Given a metric space (loosely, a set and a scheme for assigning distances between elements of the set), an isometry is a transformation which maps elements to the same or another metric space such that the distance between the image elements in the new metric space is equal to the distance between the elements in the original metric space.
In a two-dimensional or three-dimensional Euclidean space, two geometric figures are congruent if they are related by an isometry; the isometry that relates them is either a rigid motion (translation or rotation), or a composition of a rigid motion and a reflection.
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 isometry is automatically injective; otherwise two distinct points, a and b, could be mapped to the same point, thereby contradicting the coincidence axiom of the metric d.
This proof is similar to the proof that an order embedding between partially ordered sets is injective. Clearly, every isometry between metric spaces is a topological embedding.
A global isometry, isometric isomorphism or congruence mapping is a bijective isometry. Like any other bijection, a global isometry has a function inverse.
The inverse of a global isometry is also a global 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; such a map is not necessarily an isometry in the distance preserving sense, and it need not necessarily be bijective, or even injective.
This term is often abridged to simply isometry, so one should take care to determine from context which type is intended.
The map in is a path isometry but not an isometry. Note that unlike an isometry, it is not injective.
Isometries between normed spaces
The following theorem is due to Mazur and Ulam.
Definition: The midpoint of two elements x and y in a vector space is the vector (x + y).
Theorem — Let A : X → Y be a surjective isometry between normed spaces that maps 0 to 0 (Stefan Banach called such maps rotations) where note that A is not assumed to be a linear isometry.
Then A maps midpoints to midpoints and is linear as a map over the real numbers R.
If X and Y are complex vector spaces then A may fail to be linear as a map over C.
An isometry of a manifold is any (smooth) mapping of that manifold into itself, or into another manifold that preserves the notion of distance between points.
The definition of an isometry requires the notion of a metric on the manifold; a manifold with a (positive-definite) metric is a Riemannian manifold, one with an indefinite metric is a pseudo-Riemannian manifold. Thus, isometries are studied in Riemannian geometry.
Given a positive real number ?, an ?-isometry or almost isometry (also called a Hausdorff approximation) is a map between metric spaces such that
for x,x′ ? X one has |dY(?(x),?(x′))−dX(x,x′)| < ?, and
for any point y ? Y there exists a point x ? X with dY(y,?(x)) < ?
That is, an ?-isometry preserves distances to within ? and leaves no element of the codomain further than ? away from the image of an element of the domain. Note that ?-isometries are not assumed to be continuous.
"We shall find it convenient to use the word transformation in the special sense of a one-to-one correspondence among all points in the plane (or in space), that is, a rule for associating pairs of points, with the understanding that each pair has a first member P and a second member P' and that every point occurs as the first member of just one pair and also as the second member of just one pair...
In particular, an isometry (or "congruent transformation," or "congruence") is a transformation which preserves length..."
^Saul, Lawrence K.; Roweis, Sam T. (2003). "Think globally, fit locally: Unsupervised learning of nonlinear manifolds". Journal of Machine Learning Research. 4 (June): 119-155. Quadratic optimisation of (page 135) such that