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 which 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 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.
SO(3) is a subgroup of E+(3), which consists of direct isometries, i.e., isometries preserving orientation; it contains those which leave the origin fixed.
O(3) = SO(3) ×
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 and all groups K of isometries which contain inversion:
K = H ×
H = K ∩ SO(3)
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) ×)where isometry (A , I) is identified with A.
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 which 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 Ck 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, and also of the group obtained by adding reflections in planes through the axis.
The isometries of R3 which leave the origin fixed, forming the group O(3,R), can be categorized as follows:
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.
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 H1, H2 of a group G are conjugate, if there exists g ∈ G such that H1 = g-1H2g).
Thus 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 single rotation mapping this whole structure of the first symmetry group to that of the second. 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.)
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.
See also rotational symmetry with respect to any angle.
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:
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 infinite series 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.
For n = ∞ they correspond to the frieze groups. Schönflies notation is used, and, in parentheses, orbifold notation; the latter is not only conveniently related to its properties, but also to the order of the group, see below; it is a unified notation, also applicable for wallpaper groups and frieze groups.
The 7 infinite series are:
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).
Involutional symmetry (abstract group Z2):
The second of these is the first of the uniaxial groups (cyclic groups) Cn 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 Cnh of order 2n, or a set of n mirror planes containing the axis, giving the group Cnv, also of order 2n. The latter is the symmetry group for a regular n-sided pyramid. A typical object with symmetry group Cn or Dn is a propellor.
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 Dnh. Its subgroup of rotations is the dihedral group Dn of order 2n which still has the 2-fold rotation axes perpendicular to the primary rotation axis, but no mirror planes. Note that in 2D Dn 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.
There is one more group in this family, called Dnd (or Dnv), which has vertical mirror planes containing the main rotation axis, but instead of having a horizontal mirror plane it has an isometry which is the combination of a reflection in the horizontal plane and a rotation by an angle 180°/n. Dnh is the symmetry group for a regular n-sided prisms and also for a regular n-sided bipyramid. Dnd is the symmetry group for a regular n-sided antiprism, and also for a regular n-sided trapezohedron. Dn is the symmetry group of a partially rotated prism.
Sn 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, Cnh of order 2n, and therefore the notation Sn is not needed; however, for n even it is distinct, and of order n. Like Dnd 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:
S2 is the group of order 2 with a single inversion (Ci)
"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. S2n is algebraically isomorphic with Z2n.
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. Using Cn to denote an axis of rotation through 360°/n and Sn to denote an axis of improper rotation through the same, the groups are:
The rotation groups, i.e. the finite subgroups of SO(3), are: the cyclic groups Cn (the rotation group of a regular pyramid), the dihedral groups Dn (the rotation group of a regular prism, or regular bipyramid), and the rotation groups T, O and I of a regular tetrahedron, octahedron/cube and icosahedron/dodecahedron.
In particular, the dihedral groups D3, D4 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.
The following groups contain inversion:
As explained above, there is a 1-to-1 correspondence between these groups and all rotation groups:
The other groups contain indirect isometries, but not inversion:
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:
There are two discrete point groups with the property that no discrete point group has it as proper subgroup: Oh and Ih. Their largest common subgroup is Th. 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 Th.
There are two crystallographic point groups with the property that no crystallographic point group has it as proper subgroup: Oh and D6h. Their maximal common subgroups, depending on orientation, are D3d and D2h.
Below the groups explained above are arranged by abstract group type.
The column "# of order 2 elements" in the following tables shows the total number of isometry subgroups of types C2 , Ci , Cs. 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 2n + 1 elements of order 2, and there are three with 2n + 3 elements of order 2 (for each n ≥ 2). There is never a positive even number of elements of order 2.
The symmetry group for n-fold rotational symmetry is Cn; its abstract group type is cyclic group Zn , which is also denoted by Cn. 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|
|2||C2 , Ci , Cs||Z2||1|
|4||C4 , S4||Z4||1|
|6||C6 , S6 , C3h||Z6 = Z3 × Z2||1|
|8||C8 , S8||Z8||1|
|10||C10 , S10 , C5h||Z10 = Z5 × Z2||1|
In 2D dihedral group Dn 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 Dn contains n 2-fold axes perpendicular to the n-fold axis, not reflections. Dn 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 Dihn, which is also denoted by Dn. 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 D1d as the equivalent C2h:
|Order||Isometry groups||Abstract group|
|4||D2 , C2v , C2h||Dih2 = Z2 × Z2||3|
|6||D3 , C3v||Dih3||3|
|8||D4 , C4v , D2d||Dih4||5|
|10||D5 , C5v||Dih5||5|
|12||D6 , C6v , D3d , D3h||Dih6 = Dih3 × Z2||7|
|14||D7 , C7v||Dih7||7|
|16||D8 , C8v , D4d||Dih8||9|
|18||D9 , C9v||Dih9||9|
|20||D10 , C10v , D5h , D5d||Dih10 = D5 × Z2||11|
C2n,h of order 4n is of abstract group type Z2n × Z2. For n = 1 we get Dih2 , already covered above, so n ≥ 2.
Thus we have, with bolding of the 2 cyclic crystallographic point groups:
|Order||Isometry group||Abstract group||Cycle diagram|
|8||C4h||Z4 × Z2||3|
|12||C6h||Z6 × Z2 = Z3 × Z2 × Z2 = Z3 × Dih2||3|
|16||C8h||Z8 × Z2||3|
|20||C10h||Z10 × Z2 = Z5 × Z2 × Z2||3|
Dnh of order 4n is of abstract group type Dihn × Z2. For odd n this is already covered above, so we have here D2nh of order 8n, which is of abstract group type Dih2n × Z2 (n≥1).
Thus we have, with bolding of the 3 dihedral crystallographic point groups:
|Order||Isometry group||Abstract group||Cycle diagram|
|8||D2h||Dih2 × Z2||7|
|16||D4h||Dih4 × Z2||11|
|24||D6h||Dih6 × Z2||15|
|32||D8h||Dih8 × Z2||19|
The remaining seven are, with bolding of the 5 crystallographic point groups (see also above):
See also icosahedral symmetry.
Since the overview is exhaustive, it also shows implicitly what is not possible as discrete symmetry group. For example:
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. 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 preimage of a finite point group is called a binary polyhedral group, and is called by the same name as its point group, with the prefix binary. For instance, the preimage of the icosahedral group is the binary icosahedral group.
The binary polyhedral groups are:
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.