 Symmetry group

Not to be confused with Symmetric group.This article is about the abstract algebraic structures. For other meanings, see Symmetry group (disambiguation).
The symmetry group of an object (image, signal, etc.) is the group of all isometries under which it is invariant with composition as the operation. It is a subgroup of the isometry group of the space concerned.
If not stated otherwise, this article considers symmetry groups in Euclidean geometry, but the concept may also be studied in wider contexts; see below.
Contents
Introduction
The "objects" may be geometric figures, images, and patterns, such as a wallpaper pattern. The definition can be made more precise by specifying what is meant by image or pattern, e.g., a function of position with values in a set of colors. For symmetry of physical objects, one may also want to take physical composition into account. The group of isometries of space induces a group action on objects in it.
The symmetry group is sometimes also called full symmetry group in order to emphasize that it includes the orientationreversing isometries (like reflections, glide reflections and improper rotations) under which the figure is invariant. The subgroup of orientationpreserving isometries (i.e. translations, rotations, and compositions of these) which leave the figure invariant is called its proper symmetry group. The proper symmetry group of an object is equal to its full symmetry group if and only if the object is chiral (and thus there are no orientationreversing isometries under which it is invariant).
Any symmetry group whose elements have a common fixed point, which is true for all finite symmetry groups and also for the symmetry groups of bounded figures, can be represented as a subgroup of orthogonal group O(n) by choosing the origin to be a fixed point. The proper symmetry group is a subgroup of the special orthogonal group SO(n) then, and therefore also called rotation group of the figure.
Discrete symmetry groups come in three types: (1) finite point groups, which include only rotations, reflections, inversion and rotoinversion  they are in fact just the finite subgroups of O(n), (2) infinite lattice groups, which include only translations, and (3) infinite space groups which combines elements of both previous types, and may also include extra transformations like screw axis and glide reflection. There are also continuous symmetry groups, which contain rotations of arbitrarily small angles or translations of arbitrarily small distances. The group of all symmetries of a sphere O(3) is an example of this, and in general such continuous symmetry groups are studied as Lie groups. With a categorization of subgroups of the Euclidean group corresponds a categorization of symmetry groups.
Two geometric figures are considered to be of the same symmetry type if their symmetry groups are conjugate subgroups of the Euclidean group E(n) (the isometry group of R^{n}), where two subgroups H_{1}, H_{2} of a group G are conjugate, if there exists g ∈ G such that H_{1}=g^{−1}H_{2}g. For example:
 two 3D figures have mirror symmetry, but with respect to different mirror planes.
 two 3D figures have 3fold rotational symmetry, but with respect to different axes.
 two 2D patterns have translational symmetry, each in one direction; the two translation vectors have the same length but a different direction.
When considering isometry groups, one may restrict oneself to those where for all points the set of images under the isometries is topologically closed. This excludes for example in 1D the group of translations by a rational number. A "figure" with this symmetry group is nondrawable and up to arbitrarily fine detail homogeneous, without being really homogeneous.
One dimension
The isometry groups in 1D where for all points the set of images under the isometries is topologically closed are:
 the trivial group C_{1}
 the groups of two elements generated by a reflection in a point; they are isomorphic with C_{2}
 the infinite discrete groups generated by a translation; they are isomorphic with Z
 the infinite discrete groups generated by a translation and a reflection in a point; they are isomorphic with the generalized dihedral group of Z, Dih(Z), also denoted by D_{∞} (which is a semidirect product of Z and C_{2}).
 the group generated by all translations (isomorphic with R); this group cannot be the symmetry group of a "pattern": it would be homogeneous, hence could also be reflected. However, a uniform 1D vector field has this symmetry group.
 the group generated by all translations and reflections in points; they are isomorphic with the generalized dihedral group of R, Dih(R).
See also symmetry groups in one dimension.
Two dimensions
Up to conjugacy the discrete point groups in 2 dimensional space are the following classes:
 cyclic groups C_{1}, C_{2}, C_{3}, C_{4},... where C_{n} consists of all rotations about a fixed point by multiples of the angle 360°/n
 dihedral groups D_{1}, D_{2}, D_{3}, D_{4},... where D_{n} (of order 2n) consists of the rotations in C_{n} together with reflections in n axes that pass through the fixed point.
C_{1} is the trivial group containing only the identity operation, which occurs when the figure has no symmetry at all, for example the letter F. C_{2} is the symmetry group of the letter Z, C_{3} that of a triskelion, C_{4} of a swastika, and C_{5}, C_{6} etc. are the symmetry groups of similar swastikalike figures with five, six etc. arms instead of four.
D_{1} is the 2element group containing the identity operation and a single reflection, which occurs when the figure has only a single axis of bilateral symmetry, for example the letter A. D_{2}, which is isomorphic to the Klein fourgroup, is the symmetry group of a nonequilateral rectangle, and D_{3}, D_{4} etc. are the symmetry groups of the regular polygons.
The actual symmetry groups in each of these cases have two degrees of freedom for the center of rotation, and in the case of the dihedral groups, one more for the positions of the mirrors.
The remaining isometry groups in 2D with a fixed point, where for all points the set of images under the isometries is topologically closed are:
 the special orthogonal group SO(2) consisting of all rotations about a fixed point; it is also called the circle group S^{1}, the multiplicative group of complex numbers of absolute value 1. It is the proper symmetry group of a circle and the continuous equivalent of C_{n}. There is no figure which has as full symmetry group the circle group, but for a vector field it may apply (see the 3D case below).
 the orthogonal group O(2) consisting of all rotations about a fixed point and reflections in any axis through that fixed point. This is the symmetry group of a circle. It is also called Dih(S^{1}) as it is the generalized dihedral group of S^{1}.
For nonbounded figures, the additional isometry groups can include translations; the closed ones are:
 the 7 frieze groups
 the 17 wallpaper groups
 for each of the symmetry groups in 1D, the combination of all symmetries in that group in one direction, and the group of all translations in the perpendicular direction
 ditto with also reflections in a line in the first direction
Three dimensions
See also: Point groups in three dimensionsUp to conjugacy the set of 3D point groups consists of 7 infinite series, and 7 separate ones. In crystallography they are restricted to be compatible with the discrete translation symmetries of a crystal lattice. This crystallographic restriction of the infinite families of general point groups results in 32 crystallographic point groups (27 from the 7 infinite series, and 5 of the 7 others).
The continuous symmetry groups with a fixed point include those of:
 cylindrical symmetry without a symmetry plane perpendicular to the axis, this applies for example often for a bottle
 cylindrical symmetry with a symmetry plane perpendicular to the axis
 spherical symmetry
For objects and scalar fields the cylindrical symmetry implies vertical planes of reflection. However, for vector fields it does not: in cylindrical coordinates with respect to some axis, has cylindrical symmetry with respect to the axis if and only if A_{ρ},A_{ϕ}, and A_{z} have this symmetry, i.e., they do not depend on φ. Additionally there is reflectional symmetry if and only if A_{ϕ} = 0.
For spherical symmetry there is no such distinction, it implies planes of reflection.
The continuous symmetry groups without a fixed point include those with a screw axis, such as an infinite helix. See also subgroups of the Euclidean group.
Symmetry groups in general
See also: AutomorphismIn wider contexts, a symmetry group may be any kind of transformation group, or automorphism group. Once we know what kind of mathematical structure we are concerned with, we should be able to pinpoint what mappings preserve the structure. Conversely, specifying the symmetry can define the structure, or at least clarify what we mean by an invariant, geometric language in which to discuss it; this is one way of looking at the Erlangen programme.
For example, automorphism groups of certain models of finite geometries are not "symmetry groups" in the usual sense, although they preserve symmetry. They do this by preserving families of pointsets rather than pointsets (or "objects") themselves.
Like above, the group of automorphisms of space induces a group action on objects in it.
For a given geometric figure in a given geometric space, consider the following equivalence relation: two automorphisms of space are equivalent if and only if the two images of the figure are the same (here "the same" does not mean something like e.g. "the same up to translation and rotation", but it means "exactly the same"). Then the equivalence class of the identity is the symmetry group of the figure, and every equivalence class corresponds to one isomorphic version of the figure.
There is a bijection between every pair of equivalence classes: the inverse of a representative of the first equivalence class, composed with a representative of the second.
In the case of a finite automorphism group of the whole space, its order is the order of the symmetry group of the figure multiplied by the number of isomorphic versions of the figure.
Examples:
 Isometries of the Euclidean plane, the figure is a rectangle: there are infinitely many equivalence classes; each contains 4 isometries.
 The space is a cube with Euclidean metric; the figures include cubes of the same size as the space, with colors or patterns on the faces; the automorphisms of the space are the 48 isometries; the figure is a cube of which one face has a different color; the figure has a symmetry group of 8 isometries, there are 6 equivalence classes of 8 isometries, for 6 isomorphic versions of the figure.
Compare Lagrange's theorem (group theory) and its proof.
See also
Further reading
 Burns, G.; Glazer, A.M. (1990). Space Groups for Scientists and Engineers (2nd ed.). Boston: Academic Press, Inc. ISBN 0121457613.
 Clegg, W (1998). Crystal Structure Determination (Oxford Chemistry Primer). Oxford: Oxford University Press. ISBN 0198559011.
 O'Keeffe, M.; Hyde, B.G. (1996). Crystal Structures; I. Patterns and Symmetry. Washington, DC: Mineralogical Society of America, Monograph Series. ISBN 0939950405.
 Miller, Willard Jr. (1972). Symmetry Groups and Their Applications. New York: Academic Press. OCLC 589081. http://www.ima.umn.edu/~miller/symmetrygroups.html. Retrieved 20090928.
External links
 Weisstein, Eric W., "Symmetry Group" from MathWorld.
 Weisstein, Eric W., "Tetrahedral Group" from MathWorld.
 Overview of the 32 crystallographic point groups  form the first parts (apart from skipping n=5) of the 7 infinite series and 5 of the 7 separate 3D point groups
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