Английская Википедия:Isometry group
Шаблон:Short description In mathematics, the isometry group of a metric space is the set of all bijective isometries (that is, bijective, distance-preserving maps) from the metric space onto itself, with the function composition as group operation.[1] Its identity element is the identity function.[2] The elements of the isometry group are sometimes called motions of the space.
Every isometry group of a metric space is a subgroup of isometries. It represents in most cases a possible set of symmetries of objects/figures in the space, or functions defined on the space. See symmetry group.
A discrete isometry group is an isometry group such that for every point of the space the set of images of the point under the isometries is a discrete set.
In pseudo-Euclidean space the metric is replaced with an isotropic quadratic form; transformations preserving this form are sometimes called "isometries", and the collection of them is then said to form an isometry group of the pseudo-Euclidean space.
Examples
- The isometry group of the subspace of a metric space consisting of the points of a scalene triangle is the trivial group. A similar space for an isosceles triangle is the cyclic group of order two, C2. A similar space for an equilateral triangle is D3, the dihedral group of order 6.
- The isometry group of a two-dimensional sphere is the orthogonal group O(3).[3]
- The isometry group of the n-dimensional Euclidean space is the Euclidean group E(n).[4]
- The isometry group of the Poincaré disc model of the hyperbolic plane is the projective special unitary group PSU(1,1).
- The isometry group of the Poincaré half-plane model of the hyperbolic plane is PSL(2,R).
- The isometry group of Minkowski space is the Poincaré group.[5]
- Riemannian symmetric spaces are important cases where the isometry group is a Lie group.
See also
- Point group
- Point groups in two dimensions
- Point groups in three dimensions
- Fixed points of isometry groups in Euclidean space
References
