# Point groups in three dimensions

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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 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.

Group structure

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) is the direct product of SO(3) and the group generated by inversion (denoted by its matrix −"I")::O(3) = SO(3) &times; { "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" and all groups "K" of isometries which contain inversion::"K" = "H" &times; { "I" , −"I" }:"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") &times; { − "I" } )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.

3D isometries which leave the origin fixed

The isometries of R"3" which 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 which is 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.

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" ).

Thus 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 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.)

Infinite isometry groups

We restrict ourselves to isometry groups which are closed as topological subgroups of O(3). This excludes for example the group of rotations by an irrational number of turns about an 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.

Finite isometry groups

For point groups, being finite corresponds to being discrete; infinite discrete groups as in the case of translational symmetry and glide reflectional symmetry do not apply.

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.
*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&times;3
**4&times;3 and 3&times;4
**10&times;3 and 6&times;5A 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

The infinite series 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.

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:
*"Cn" ("nn" ) of order "n" - "n"-fold rotational symmetry (abstract group Z"n" ); for "n" = 1: no symmetry"' (trivial group)
*"Cnh" ("n"* ) of order 2"n" (for odd "n" abstract group Z"2n" = Z"n" &times; Z2 , for even "n" abstract group Z"n" &times; Z2 )
*"Cnv" (*"nn" ) of order 2"n" - pyramidal symmetry (abstract group Dih"n" ); in biology "C2v" is called biradial symmetry.
*"Dn" (22"n" ) of order 2"n" - dihedral symmetry (abstract group Dih"n" )
*"S2n" ("n"x ) of order 2"n" (not to be confused with symmetric groups, for which the same notation is used; abstract group Z"2n" )
*"Dnh" (*22"n" ) of order 4"n" - prismatic symmetry (for odd "n" abstract group Dih"2n" = Dih"n" &times; Z2 ; for even "n" abstract group Dih"n" &times; Z2 )
*"Dnd" (or "Dnv" ) (2*"n" ) - antiprismatic symmetry of order 4"n" (abstract group Dih"2n" )

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 ):
*"Ci" - inversion symmetry
*"C2" - 2-fold rotational symmetry
*"Cs" - reflection symmetry, also called bilateral symmetry.

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 2"n", or a set of "n" mirror planes containing the axis, giving the group "Cnv", also of order 2"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 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 4"n" is called "Dnh". Its subgroup of rotations is the dihedral group "Dn" of order 2"n" 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 2"n", 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:
*"C1h" and "C1v": group of order 2 with a single reflection ("Cs" )
*"D1" and "C2": group of order 2 with a single 180° rotation
*"D1h" and "C2v": group of order 4 with a reflection in a plane and a 180° rotation through a line in that plane
*"D1d" and "C2h": group of order 4 with a reflection in a plane and a 180° rotation through a line perpendicular to that plane

"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 Z"2n".

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. 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:

*T (332) of order 12 - chiral tetrahedral symmetry. There are four C3 axes, each through two vertices of a cube (body diagonals) or one of a regular tetrahedron, and three C2 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.

*Td (*332) of 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 C2 axis and two C3 axes. The C2 axes are now actually S4 axes. This group is the symmetry group for a regular tetrahedron. Td is isomorphic to "S"4, the symmetric group on 4 letters. See also the isometries of the regular tetrahedron.

*Th (3*2) of order 24 - pyritohedral symmetry., 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 of the full icosahedral symmetry group (as isometry group, not just as abstract group), with 4 of the 10 3-fold axes.

*O (432) of order 24 - chiral octahedral symmetry. This group is like "T", but the C2 axes are now C4 axes, and additionally there are 6 C2 axes, through the midpoints of the edges of the cube. This group is also isomorphic to "S"4, and is the rotation group of the cube and octahedron.

*Oh (*432) of order 48 - full octahedral symmetry. This group has the same rotation axes as "O", but with mirror planes, comprising both the mirror planes of "Td" and "Th". This group is isomorphic to "S"4 &times; "C"2, and is the symmetry group of the cube and octahedron. See also the isometries of the cube.

*I (532) of 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 Ih. The group I is isomorphic to "A"5, the alternating group on 5 letters. The group contains 10 versions of "D3" and 6 versions of "D5" (rotational symmetries like prisms and antiprisms).

*Ih (*532) of order 120 - full icosahedral symmetry; the symmetry group of the icosahedron and the dodecahedron. The group Ih is isomorphic to "A"5 &times; "C"2. The group contains 10 versions of "D3d" and 6 versions of "D5d" (symmetries like antiprisms).

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 x count as 1This 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

Rotation groups

The rotation groups, i.e. the finite subgroups of SO(3), are: the cyclic groups "C"n (the rotation group of a regular pyramid), the dihedral groups "D"n (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 "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 "Cn", "Cnh", "Cnv" or "S2n" has rotation group "Cn.
*An object with symmetry group "Dn", "Dnh", or "Dnd" has rotation group "Dn.
*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.

Correspondence between rotation groups and other groups

The following groups contain inversion:
*"C""nh" and "D""nh" for even "n"
*"S"2"n" and "D""nd" for odd "n" ("S2" = "Ci" is the group generated by inversion; "D""1d" = "C""2h")
*"Th", "Oh", and "Ih"

As explained above, there is a 1-to-1 correspondence between these groups and all rotation groups:
*"C""nh" for even "n" and "S"2"n" for odd "n" correspond to "C""n"
*"D""nh" for even "n" and "D""nd" for odd "n" correspond to "D""n"
*"Th", "Oh", and "Ih" correspond to "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"2"n" and "D""nd" for even "n"
*"Td"

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"2"n" 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 "Td"

Maximal symmetries

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".

The groups arranged by abstract group type

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

The smallest abstract groups which are "not" any symmetry group in 3D, are the quaternion group (of order 8), the dicyclic group Dic3 (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 "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 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.

ymmetry groups in 3D which are cyclic as abstract group

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:
*For even order 2"n" there is the group "S2n" (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 "S2" the notation "Ci" is used; it is generated by inversion.
*For any order 2"n" where "n" is odd, we have "Cnh"; it has an 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 "C1h" the notation "Cs" is used; it is generated by reflection in a plane.

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

"Dnh" of order 4"n" is of abstract group type Dihn &times; Z2. For odd "n" this is already covered above, so we have here "D"2"n"h of order 8"n", which is of abstract group type Dih2"n" &times; Z2 ("n"≥1).

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

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

*order 12: of type "A"4 (alternating group): T
*order 24:
**of type "S"4 (symmetric group, not to be confused with the symmetry group with this notation): Td, O
**of type "A"4 &times; Z2: Th .
*order 48, of type "S"4 &times; Z2: Oh
*order 60, of type "A"5: "I"

Impossible discrete symmetries

Since the overview is exhaustive, it also shows implicitly what is "not" possible as discrete symmetry group. For example:
*a "C"6 axis in one direction and a "C"3 in another
*a "C"5 axis in one direction and a "C"4 in another
*a "C"3 axis in one direction and another "C"3 axis in a perpendicular directionetc.

Fundamental domain

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.

Binary polyhedral groups

The map $mbox\left\{Spin\right\}\left(3\right) o SO\left(3\right)$ is the double cover of the rotation group by the spin group in 3 dimensions. (This is the only connected cover of $SO\left(3\right)$, 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, 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:
* $A_n$: binary cyclic group of an "(n+1)"-gon
* $D_n$: binary dihedral group of an "n"-gon
* $E_6$: binary tetrahedral group
* $E_7$: binary octahedral group
* $E_8$: binary icosahedral groupThese are classified by the ADE classification, and the quotient of $mathbf\left\{C\right\}^2$ by the action of a binary polyhedral group is a Du Val singularity [ [http://enriques.mathematik.uni-mainz.de/burban/singul.pdf Du Val Singularities, by Igor Burban] ] .

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.

ee also

*list of spherical symmetry groups
*point groups in two dimensions
*symmetry
*Euclidean plane isometry
*group action
*point group
*crystal system
*space group
*list of small groups
*molecular symmetry

Footnotes

* [http://newton.ex.ac.uk/research/qsystems/people/goss/symmetry/Solids.html 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
* [http://newton.ex.ac.uk/research/qsystems/people/goss/symmetry/CC_All.html Overview of properties of point groups]

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