# Point groups in three dimensions

In geometry, a **point group in three dimensions** is an isometry group in three dimensions that leaves the origin fixed, or correspondingly, an isometry group of a sphere. It is a subgroup of the orthogonal group O(3), the group of all isometries that leave the origin fixed, or correspondingly, the group of orthogonal matrices. O(3) itself is a subgroup of the Euclidean group E(3) of all isometries.

Involutional symmetry C _{s}, (*)[ ] = |
Cyclic symmetry C _{nv}, (*nn)[n] = |
Dihedral symmetry D _{nh}, (*n22)[n,2] = | |

Polyhedral group, [n,3], (*n32) | |||
---|---|---|---|

Tetrahedral symmetry T _{d}, (*332)[3,3] = |
Octahedral symmetry O _{h}, (*432)[4,3] = |
Icosahedral symmetry I _{h}, (*532)[5,3] = |

Symmetry groups of objects are isometry groups. Accordingly, analysis of isometry groups is analysis of possible symmetries. All isometries of a bounded 3D object have one or more common fixed points. We choose the origin as one of them.

The symmetry group of an object is sometimes also called **full symmetry group**, as opposed to its **rotation group** or **proper symmetry group**, the intersection of its full symmetry group and the rotation group SO(3) of the 3D space itself. The rotation group of an object is equal to its full symmetry group if and only if the object is chiral.

The point groups in three dimensions are heavily used in chemistry, especially to describe the symmetries of a molecule and of molecular orbitals forming covalent bonds, and in this context they are also called **molecular point groups**.

Finite Coxeter groups are a special set of *point groups* generated purely by a set of reflectional mirrors passing through the same point. A rank *n* Coxeter group has *n* mirrors and is represented by a Coxeter–Dynkin diagram. Coxeter notation offers a bracketed notation equivalent to the Coxeter diagram, with markup symbols for rotational and other subsymmetry point groups.

## Group structure

SO(3) is a subgroup of E^{+}(3), which consists of *direct isometries*, i.e., isometries preserving orientation; it contains those that leave the origin fixed.

O(3) is the direct product of SO(3) and the group generated by inversion (denoted by its matrix −*I*):

- O(3) = SO(3) × {
*I*, −*I*}

Thus there is a 1-to-1 correspondence between all direct isometries and all indirect isometries, through inversion. Also there is a 1-to-1 correspondence between all groups of direct isometries *H* in O(3) and all groups *K* of isometries in O(3) that contain inversion:

*K*=*H*× {*I*, −*I*}*H*=*K*∩ SO(3)

For instance, if *H* is *C*_{2}, then *K* is *C*_{2h}, or if *H* is *C*_{3}, then *K* is *S*_{6}. (See lower down for the definitions of these groups.)

If a group of direct isometries *H* has a subgroup *L* of index 2, then, apart from the corresponding group containing inversion there is also a corresponding group that contains indirect isometries but no inversion:

*M*=*L*∪ ( (*H*∖*L*) × { −*I*} )

where isometry ( *A*, *I* ) is identified with *A*. An example would be *C*_{4} for *H* and *S*_{4} for *M*.

Thus *M* is obtained from *H* by inverting the isometries in *H* ∖ *L*. This group *M* is as abstract group isomorphic with *H*. Conversely, for all isometry groups that contain indirect isometries but no inversion we can obtain a rotation group by inverting the indirect isometries. This is clarifying when categorizing isometry groups, see below.

In 2D the cyclic group of *k*-fold rotations *C _{k}* is for every positive integer

*k*a normal subgroup of O(2,

**R**) and SO(2,

**R**). Accordingly, in 3D, for every axis the cyclic group of

*k*-fold rotations about that axis is a normal subgroup of the group of all rotations about that axis. Since any subgroup of index two is normal, the group of rotations (

*C*) is normal both in the group obtained by adding reflections in planes through the axis (

_{n}*C*) and in the group obtained by adding a reflection plane perpendicular to the axis (

_{nv}*C*).

_{nh}## 3D isometries that leave origin fixed

The isometries of **R**^{3} that leave the origin fixed, forming the group O(3,**R**), can be categorized as follows:

- SO(3,
**R**):- identity
- rotation about an axis through the origin by an angle not equal to 180°
- rotation about an axis through the origin by an angle of 180°

- the same with inversion (
**x**is mapped to −**x**), i.e. respectively:- inversion
- rotation about an axis by an angle not equal to 180°, combined with reflection in the plane through the origin perpendicular to the axis
- reflection in a plane through the origin

The 4th and 5th in particular, and in a wider sense the 6th also, are called improper rotations.

See also the similar overview including translations.

## Conjugacy

When comparing the symmetry type of two objects, the origin is chosen for each separately, i.e. they need not have the same center. Moreover, two objects are considered to be of the same symmetry type if their symmetry groups are conjugate subgroups of O(3) (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 objects have the same symmetry type:

- if both have mirror symmetry, but with respect to a different mirror plane
- if both have 3-fold rotational symmetry, but with respect to a different axis.

In the case of multiple mirror planes and/or axes of rotation, two symmetry groups are of the same symmetry type if and only if there is a rotation mapping the whole structure of the first symmetry group to that of the second. (In fact there will be more than one such rotation, but not an infinite number as when there is only one mirror or axis.) The conjugacy definition would also allow a mirror image of the structure, but this is not needed, the structure itself is achiral. For example, if a symmetry group contains a 3-fold axis of rotation, it contains rotations in two opposite directions. (The structure * is* chiral for 11 pairs of

*with a screw axis.)*

**space groups**## Infinite isometry groups

There are many **infinite isometry groups**; for example, the "cyclic group" (meaning that it is generated by one element – not to be confused with a torsion group) generated by a rotation by an irrational number of turns about an axis. We may create non-cyclical abelian groups by adding more rotations around the same axis. There are also non-abelian groups generated by rotations around different axes. These are usually (generically) free groups. They will be infinite unless the rotations are specially chosen.

All the infinite groups mentioned so far are not closed as topological subgroups of O(3). We now discuss topologically closed subgroups of O(3).

The whole O(3) is the symmetry group of spherical symmetry; SO(3) is the corresponding rotation group. The other infinite isometry groups consist of all rotations about an axis through the origin, and those with additionally reflection in the planes through the axis, and/or reflection in the plane through the origin, perpendicular to the axis. Those with reflection in the planes through the axis, with or without reflection in the plane through the origin perpendicular to the axis, are the symmetry groups for the two types of cylindrical symmetry. Note that any physical object having infinite rotational symmetry will also have the symmetry of mirror planes through the axis.

## Finite isometry groups

Symmetries in 3D that leave the origin fixed are fully characterized by symmetries on a sphere centered at the origin. For finite 3D point groups, see also spherical symmetry groups.

Up to conjugacy the set of finite 3D point groups consists of:

- 7 infinite series with at most one more-than-2-fold rotation axis; they are the finite symmetry groups on an infinite cylinder, or equivalently, those on a finite cylinder. They are sometimes called the axial or prismatic point groups.
- 7 point groups with multiple 3-or-more-fold rotation axes; they can also be characterized as point groups with multiple 3-fold rotation axes, because all 7 include these axes; with regard to 3-or-more-fold rotation axes the possible combinations are:
- 4 3-fold axes
- 4 3-fold axes and 3 4-fold axes
- 10 3-fold axes and 6 5-fold axes

A selection of point groups is compatible with discrete translational symmetry: 27 from the 7 infinite series, and 5 of the 7 others, the 32 so-called crystallographic point groups. See also the crystallographic restriction theorem.

## The seven infinite series of axial groups

The infinite series of axial or prismatic groups have an index *n*, which can be any integer; in each series, the *n*th symmetry group contains *n*-fold rotational symmetry about an axis, i.e. symmetry with respect to a rotation by an angle 360°/*n*. *n*=1 covers the cases of no rotational symmetry at all. There are four series with no other axes of rotational symmetry (see cyclic symmetries) and three with additional axes of 2-fold symmetry (see dihedral symmetry). They can be understood as point groups in two dimensions extended with an axial coordinate and reflections in it. They are related to the frieze groups;[1] they can be interpreted as frieze-group patterns repeated *n* times around a cylinder.

The following table lists several notations for point groups: Hermann–Mauguin notation (used in crystallography), Schönflies notation (used to describe molecular symmetry), orbifold notation, and Coxeter notation. The latter three are not only conveniently related to its properties, but also to the order of the group. It is a unified notation, also applicable for wallpaper groups and frieze groups. The crystallographic groups have *n* restricted to 1, 2, 3, 4, and 6; removing crystallographic restriction allows any positive integer.
The series are:

H–M | Schön. | Orb. | Cox. | Frieze | Struct. (Order) |
Example | Comments | |
---|---|---|---|---|---|---|---|---|

Even n | Odd n | (cylinder) | ||||||

n | C_{n} | nn | [n]^{+} | p1 | Z_{n}( n) | n-fold rotational symmetry | ||

2n | n | S_{2n} | n× | [2n^{+},2^{+}] | p11g | Z_{2n}(2 n) | n-fold rotoreflection symmetryNot to be confused with the abstract symmetric group | |

n/m | 2n | C_{nh} | n* | [n^{+},2] | p11m | Z_{n}×Dih_{1}(2 n) | ||

nmm | nm | C_{nv} | *nn | [n] | p1m1 | Dih_{n}(2 n) | Pyramidal symmetry; in biology, biradial symmetry | |

n22 | n2 | D_{n} | 22n | [n,2]^{+} | p211 | Dih_{n}(2 n) | Dihedral symmetry | |

2n2m | nm | D_{nd} | 2*n | [2n,2^{+}] | p2mg | Dih_{2n}(4 n) | Antiprismatic symmetry | |

n/mmm | 2n2m | D_{nh} | *22n | [n,2] | p2mm | Dih_{n}×Dih_{1}(4 n) | Prismatic symmetry |

For odd *n* we have Z_{2n} = Z_{n} × Z_{2} and Dih_{2n} = Dih_{n} × Dih_{1}.

The terms horizontal (h) and vertical (v), and the corresponding subscripts, refer to the additional mirror plane, that can be parallel to the rotation axis (vertical) or perpendicular to the rotation axis (horizontal).

The simplest nontrivial ones have involutional symmetry (abstract group Z_{2} or Dih_{1}):

*C*_{i}–**inversion symmetry***C*_{2}–**2-fold rotational symmetry***C*_{s}–**reflection symmetry**, also called**bilateral symmetry**.

The second of these is the first of the uniaxial groups (cyclic groups) *C _{n}* of order

*n*(also applicable in 2D), which are generated by a single rotation of angle 360°/

*n*. In addition to this, one may add a mirror plane perpendicular to the axis, giving the group

*C*of order 2

_{nh}*n*, or a set of

*n*mirror planes containing the axis, giving the group

*C*, also of order 2

_{nv}*n*. The latter is the symmetry group for a regular

*n*-sided pyramid. A typical object with symmetry group

*C*

_{n}or

*D*

_{n}is a propeller.

If both horizontal and vertical reflection planes are added, their intersections give *n* axes of rotation through 180°, so the group is no longer uniaxial. This new group of order 4*n* is called *D _{nh}*. Its subgroup of rotations is the dihedral group

*D*of order 2

_{n}*n*, which still has the 2-fold rotation axes perpendicular to the primary rotation axis, but no mirror planes. Note that in 2D

*D*includes reflections, which can also be viewed as flipping over flat objects without distinction of front- and backside, but in 3D the two operations are distinguished: the group contains "flipping over", not reflections.

_{n}There is one more group in this family, called *D _{nd}* (or

*D*), which has vertical mirror planes containing the main rotation axis, but instead of having a horizontal mirror plane, it has an isometry that combines a reflection in the horizontal plane and a rotation by an angle 180°/

_{nv}*n*.

*D*is the symmetry group for a regular

_{nh}*(n+2)*-sided prisms and also for a regular (2n)-sided bipyramid.

*D*is the symmetry group for a regular

_{nd}*(n+2)*-sided antiprism, and also for a regular

*(2n)*-sided trapezohedron.

*D*is the symmetry group of a partially rotated prism.

_{n}The groups *D*_{2} and *D*_{2h} are noteworthy in that there is no special rotation axis. Rather, there are three perpendicular 2-fold axes. *D*_{2} is a subgroup of all the polyhedral symmetries (see below), and *D*_{2h} is a subgroup of the polyhedral groups T_{h} and O_{h}. *D*_{2} can occur in homotetramers such as Concanavalin A, in tetrahedral coordination compounds with four identical chiral ligands, or in a molecule such as tetrakis(chlorofluoromethyl)methane if all the chlorofluoromethyl groups have the same chirality. The elements of *D*_{2} are in 1-to-2 correspondence with the rotations given by the unit Lipschitz quaternions.

The group *S _{n}* is generated by the combination of a reflection in the horizontal plane and a rotation by an angle 360°/n. For

*n*odd this is equal to the group generated by the two separately,

*C*of order 2

_{nh}*n*, and therefore the notation

*S*is not needed; however, for

_{n}*n*even it is distinct, and of order

*n*. Like

*D*it contains a number of improper rotations without containing the corresponding rotations.

_{nd}All symmetry groups in the 7 infinite series are different, except for the following four pairs of mutually equal ones:

*C*and_{1h}*C*: group of order 2 with a single reflection (_{1v}*C*)_{s}*D*_{1}and*C*_{2}: group of order 2 with a single 180° rotation*D*_{1h}and*C*_{2v}: group of order 4 with a reflection in a plane and a 180° rotation through a line in that plane*D*_{1d}and*C*_{2h}: group of order 4 with a reflection in a plane and a 180° rotation through a line perpendicular to that plane

*S*_{2} is the group of order 2 with a single inversion (*C _{i}* )

"Equal" is meant here as the same up to conjugacy in space. This is stronger than "up to algebraic isomorphism". For example, there are three different groups of order two in the first sense, but there is only one in the second sense. Similarly, e.g. *S _{2n}* is algebraically isomorphic with Z

*.*

_{2n}The groups may be constructed as follows:

- C
_{n}. Generated by an element also called C_{n}, which corresponds to a rotation by angle 2π/*n*around the axis. Its elements are E (the identity), C_{n}, C_{n}^{2}, ..., C_{n}^{n−1}, corresponding to rotation angles 0, 2π/*n*, 4π/*n*, ..., 2(*n*− 1)π/*n*. - S
_{2n}. Generated by element C_{2n}σ_{h}, where σ_{h}is a reflection in the direction of the axis. Its elements are the elements of C_{n}with C_{2n}σ_{h}, C_{2n}^{3}σ_{h}, ..., C_{2n}^{2n−1}σ_{h}added. - C
_{nh}. Generated by element C_{n}and reflection σ_{h}. Its elements are the elements of group C_{n}, with elements σ_{h}, C_{n}σ_{h}, C_{n}^{2}σ_{h}, ..., C_{n}^{n−1}σ_{h}added. - C
_{nv}. Generated by element C_{n}and reflection σ_{v}in a direction in the plane perpendicular to the axis. Its elements are the elements of group C_{n}, with elements σ_{v}, C_{n}σ_{v}, C_{n}^{2}σ_{v}, ..., C_{n}^{n−1}σ_{v}added. - D
_{n}. Generated by element C_{n}and 180° rotation U = σ_{h}σ_{v}around a direction in the plane perpendicular to the axis. Its elements are the elements of group C_{n}, with elements U, C_{n}U, C_{n}^{2}U, ..., C_{n}^{n − 1}U added. - D
_{nd}. Generated by elements C_{2n}σ_{h}and σ_{v}. Its elements are the elements of group C_{n}and the additional elements of S_{2n}and C_{nv}, with elements C_{2n}σ_{h}σ_{v}, C_{2n}^{3}σ_{h}σ_{v}, ..., C_{2n}^{2n − 1}σ_{h}σ_{v}added. - D
_{nh}. Generated by elements C_{n}, σ_{h}, and σ_{v}. Its elements are the elements of group C_{n}and the additional elements of C_{nh}, C_{nv}, and D_{n}.

Taking *n* to ∞ yields groups with continuous axial rotations:

H–M | Schönflies | Orbifold | Coxeter | Limit of | Abstract group | ||
---|---|---|---|---|---|---|---|

∞ | C_{∞} | ∞∞ | [∞]^{+} | C_{n} | Z_{∞} | SO(2) | |

∞, ∞/m | C_{∞h} | ∞* | [2,∞^{+}] | C_{nh}, S_{2n} | Dih_{1}×Z_{∞} | Z_{2}×SO(2) | |

∞m | C_{∞v} | *∞∞ | [∞] | C_{nv} | Dih_{∞} | O(2) | |

∞2 | D_{∞} | 22∞ | [2,∞]^{+} | D_{n} | Dih_{∞} | O(2) | |

∞m, ∞/mm | D_{∞h} | *22∞ | [2,∞] | D_{nh}, D_{nd} | Dih_{1}×Z_{∞} | Z_{2}×O(2) |

## The seven remaining point groups

The remaining point groups are said to be of very high or polyhedral symmetry because they have more than one rotation axis of order greater than 2. Here, C_{n} denotes an axis of rotation through 360°/n and S_{n} denotes an axis of improper rotation through the same. In parentheses are the orbifold notation, Coxeter notation (Coxeter diagram), the full Hermann–Mauguin notation, and the abbreviated one if different. The groups are:

T, (332)[3,3] ^{+} (23 order 12 | chiral tetrahedral symmetry | There are four C_{3} axes, each through two vertices of a cube (body diagonals) or one of a regular tetrahedron, and three C_{2} axes, through the centers of the cube's faces, or the midpoints of the tetrahedron's edges. This group is isomorphic to A_{4}, the alternating group on 4 elements, and is the rotation group for a regular tetrahedron. It is a normal subgroup of T_{d}, T_{h}, and the octahedral symmetries. The elements of the group correspond 1-to-2 to the rotations given by the 24 unit Hurwitz quaternions (the "binary tetrahedral group"). |

T, (*332)_{d}[3,3] ( 43m order 24 | full tetrahedral symmetry | This group has the same rotation axes as T, but with six mirror planes, each containing two edges of the cube or one edge of the tetrahedron, a single C_{2} axis and two C_{3} axes. The C_{2} axes are now actually S_{4} axes. This group is the symmetry group for a regular tetrahedron. T_{d} is isomorphic to S_{4}, the symmetric group on 4 letters, because there is a 1-to-1 correspondence between the elements of T_{d} and the 24 permutations of the four 3-fold axes. An object of C_{3v} symmetry under one of the 3-fold axes gives rise under the action of T_{d} to an orbit consisting of four such objects, and T_{d} corresponds to the set of permutations of these four objects. T_{d} is a normal subgroup of O_{h}. See also the isometries of the regular tetrahedron. |

T, (3*2)_{h}[3 ^{+},4] (2/m3, m3 order 24 | pyritohedral symmetry | This group has the same rotation axes as T, with mirror planes parallel to the cube faces. The C_{3} axes become S_{6} axes, and there is inversion symmetry. T_{h} is isomorphic to A_{4} × Z_{2} (since T and C_{i} are both normal subgroups), and not to the symmetric group S_{4}. It is the symmetry of a cube with on each face a line segment dividing the face into two equal rectangles, such that the line segments of adjacent faces do not meet at the edge. The symmetries correspond to the even permutations of the body diagonals and the same combined with inversion. It is also the symmetry of a pyritohedron, which is similar to the cube described, with each rectangle replaced by a pentagon with one symmetry axis and 4 equal sides and 1 different side (the one corresponding to the line segment dividing the cube's face); i.e., the cube's faces bulge out at the dividing line and become narrower there. It is a subgroup (but not a normal subgroup) of the full icosahedral symmetry group (as isometry group, not just as abstract group), with 4 of the 10 3-fold axes. It is a normal subgroup of O_{h}. |

O, (432)[4,3] ^{+} (432 order 24 | chiral octahedral symmetry | This group is like T, but the C_{2} axes are now C_{4} axes, and additionally there are 6 C_{2} axes, through the midpoints of the edges of the cube. This group is also isomorphic to S_{4} because its elements are in 1-to-1 correspondence to the 24 permutations of the 3-fold axes, as with T. An object of D_{3} symmetry under one of the 3-fold axes gives rise under the action of O to an orbit consisting of four such objects, and O corresponds to the set of permutations of these four objects. It is the rotation group of the cube and octahedron. Representing rotations with quaternions, O is made up of the 24 unit Hurwitz quaternions and the 24 Lipschitz quaternions of squared norm 2 normalized by dividing by . As before, this is a 1-to-2 correspondence. |

O, (*432)_{h}[4,3] ( 4/m32/m, m3m order 48 | full octahedral symmetry | This group has the same rotation axes as O, but with mirror planes, comprising both the mirror planes of T and _{d}T. This group is isomorphic to _{h}S_{4} × Z_{2} (because both O and C are normal subgroups), and is the symmetry group of the cube and octahedron. See also the isometries of the cube._{i} |

I, (532)[5,3] ^{+} (532 order 60 | chiral icosahedral symmetry | the rotation group of the icosahedron and the dodecahedron. It is a normal subgroup of index 2 in the full group of symmetries I. The group contains 10 versions of _{h}D and 6 versions of _{3}D (rotational symmetries like prisms and antiprisms). It also contains five versions of _{5}T (see Compound of five tetrahedra). The group I is isomorphic to A_{5}, the alternating group on 5 letters, since its elements correspond 1-to-1 with even permutations of the five T symmetries (or the five tetrahedra just mentioned). Representing rotations with quaternions, _{h}I is made up of the 120 unit icosians. As before, this is a 1-to-2 correspondence. |

I, (*532)_{h}[5,3] ( 532/m, 53m order 120 | full icosahedral symmetry | the symmetry group of the icosahedron and the dodecahedron. The group I is isomorphic to _{h}A_{5} × Z_{2} because I and C_{i} are both normal subgroups. The group contains 10 versions of D, 6 versions of _{3d}D (symmetries like antiprisms), and 5 versions of T_{5d}_{h}. |

The continuous groups related to these groups are:

As noted above for the infinite isometry groups, any physical object having K symmetry will also have K_{h} symmetry.

## Relation between orbifold notation and order

The order of each group is 2 divided by the orbifold Euler characteristic; the latter is 2 minus the sum of the feature values, assigned as follows:

*n*without or before * counts as (*n*−1)/*n**n*after * counts as (*n*−1)/(2*n*)- * and × count as 1

This can also be applied for wallpaper groups and frieze groups: for them, the sum of the feature values is 2, giving an infinite order; see orbifold Euler characteristic for wallpaper groups

## Reflective Coxeter groups

A_{3}, [3,3], |
B_{3}, [4,3], |
H_{3}, [5,3], |
---|---|---|

6 mirrors |
3+6 mirrors |
15 mirrors |

2A_{1}, [1,2], |
3A_{1}, [2,2], |
A_{1}A_{2}, [2,3], |

2 mirrors |
3 mirrors |
4 mirrors |

A_{1}, [1], |
2A_{1}, [2], |
A_{2}, [3], |

1 mirror |
2 mirrors |
3 mirrors |

The reflective point groups in three dimensions are also called Coxeter groups and can be given by a Coxeter-Dynkin diagram and represent a set of mirrors that intersect at one central point, and bound a spherical triangle domain on the surface of a sphere. Coxeter groups with less than 3 generators have degenerate spherical triangle domains, as lunes or a hemisphere. In Coxeter notation these groups are tetrahedral symmetry [3,3], octahedral symmetry [4,3], icosahedral symmetry [5,3], and dihedral symmetry [p,2]. The number of mirrors for an irreducible group is *nh/2*, where *h* is the Coxeter group's Coxeter number, *n* is the dimension (3).[2]

Weyl group |
Coxeter notation |
Order | Coxeter number (h) |
Mirrors (m) | |
---|---|---|---|---|---|

Polyhedral groups | |||||

A_{3} | [3,3] | 24 | 4 | 6 | |

B_{3} | [4,3] | 48 | 6 | 3+6 | |

H_{3} | [5,3] | 120 | 10 | 15 | |

Dihedral groups | |||||

2A_{1} | [1,2] | 4 | 1+1 | ||

3A_{1} | [2,2] | 8 | 2+1 | ||

I_{2}(p)A_{1} | [p,2] | 4p | p+1 | ||

Cyclic groups | |||||

2A_{1} | [2] | 4 | 2 | ||

I_{2}(p) | [p] | 2p | p | ||

Single mirror | |||||

A_{1} | [ ] | 2 | 1 |

## Rotation groups

The rotation groups, i.e. the finite subgroups of SO(3), are: the cyclic groups *C*_{n} (the rotation group of a canonical pyramid), the dihedral groups *D*_{n} (the rotation group of a uniform prism, or canonical bipyramid), and the rotation groups *T*, *O* and *I* of a regular tetrahedron, octahedron/cube and icosahedron/dodecahedron.

In particular, the dihedral groups *D*_{3}, *D*_{4} etc. are the rotation groups of plane regular polygons embedded in three-dimensional space, and such a figure may be considered as a degenerate regular prism. Therefore, it is also called a *dihedron* (Greek: solid with two faces), which explains the name *dihedral group*.

- An object with symmetry group
*C*,_{n}*C*,_{nh}*C*or_{nv}*S*has rotation group_{2n}*C*_{n}. - An object with symmetry group
*D*,_{n}*D*, or_{nh}*D*has rotation group_{nd}*D*_{n}. - An object with one of the other seven symmetry groups has as rotation group the corresponding one without subscript:
*T*,*O*or*I*.

The rotation group of an object is equal to its full symmetry group if and only if the object is chiral. In other words, the chiral objects are those with their symmetry group in the list of rotation groups.

Given in Schönflies notation, Coxeter notation, (orbifold notation), the rotation subgroups are:

Reflection | Reflection/rotational | Improper rotation | Rotation |
---|---|---|---|

C, [n], (*nn)_{nv} |
C, [n_{nh}^{+},2], (n*) |
S, [2n_{2n}^{+},2^{+}], (n×) |
C, [n]_{n}^{+}, (nn) |

D, [2,n], (*n22)_{nh} |
D, [2_{nd}^{+},2n], (2*n) |
D, [2,n]_{n}^{+}, (n22) | |

T, [3,3], (*332)_{d} |
T, [3,3]^{+}, (332) | ||

O, [4,3], (*432)_{h} |
T, [3_{h}^{+},4], (3*2) |
O, [4,3]^{+}, (432) | |

I, [5,3], (*532)_{h} |
I, [5,3]^{+}, (532) | ||

## Correspondence between rotation groups and other groups

The following groups contain inversion:

*C*_{nh}and*D*_{nh}for even*n**S*_{2n}and*D*_{nd}for odd*n*(*S*=_{2}*C*is the group generated by inversion;_{i}*D*_{1d}=*C*_{2h})*T*,_{h}*O*, and_{h}*I*_{h}

As explained above, there is a 1-to-1 correspondence between these groups and all rotation groups:

*C*_{nh}for even*n*and*S*_{2n}for odd*n*correspond to*C*_{n}*D*_{nh}for even*n*and*D*_{nd}for odd*n*correspond to*D*_{n}*T*,_{h}*O*, and_{h}*I*correspond to_{h}*T*,*O*, and*I*, respectively.

The other groups contain indirect isometries, but not inversion:

*C*_{nv}*C*_{nh}and*D*_{nh}for odd*n**S*_{2n}and*D*_{nd}for even*n**T*_{d}

They all correspond to a rotation group *H* and a subgroup *L* of index 2 in the sense that they are obtained from *H* by inverting the isometries in *H* \ *L*, as explained above:

*C*_{n}is subgroup of*D*_{n}of index 2, giving*C*_{nv}*C*_{n}is subgroup of*C*_{2n}of index 2, giving*C*_{nh}for odd*n*and*S*_{2n}for even*n**D*_{n}is subgroup of*D*_{2n}of index 2, giving*D*_{nh}for odd*n*and*D*_{nd}for even*n*- T is subgroup of
*O*of index 2, giving*T*_{d}

## Maximal symmetries

There are two discrete point groups with the property that no discrete point group has it as proper subgroup: *O _{h}* and

*I*. Their largest common subgroup is

_{h}*T*. The two groups are obtained from it by changing 2-fold rotational symmetry to 4-fold, and adding 5-fold symmetry, respectively. Alternatively the two groups are generated by adding for each a reflection plane to

_{h}*T*.

_{h}There are two crystallographic point groups with the property that no crystallographic point group has it as proper subgroup: *O _{h}* and

*D*. Their maximal common subgroups, depending on orientation, are

_{6h}*D*and

_{3d}*D*.

_{2h}## The groups arranged by abstract group type

Below the groups explained above are arranged by abstract group type.

The smallest abstract groups that are *not* any symmetry group in 3D, are the quaternion group (of order 8), Z_{3} × Z_{3} (of order 9), the dicyclic group Dic_{3} (of order 12), and 10 of the 14 groups of order 16.

The column "# of order 2 elements" in the following tables shows the total number of isometry subgroups of types *C _{2}*,

*C*,

_{i}*C*. This total number is one of the characteristics helping to distinguish the various abstract group types, while their isometry type helps to distinguish the various isometry groups of the same abstract group.

_{s}Within the possibilities of isometry groups in 3D, there are infinitely many abstract group types with 0, 1 and 3 elements of order 2, there are two with 2*n* + 1 elements of order 2, and there are three with 2*n* + 3 elements of order 2 (for each *n* ≥ 2 ). There is never a positive even number of elements of order 2.

### Symmetry groups in 3D that are cyclic as abstract group

The symmetry group for *n*-fold rotational symmetry is *C _{n}*; its abstract group type is cyclic group Z

_{n}, which is also denoted by

*C*. However, there are two more infinite series of symmetry groups with this abstract group type:

_{n}- For even order 2
*n*there is the group*S*(Schoenflies notation) generated by a rotation by an angle 180°/n about an axis, combined with a reflection in the plane perpendicular to the axis. For_{2n}*S*the notation_{2}*C*is used; it is generated by inversion._{i} - For any order 2
*n*where*n*is odd, we have*C*; it has an_{nh}*n*-fold rotation axis, and a perpendicular plane of reflection. It is generated by a rotation by an angle 360°/*n*about the axis, combined with the reflection. For*C*_{1h}the notation*C*is used; it is generated by reflection in a plane._{s}

Thus we have, with bolding of the 10 cyclic crystallographic point groups, for which the crystallographic restriction applies:

Order | Isometry groups | Abstract group | # of order 2 elements | Cycle diagram |
---|---|---|---|---|

1 | C_{1} | Z_{1} | 0 | |

2 | , C_{2}, C_{i}C_{s} | Z_{2} | 1 | |

3 | C_{3} | Z_{3} | 0 | |

4 | , C_{4}S_{4} | Z_{4} | 1 | |

5 | C_{5} | Z_{5} | 0 | |

6 | , C_{6}, S_{6}C_{3h} | Z_{6} = Z_{3} × Z_{2} | 1 | |

7 | C_{7} | Z_{7} | 0 | |

8 | C, _{8}S_{8} | Z_{8} | 1 | |

9 | C_{9} | Z_{9} | 0 | |

10 | C, _{10}S, _{10}C_{5h} | Z_{10} = Z_{5} × Z_{2} | 1 |

etc.

### Symmetry groups in 3D that are dihedral as abstract group

In 2D dihedral group *D _{n}* includes reflections, which can also be viewed as flipping over flat objects without distinction of front- and backside.

However, in 3D the two operations are distinguished: the symmetry group denoted by *D _{n}* contains

*n*2-fold axes perpendicular to the

*n*-fold axis, not reflections.

*D*is the rotation group of the

_{n}*n*-sided prism with regular base, and

*n*-sided bipyramid with regular base, and also of a regular,

*n*-sided antiprism and of a regular,

*n*-sided trapezohedron. The group is also the full symmetry group of such objects after making them chiral by e.g. an identical chiral marking on every face, or some modification in the shape.

The abstract group type is dihedral group Dih_{n}, which is also denoted by *D _{n}*. However, there are three more infinite series of symmetry groups with this abstract group type:

*C*of order 2_{nv}*n*, the symmetry group of a regular*n*-sided pyramid*D*of order 4_{nd}*n*, the symmetry group of a regular*n*-sided antiprism*D*of order 4_{nh}*n*for odd*n*. For*n*= 1 we get*D*_{2}, already covered above, so*n*≥ 3.

Note the following property:

- Dih
_{4n+2}Dih_{2n+1}× Z_{2}

Thus we have, with bolding of the 12 crystallographic point groups, and writing *D _{1d}* as the equivalent

*C*:

_{2h}Order | Isometry groups | Abstract group | # of order 2 elements | Cycle diagram |
---|---|---|---|---|

4 | , D_{2}, C_{2v}C_{2h} | Dih_{2} = Z_{2} × Z_{2} | 3 | |

6 | , D_{3}C_{3v} | Dih_{3} | 3 | |

8 | , D_{4}, C_{4v}D_{2d} | Dih_{4} | 5 | |

10 | D_{5}, C_{5v} | Dih_{5} | 5 | |

12 | , D_{6}, C_{6v}, D_{3d}D_{3h} | Dih_{6} = Dih_{3} × Z_{2} | 7 | |

14 | D_{7}, C_{7v} | Dih_{7} | 7 | |

16 | D_{8}, C_{8v}, D_{4d} | Dih_{8} | 9 | |

18 | D_{9}, C_{9v} | Dih_{9} | 9 | |

20 | D_{10}, C_{10v}, D_{5h}, D_{5d} | Dih_{10} = D_{5} × Z_{2} | 11 |

etc.

### Other

*C _{2n,h}* of order 4

*n*is of abstract group type Z

_{2n}× Z

_{2}. For

*n*= 1 we get Dih

_{2}, already covered above, so

*n*≥ 2.

Thus we have, with bolding of the 2 cyclic crystallographic point groups:

Order | Isometry group | Abstract group | # of order 2 elements | Cycle diagram |
---|---|---|---|---|

8 | C_{4h} | Z_{4} × Z_{2} | 3 | |

12 | C_{6h} | Z_{6} × Z_{2} = Z_{3} × Z_{2}^{2} = Z_{3} × Dih_{2} | 3 | |

16 | C_{8h} | Z_{8} × Z_{2} | 3 | |

20 | C_{10h} | Z_{10} × Z_{2} = Z_{5} × Z_{2}^{2} = Z_{5} × Dih_{2} | 3 |

etc.

*D _{nh}* of order 4

*n*is of abstract group type Dih

_{n}× Z

_{2}. For odd

*n*this is already covered above, so we have here

*D*

_{2nh}of order 8

*n*, which is of abstract group type Dih

_{2n}× Z

_{2}(

*n*≥1).

Thus we have, with bolding of the 3 dihedral crystallographic point groups:

Order | Isometry group | Abstract group | # of order 2 elements | Cycle diagram |
---|---|---|---|---|

8 | D_{2h} | Dih_{2} × Z_{2} | 7 | |

16 | D_{4h} | Dih_{4} × Z_{2} | 11 | |

24 | D_{6h} | Dih_{6} × Z_{2} = Dih_{3} × Z_{2}^{2} | 15 | |

32 | D_{8h} | Dih_{8} × Z_{2} | 19 |

etc.

The remaining seven are, with bolding of the 5 crystallographic point groups (see also above):

Order | Isometry group | Abstract group | # of order 2 elements | Cycle diagram |
---|---|---|---|---|

12 | T | A_{4} | 3 | |

24 | T, _{d}O | S_{4} | 6 | |

24 | T_{h} | A_{4} × Z_{2} | 6 | |

48 | O_{h} | S_{4} × Z_{2} | 6 | |

60 | I | A_{5} | ||

120 | I_{h} | A_{5} × Z_{2} |

## Fundamental domain

The planes of reflection for icosahedral symmetry intersect the sphere on great circles, with right spherical triangle fundamental domains |

The fundamental domain of a point group is a conic solid. An object with a given symmetry in a given orientation is characterized by the fundamental domain. If the object is a surface it is characterized by a surface in the fundamental domain continuing to its radial bordal faces or surface. If the copies of the surface do not fit, radial faces or surfaces can be added. They fit anyway if the fundamental domain is bounded by reflection planes.

For a polyhedron this surface in the fundamental domain can be part of an arbitrary plane. For example, in the disdyakis triacontahedron one full face is a fundamental domain of icosahedral symmetry. Adjusting the orientation of the plane gives various possibilities of combining two or more adjacent faces to one, giving various other polyhedra with the same symmetry. The polyhedron is convex if the surface fits to its copies and the radial line perpendicular to the plane is in the fundamental domain.

Also the surface in the fundamental domain may be composed of multiple faces.

## Binary polyhedral groups

The map Spin(3) → SO(3) is the double cover of the rotation group by the spin group in 3 dimensions. (This is the only connected cover of SO(3), since Spin(3) is simply connected.) By the lattice theorem, there is a Galois connection between subgroups of Spin(3) and subgroups of SO(3) (rotational point groups): the image of a subgroup of Spin(3) is a rotational point group, and the preimage of a point group is a subgroup of Spin(3).

The preimage of a finite point group is called a **binary polyhedral group**, represented as <l,n,m>, and is called by the same name as its point group, with the prefix **binary**, with double the order of the related polyhedral group (l,m,n). For instance, the preimage of the icosahedral group (2,3,5) is the binary icosahedral group, <2,3,5>.

The binary polyhedral groups are:

- : binary cyclic group of an (
*n*+ 1)-gon, order 2*n* - : binary dihedral group of an
*n*-gon, <2,2,n>, order 4*n* - : binary tetrahedral group, <2,3,3>, order 24
- : binary octahedral group, <2,3,4>, order 48
- : binary icosahedral group, <2,3,5>, order 120

These are classified by the ADE classification, and the quotient of **C**^{2} by the action of a binary polyhedral group is a Du Val singularity.[3]

For point groups that reverse orientation, the situation is more complicated, as there are two pin groups, so there are two possible binary groups corresponding to a given point group.

Note that this is a covering of *groups,* not a covering of *spaces* – the sphere is simply connected, and thus has no covering spaces. There is thus no notion of a "binary polyhedron" that covers a 3-dimensional polyhedron. Binary polyhedral groups are discrete subgroups of a Spin group, and under a representation of the spin group act on a vector space, and may stabilize a polyhedron in this representation – under the map Spin(3) → SO(3) they act on the same polyhedron that the underlying (non-binary) group acts on, while under spin representations or other representations they may stabilize other polyhedra.

This is in contrast to projective polyhedra – the sphere does cover projective space (and also lens spaces), and thus a tessellation of projective space or lens space yields a distinct notion of polyhedron.

## See also

## Footnotes

- Fisher, G.L.; Mellor, B. (2007), "Three-dimensional finite point groups and the symmetry of beaded beads" (PDF),
*Journal of Mathematics and the Arts* - Coxeter,
*Regular polytopes', §12.6 The number of reflections, equation 12.61* - Du Val Singularities, by Igor Burban

## References

- Coxeter, H. S. M. (1974), "7 The Binary Polyhedral Groups",
*Regular Complex Polytopes*, Cambridge University Press, pp. 73–82. - Coxeter, H. S. M. & Moser, W. O. J. (1980).
*Generators and Relations for Discrete Groups, 4th edition*. New York: Springer-Verlag. ISBN 0-387-09212-9. 6.5 The binary polyhedral groups, p. 68 - Conway, John Horton; Huson, Daniel H. (2002), "The Orbifold Notation for Two-Dimensional Groups",
*Structural Chemistry*, Springer Netherlands,**13**(3): 247–257, doi:10.1023/A:1015851621002

## External links

- Graphic 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 - Overview of properties of point groups
- Simplest Canonical Polyhedra of Each Symmetry Type (uses Java)
- Point Groups and Crystal Systems, by Yi-Shu Wei, pp. 4–6
- The Geometry Center: 10.1 Formulas for Symmetries in Cartesian Coordinates (three dimensions)