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In mathematics, the 'dihedral group' of order 2''n'' is the abstract group of which one representation is the symmetry group in 2D of a regular polygon with ''n'' sides. The group consists of ''n'' elements corresponding to rotations of the polygon, and ''n'' corresponding to reflections.

Contents

Notation

Small dihedral groups

The dihedral group as symmetry group in 2D and rotation group in 3D

Equivalent definitions and properties

Examples of automorphism groups

Infinite dihedral group

Generalized dihedral group

Topology

See also

Notation

In this article the notation Dih_''n'' is used for the dihedral group of order 2''n'' as abstract group. The notations ''D''_''n'' and ''D''_''2n'' are also seen.
For the isometry group in 2D of this abstract group type, the notation ''D''_''n'' is used. There are four series of isometry groups in 3D which are dihedral as abstract group. Only for one of them the notation ''D''_''n'' is used.

Small dihedral groups

For ''n'' = 1 we have Dih₁. This notation is rarely used except in the framework of the series, because it is equal to Z₂. For ''n'' = 2 we have Dih₂, the Klein four-group. Both are exceptional within the series:

★ they are abelian; for all other values of ''n'' the group Dih_''n'' is ''not'' abelian

★ they are ''not'' subgroups of the symmetric group S_''n'', corresponding to the fact that 2''n'' > ''n'' ! for these ''n''.
The cycle graphs of dihedral groups consist of an n-element cycle and n 2-element cycles. The dark vertex in the cycle graphs below of various dihedral groups stand for the identity element, and the other vertices are the other elements of the group. A cycle consists of successive powers of either of the elements connected to the identity element.

Dih1	Dih2	Dih3	Dih4	Dih5	Dih6	Dih7

The dihedral group as symmetry group in 2D and rotation group in 3D

An example of abstract group Dih_''n'', and a common way to visualize it, is the group ''D_n'' of Euclidean plane isometries which keep the origin fixed. These groups form one of the two series of discrete point groups in two dimensions. ''D_n'' consists of ''n'' rotations of multiples of 360°/''n'' about the origin, and reflections across ''n'' lines through the origin, making angles of multiples of 180°/''n'' with each other. This is the symmetry group of a regular polygon with ''n'' sides (for n ≥3, and also for the degenerate case ''n'' = 2, where we have a line segment in the plane).
Dihedral group ''D''_''n'' is generated by a rotation ''r'' of order ''n'' and a reflection ''f'' of order 2 such that
:$frf\; =\; r^\{-1\}$ (in geometric terms: in the mirror a rotation looks like an inverse rotation)
In matrix form, an anti-clockwise rotation and a reflection in the ''x''-axis are given by
:
(in terms of complex numbers: multiplication by $e^\{2pi\; i\; over\; n\}$ and complex conjugation).
By setting
:
and defining $r\_j\; =\; r\_0^j$ and $f\_j\; =\; r\_j\; ,\; f\_0$ for $j\; in\; \{1,ldots,n-1\}$ we can write the product rules for $D\_n$ as
:$r\_j\; ,\; r\_k\; =\; r\_\{(j+k)\; mbox\{\; mod\; n\}\}$
:$r\_j\; ,\; f\_k\; =\; f\_\{(j+k)\; mbox\{\; mod\; n\}\}$
:$f\_j\; ,\; r\_k\; =\; f\_\{(j-k)\; mbox\{\; mod\; n\}\}$
:$f\_j\; ,\; f\_k\; =\; r\_\{(j-k)\; mbox\{\; mod\; n\}\}$
(Compare coordinate rotations and reflections.)
The dihedral group D₂ is generated by the rotation ''r'' of 180 degrees, and the reflection ''f'' across the x-axis. The elements of D₂ can then be represented as {''e'', ''r'', ''f'', ''rf''}, where ''e'' is the identity or null transformation and ''rf'' is the reflection across the y-axis.
D₂ is isomorphic to the Klein four-group.
If the order of D_''n'' is greater than 4, the operations of rotation and reflection in general do not commute and D_''n'' is not abelian; for example, in D₄, a rotation of 90 degrees followed by a reflection yields a different result from a reflection followed by a rotation of 90 degrees:
Thus, beyond their obvious application to problems of symmetry in the plane, these groups are among the simplest examples of non-abelian groups, and as such arise frequently as easy counterexamples to theorems which are restricted to abelian groups.
The 2''n'' elements of D_''n'' can be written as ''e'', ''r'', ''r''²,...,''r''^''n''−1, ''f'', ''r f'', ''r² f'',...,''r''^''n''−1 f. The first ''n'' listed elements are rotations and the remaining ''n'' elements are axis-reflections (all of which have order 2). The product of two rotations or two reflections is a rotation; the product of a rotation and a reflection is a reflection.
So far, we have considered ''D''_''n'' to be a subgroup of O(2), i.e. the group of rotations (about the origin) and reflections (across axes through the origin) of the plane. However, notation ''D''_''n'' is also used for a subgroup of SO(3) which is also of abstract group type Dih_''n'': the proper symmetry group of a ''regular polygon embedded in three-dimensional space'' (if ''n'' ≥ 3). Such a figure may be considered as a degenerate regular solid with its face counted twice. Therefore it is also called a ''dihedron'' (Greek: solid with two faces), which explains the name ''dihedral group'' (in analogy to ''tetrahedral'', ''octahedral'' and ''icosahedral group'', referring to the proper symmetry groups of a regular tetrahedron, octahedron, and icosahedron respectively).

Equivalent definitions and properties

Further equivalent definitions of Dih_''n'' are:

★ The automorphism group of the graph consisting only of a cycle with ''n'' vertices (if ''n'' ≥ 3).

★ The group with presentation
::$langle\; r,\; f\; mid\; r^n\; =\; 1,\; f^2\; =\; 1,\; frf\; =\; r^\{-1\}$
angle
:or
::$langle\; x,\; y\; mid\; x^2\; =\; y^2\; =\; (xy)^n\; =\; 1$
angle
:(Indeed the only finite groups that can be generated by two elements of order 2 are the dihedral groups and the cyclic groups)
:From the second presentation follows that Dih_''n'' belongs to the class of coxeter groups.

★ The semidirect product of cyclic groups Z_''n'' and Z₂, with Z₂ acting on Z_''n'' by inversion (thus, Dih_''n'' always has a normal subgroup isomorphic to Z_''n'' ):
$Z\_n$
times_phi Z_2 is isomorphic to Dih_''n'' if φ(0) is the identity and φ(1) is inversion.
If we consider Dih_''n'' (''n'' ≥ 3) as the symmetry group of a regular ''n''-gon and number the polygon's vertices, we see that Dih_''n'' is a subgroup of the symmetric group S_''n''.
The properties of the dihedral groups Dih_''n'' with ''n'' ≥ 3 depend on whether ''n'' is even or odd. For example, the center of Dih_''n'' consists only of the identity if ''n'' is odd, but if ''n'' is even the center has two elements, namely the identity and the element ''r''^{''n /'' 2} (with D_''n'' as a subgroup of O(2), this is inversion; since it is scalar multiplication by −1, it is clear that it commutes with any linear transformation).
For odd ''n'', abstract group Dih_2''n'' is isomorphic with the direct product of Dih_''n'' and Z₂.
In the case of 2D isometries, this corresponds to adding inversion, giving rotations and mirrors in between the existing ones.
All the reflections are conjugate to each other in case ''n'' is odd, but they fall into two conjugacy classes if ''n'' is even. If we think of the isometries of a regular ''n''-gon: for odd ''n'' there are rotations in the group between every pair of mirrors, while for even ''n'' only half of the mirrors can be reached from one by these rotations.
If ''m'' divides ''n'', then Dih_''n'' has ''n / m'' subgroups of type Dih_''m'', and one subgroup Z_''m''. Therefore the total number of subgroups of Dih_''n'' (''n'' ≥ 1), is equal to ''d'' (''n'') + σ (''n''), where ''d'' (''n'') is the number of positive divisors of ''n'' and σ (''n'') is the sum of the positive divisors of ''n''. See List of small groups for the cases ''n'' ≤ 8.

Examples of automorphism groups

Dih₉ has 18 inner automorphisms. As 2D isometry group ''D''₉, the group has mirrors at 20° intervals. The 18 inner automorphisms provide rotation of the mirrors by multiples of 20°, and reflections. As isometry group these are all automorphisms. As abstract group there are in addition to these, 36 outer automorphisms, e.g. multiplying angles of rotation by 2.
Dih₁₀ has 10 inner automorphisms. As 2D isometry group ''D''₁₀, the group has mirrors at 18° intervals. The 10 inner automorphisms provide rotation of the mirrors by multiples of 36°, and reflections. As isometry group there are 10 more automorphisms; they are conjugates by isometries outside the group, rotating the mirrors 18° with respect to the inner automorphisms. As abstract group there are in addition to these 10 inner and 10 outer automorphisms, 20 more outer automorphisms, e.g. multiplying rotations by 3.
Compare the values 6 and 4 for Euler's totient function, the multiplicative group of integers modulo n for ''n'' = 9 and 10, respectively. This triples and doubles the number of automorphisms compared with the two automorphisms as isometries (keeping the order of the rotations the same or reversing the order).
In general, the automorphism group of Dih_n is isomorphic to the affine group Aff(Z/nZ).

Infinite dihedral group

In addition to the finite dihedral groups, there is the 'infinite dihedral group' Dih_∞. Every dihedral group is generated by a rotation ''r'' and a reflection; if the rotation is a rational multiple of a full rotation, then there is some integer ''n'' such that ''r''^''n'' is the identity, and we have a finite dihedral group of order 2''n''. If the rotation is ''not'' a rational multiple of a full rotation, then there is no such ''n'' and the resulting group has infinitely many elements and is called Dih_∞. It has presentations
:$langle\; r,\; f\; mid\; f^2\; =\; 1,\; frf\; =\; r^\{-1\}$
angle
:$langle\; x,\; y\; mid\; x^2\; =\; y^2\; =\; 1$
angle
and is isomorphic to a semidirect product of Z and Z₂, and to the free product Z₂
★ Z₂. It is the automorphism group of the graph consisting of a path infinite to both sides. Correspondingly, it is the isometry group of Z (see also symmetry groups in one dimension).

Generalized dihedral group

For any abelian group ''H'', the 'generalized dihedral group' of ''H'', written Dih(''H''), is the semidirect product of ''H'' and Z₂, with Z₂ acting on ''H'' by inverting elements. I.e., $mathrm\{Dih\}(H)\; =\; H$
times_phi Z_2 with φ(0) the identity and φ(1) inversion.
Thus we get:
:(''h''₁, 0)
★ (''h''₂, ''t''₂) = (''h''₁ + ''h''₂, ''t''₂)
:(''h''₁, 1)
★ (''h''₂, ''t''₂) = (''h''₁ - ''h''₂, 1 + ''t''₂)
for all ''h''₁, ''h''₂ in ''H'' and ''t''₂ in Z₂.
(Writing Z₂ multiplicatively, we have (''h''₁, ''t''₁)
★ (''h''₂, ''t''₂) = (''h''₁ + ''t''₁''h''₂, ''t''₁''t''₂) .)
Note that (''h'', 0)
★ (0,1) = (''h'',1), i.e. first the inversion and then the operation in ''H''. Also (0, 1)
★ (''h'', ''t'') = (- ''h'', 1 + ''t''); indeed (0,1) inverts ''h'', and toggles ''t'' between "normal" (0) and "inverted" (1) (this combined operation is its own inverse).
The subgroup of Dih(''H'') of elements (''h'', 0) is a normal subgroup of index 2, isomorphic to ''H'', while the elements (''h'', 1) are all their own inverse.
The conjugacy classes are:

★ the sets {(''h'',0 ), (-''h'',0 )}

★ the sets {(''h'' + ''k'' + ''k'', 1) | ''k'' in ''H'' }
Thus for every subgroup ''M'' of ''H'', the corresponding set of elements (''m'',0) is also a normal subgroup. We have:
::Dih(''H'') ''/'' ''M'' = Dih ( ''H / M'' )
Examples:

★ Dih_''n'' = Dih('Z'_''n'')

★
★ For even ''n'' there are two sets {(''h'' + ''k'' + ''k'', 1) | ''k'' in ''H'' }, and each generates a normal subgroup of type Dih_{''n /'' 2}. As subgroups of the isometry group of the set of vertices of a regular ''n''-gon they are different: the reflections in one subgroup all have two fixed points, while none in the other subgroup has (the rotations of both are the same). However, they are isomorphic as abstract groups.

★
★ For odd ''n'' there is only one set {(''h'' + ''k'' + ''k'', 1) | ''k'' in ''H'' }

★ Dih_∞ = Dih('Z'); there are two sets {(''h'' + ''k'' + ''k'', 1) | ''k'' in ''H'' }, and each generates a normal subgroup of type Dih_∞. As subgroups of the isometry group of 'Z' they are different: the reflections in one subgroup all have a fixed point, the mirrors are at the integers, while none in the other subgroup has, the mirrors are in between (the translations of both are the same: by even numbers). However, they are isomorphic as abstract groups.

★ Dih(S¹), or orthogonal group O(2,'R'), or O(2): the isometry group of a circle, or equivalently, the group of isometries in 2D that keep the origin fixed. The rotations form the circle group S¹, or equivalently SO(2,'R'), also written SO(2), and 'R'/'Z' ; it is also the multiplicative group of complex numbers of absolute value 1. In the latter case one of the reflections (generating the others) is complex conjugation. There are no proper normal subgroups with reflections. The discrete normal subgroups are cyclic groups of order ''n'' for all positive integers ''n''. The quotient groups are isomorphic with the same group Dih(S¹).

★ Dih('R'^''n'' ): the group of isometries of 'R'^''n'' consisting of all translations and inversion in all points; for ''n'' = 1 this is the Euclidean group E(1); for ''n'' > 1 the group Dih('R'^''n'' ) is a proper subgroup of E(''n'' ), i.e. it does not contain all isometries.

★ ''H'' can be any subgroup of 'R'^''n'', e.g. a discrete subgroup; in that case, if it extends in ''n'' directions it is a lattice.

★
★ Discrete subgroups of Dih('R'² ) which contain translations in one direction are of frieze group type $inftyinfty$ and 22$infty$.

★
★ Discrete subgroups of Dih('R'² ) which contain translations in two directions are of wallpaper group type p1 and p2.

★
★ Discrete subgroups of Dih('R'³ ) which contain translations in three directions are space groups of the triclinic crystal system.
Dih(''H'') is Abelian, with the semidirect product a direct product, if and only if all elements of ''H'' are their own inverse:

★ Dih('Z'₁) = Dih₁ = 'Z'₂

★ Dih('Z'₂) = Dih₂ = 'Z'₂ × 'Z'₂ (Klein four-group)

★ Dih(Dih₂) = Dih₂ × 'Z'₂ = 'Z'₂ × 'Z'₂ × 'Z'₂
etc.

Topology

Dih('R'^''n'' ) and its dihedral subgroups are disconnected topological groups. Dih('R'^''n'' ) consists of two connected components: the identity component isomorphic to 'R'^''n'', and the component with the reflections. Similarly O(2) consists of two connected components: the identity component isomorphic to the circle group, and the component with the reflections.
For the group Dih_∞ we can distinguish two cases:

★ Dih_∞ as the isometry group of 'Z'

★ Dih_∞ as a 2-dimensional isometry group generated by a rotation by an irrational number of turns, and a reflection
Both topological groups are totally disconnected, but in the first case the (singleton) components are open, while in the second case they are not. Also, the first topological group is a closed subgroup of Dih('R') but the second is not a closed subgroup of O(2).

See also

★ quasidihedral group

★ dicyclic group

★ coordinate rotations and reflections

★ dihedral group of order 6

★ dihedral group of order 8

★ dihedral symmetry in three dimensions

★ dihedral symmetry groups in 3D