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] = |
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.
Symmetry groups of geometric objects are isometry groups. Accordingly, analysis of isometry groups is analysis of possible symmetries. All isometries of a bounded (finite) 3D object have one or more common fixed points. We follow the usual convention by choosing the origin as one of them.
The symmetry group of an object is sometimes also called its full symmetry group, as opposed to its proper symmetry group, the intersection of its full symmetry group with E^{+}(3), which consists of all direct isometries, i.e., isometries preserving orientation. For a bounded object, the proper symmetry group is called its rotation group. It is the intersection of its full symmetry group with SO(3), the full rotation group of the 3D space. The rotation group of a bounded object is equal to its full symmetry group if and only if the object is chiral.
The point groups that are generated purely by a finite set of reflection mirror planes passing through the same point are the finite Coxeter groups, represented by Coxeter notation.
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.
The symmetry group operations (symmetry operations) are the isometries of three-dimensional space R^{3} that leave the origin fixed, forming the group O(3). These operations can be categorized as:
Inversion is a special case of rotation-reflection (i = S_{2}), as is reflection (σ = S_{1}), so these operations are often considered to be improper rotations.
A circumflex is sometimes added to the symbol to indicate an operator, as in Ĉ_{n} and Ŝ_{n}.
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:
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 space groups with a screw axis.)
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. The set of points on a circle at rational numbers of degrees around the circle illustrates a point group requiring an infinite number of generators. 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. Any 3D shape (subset of R^{3}) having infinite rotational symmetry must also have mirror symmetry for every plane through the axis. Physical objects having infinite rotational symmetry will also have the symmetry of mirror planes through the axis, but vector fields may not, for instance the velocity vectors of a cone rotating about its axis, or the magnetic field surrounding a wire.^{[1]}
There are seven continuous groups which are all in a sense limits of the finite isometry groups. These so called limiting point groups or Curie limiting groups are named after Pierre Curie who was the first to investigate them.^{[1]}^{[2]} The seven infinite series of axial groups lead to five limiting groups (two of them are duplicates), and the seven remaining point groups produce two more continuous groups. In international notation, the list is ∞, ∞2, ∞/m, ∞mm, ∞/mm, ∞∞, and ∞∞m.^{[3]} Not all of these are possible for physical objects, for example objects with ∞∞ symmetry also have ∞∞m symmetry. See below for other designations and more details.
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:
According to the crystallographic restriction theorem, only a limited number of point groups are compatible with discrete translational symmetry: 27 from the 7 infinite series, and 5 of the 7 others. Together, these make up the 32 so-called crystallographic point groups.
The infinite series of axial or prismatic groups have an index n, which can be any integer; in each series, the nth 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;^{[4]} 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. The orbifold notation 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:
Intl | Schoenflies | Orbifold | Coxeter | 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} | 2n | 2n-fold rotoreflection symmetry | |
n/m | 2n | C_{nh} | n* | [n^{+},2] |
p11m | Z_{n}×Z_{2} | 2n | ||
nmm | nm | C_{nv} | *nn | [n] |
p1m1 | Dih_{n} | 2n | Pyramidal symmetry; in biology, biradial symmetry | |
n22 | n2 | D_{n} | 22n | [n,2]^{+} |
p211 | Dih_{n} | 2n | Dihedral symmetry | |
2n2m | nm | D_{nd} | 2*n | [2n,2^{+}] |
p2mg | Dih_{2n} | 4n | Antiprismatic symmetry | |
n/mmm | 2n2m | D_{nh} | *22n | [n,2] |
p2mm | Dih_{n}×Z_{2} | 4n | Prismatic symmetry |
For odd n we have Z_{2n} = Z_{n} × Z_{2} and Dih_{2n} = Dih_{n} × Z_{2}.
The groups C_{n} (including the trivial C_{1}) and D_{n} are chiral, the others are achiral.
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 axial groups are equivalent to the abstract group Z_{2}:
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_{nh} of order 2n, or a set of n mirror planes containing the axis, giving the group C_{nv}, also of order 2n. 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 4n is called D_{nh}. Its subgroup of rotations is the dihedral group D_{n} of order 2n, which still has the 2-fold rotation axes perpendicular to the primary rotation axis, but no mirror planes.
Note: in 2D, D_{n} includes reflections, which can also be viewed as flipping over flat objects without distinction of frontside and backside; but in 3D, the two operations are distinguished: D_{n} contains "flipping over", not reflections.
There is one more group in this family, called D_{nd} (or D_{nv}), 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°/n. D_{nh} is the symmetry group for a "regular" n-gonal prism and also for a "regular" n-gonal bipyramid. D_{nd} is the symmetry group for a "regular" n-gonal antiprism, and also for a "regular" n-gonal trapezohedron. D_{n} is the symmetry group of a partially rotated ("twisted") prism.
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} occurs in molecules such as twistane and in homotetramers such as Concanavalin A. 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_{nh} of order 2n, and therefore the notation S_{n} is not needed; however, for n even it is distinct, and of order n. Like D_{nd} it contains a number of improper rotations without containing the corresponding rotations.
All symmetry groups in the 7 infinite series are different, except for the following four pairs of mutually equal ones:
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:
Groups with continuous axial rotations are designated by putting ∞ in place of n. Note however that C_{∞} here is not the same as the infinite cyclic group (also sometimes designated C_{∞}), which is isomorphic to the integers. The following table gives the five continuous axial rotation groups. They are limits of the finite groups only in the sense that they arise when the main rotation is replaced by rotation by an arbitrary angle, so not necessarily a rational number of degrees as with the finite groups. Physical objects can only have C_{∞v} or D_{∞h} symmetry, but vector fields can have the others.
H–M | Schönflies | Orbifold | Coxeter | Limit of | Abstract group | |
---|---|---|---|---|---|---|
∞ | C_{∞} | ∞∞ | [∞]^{+} | C_{n} | SO(2) | |
∞, ∞/m | C_{∞h} | ∞* | [2,∞^{+}] | C_{nh}, S_{2n} | SO(2)×Z_{2} | |
∞m | C_{∞v} | *∞∞ | [∞] | C_{nv} | O(2) | |
∞2 | D_{∞} | 22∞ | [2,∞]^{+} | D_{n} | O(2) | |
∞m, ∞/mm | D_{∞h} | *22∞ | [2,∞] | D_{nh}, D_{nd} | O(2)×Z_{2} |
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. On successive lines are the orbifold notation, the Coxeter notation and Coxeter diagram, and the Hermann–Mauguin notation (full, and abbreviated if different) and the order (number of elements) of the symmetry group. 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 circumscribing cube (red cube in images), or through one vertex 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_{d}, (*332) [3,3] ( ) 43m order 24 |
full tetrahedral symmetry |
This group is the symmetry group of a regular tetrahedron. This group has the same rotation axes as T, and the C_{2} axes are now D_{2d} axes, whereas the four three-fold axes now give rise to four C_{3v} subgroups. This group has six mirror planes, each containing two edges of the cube or one edge of the tetrahedron, a single S_{4} axis, and two C_{3} axes. 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_{h}, (3*2) [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. The two-fold axes give rise to three D_{2h} subgroups. 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}. In spite of being called T_{h}, it does not apply to a tetrahedron. |
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 two-fold rotation axes through the midpoints of the edges of the cube, giving rise to three D_{2} subgroups. The three-fold axes now give rise to four D_{3} subgroups. 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_{h}, (*432) [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_{d} and T_{h}. The three-fold axes give rise to four D_{3d} subgroups. The three perpendicular four-fold axes of O now give D_{4h} subgroups, while the six two-fold axes give six D_{2h} subgroups. This group is isomorphic to S_{4} × Z_{2} (because both O and C_{i} are normal subgroups), and is the symmetry group of the cube and octahedron. See also the isometries of the cube. |
I, (532) [5,3]^{+} ( ) 532 order 60 |
chiral icosahedral symmetry | This is the rotation group of the icosahedron and the dodecahedron. It is a normal subgroup of index 2 in the full group of symmetries I_{h}. The group contains 10 versions of D_{3} and 6 versions of D_{5} (rotational symmetries like prisms and antiprisms). It also contains five versions of 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, I is made up of the 120 unit icosians. As before, this is a 1-to-2 correspondence. |
I_{h}, (*532) [5,3] ( ) 532/m, 53m order 120 |
full icosahedral symmetry | This is the symmetry group of the icosahedron and the dodecahedron. The group I_{h} is isomorphic to A_{5} × Z_{2} because I and C_{i} are both normal subgroups. The group contains 10 versions of D_{3d}, 6 versions of D_{5d} (symmetries like antiprisms), and 5 versions of T_{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.
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. Coxeter notation offers a bracketed notation equivalent to the Coxeter diagram, with markup symbols for rotational and other subsymmetry point groups. In Schoenflies notation, the reflective point groups in 3D are C_{nv}, D_{nh}, and the full polyhedral groups T, O, and I.
The mirror planes bound a set of spherical triangle domains on the surface of a sphere. A rank n Coxeter group has n mirror planes. Coxeter groups having fewer 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).^{[5]}
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 |
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.
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_{nv}, [n], (*nn) | C_{nh}, [n^{+},2], (n*) | S_{2n}, [2n^{+},2^{+}], (n×) | C_{n}, [n]^{+}, (nn) |
D_{nh}, [2,n], (*n22) | D_{nd}, [2^{+},2n], (2*n) | D_{n}, [2,n]^{+}, (n22) | |
T_{d}, [3,3], (*332) | T, [3,3]^{+}, (332) | ||
O_{h}, [4,3], (*432) | T_{h}, [3^{+},4], (3*2) | O, [4,3]^{+}, (432) | |
I_{h}, [5,3], (*532) | I, [5,3]^{+}, (532) |
The rotation group SO(3) is a subgroup of O(3), the full point rotation group of the 3D Euclidean space. Correspondingly, O(3) is the direct product of SO(3) and the inversion group C_{i} (where inversion is denoted by its matrix −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 H of direct isometries in SO(3) and all groups K of isometries in O(3) that contain inversion:
where the isometry ( A, I ) is identified with A.
For finite groups, the correspondence is:
Rotation group H |
Parity of n |
Group containing inversion K |
---|---|---|
C_{n} | even | C_{nh} |
odd | S_{2n} | |
D_{n} | even | D_{nh} |
odd | D_{nd} | |
T | T_{h} | |
O | O_{h} | |
I | I_{h} |
If a group of direct isometries H has a subgroup L of index 2, then there is a corresponding group that contains indirect isometries but no inversion:
For example, H = C_{4} corresponds to M = S_{4}.
Thus M is obtained from H by inverting the isometries in H ∖ L. This group M is, when considered as an abstract group, isomorphic to H. Conversely, for all point groups M that contain indirect isometries but no inversion we can obtain a rotation group H by inverting the indirect isometries.
For finite groups, the correspondence is:
Rotation group H |
Index-2 subgroup L |
Parity of n |
Group containing indirect isometries M |
---|---|---|---|
C_{2n} | C_{n} | even | S_{2n} |
odd | C_{nh} | ||
D_{2n} | D_{n} | even | D_{nh} |
odd | D_{nd} | ||
D_{n} | C_{n} | any | C_{nv} |
O | T | T_{d} |
In 2D, the cyclic group of k-fold rotations C_{k} is for every positive integer k a normal subgroup of O(2) and SO(2). 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_{n}) is normal both in the group (C_{nv}) obtained by adding to (C_{n}) reflection planes through its axis and in the group (C_{nh}) obtained by adding to (C_{n}) a reflection plane perpendicular to its axis.
There are two discrete point groups with the property that no discrete point group has it as proper subgroup: O_{h} and I_{h}. Their largest common subgroup is T_{h}. The two groups are obtained from it by changing 2-fold rotational symmetry to 4-fold, and adding 5-fold symmetry, respectively.
There are two crystallographic point groups with the property that no crystallographic point group has it as proper subgroup: O_{h} and D_{6h}. Their maximal common subgroups, depending on orientation, are D_{3d} and D_{2h}.
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_{s}. 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.
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 4n + 1 elements of order 2, and there are three with 4n + 3 elements of order 2 (for each n ≥ 8 ). There is never a positive even number of elements of order 2.
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_{n}. However, there are two more infinite series of symmetry groups with this abstract group type:
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.
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_{n} is the rotation group of the 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:
Note the following property:
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.
C_{2n,h} of order 4n 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 4n 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 8n, 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} | Z_{2}^{3} | 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} | 9 | |
24 | T_{h} | A_{4} × Z_{2} | 7 | |
48 | O_{h} | S_{4} × Z_{2} | 19 | |
60 | I | A_{5} | 15 | |
120 | I_{h} | A_{5} × Z_{2} | 31 |
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.
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). (Note that Spin(3) has alternative descriptions as the special unitary group SU(2) and as the group of unit quaternions. Topologically, this Lie group is the 3-dimensional sphere S^{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:
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.^{[6]}
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.