.
See also
 Isometric projection
 Congruence (geometry)
 Euclidean plane isometry
 3D isometries that leave the origin fixed
 Space group
 Involution
 Isometries in physics
 Isometry group
 Homeomorphism group
 Partial isometry
References
 F. S. Beckman and D. A. Quarles, Jr., On isometries of Euclidean space, Proc. Amer. Math. Soc., 4 (1953) 810815.
Categories: Functions and mappings
 Metric geometry
 Symmetry
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