Dihedral group

This snowflake has the dihedral symmetry of a regular hexagon.

In mathematics, a dihedral group is the group of symmetries of a regular polygon, including both rotations and reflections.^[1] Dihedral groups are among the simplest examples of finite groups, and they play an important role in group theory, geometry, and chemistry.

See also: Dihedral symmetry in three dimensions.

Notation

There are two competing notations for the dihedral group associated to a polygon with n sides. In geometry the group is denoted D_n, while in algebra the same group is denoted by D_2n to indicate the number of elements.

In this article, D_n (and sometimes Dih_n) refers to the symmetries of a regular polygon with n sides.

Definition

Elements

The six reflection symmetries of a regular hexagon

A regular polygon with n sides has 2n different symmetries: n rotational symmetries and n reflection symmetries. The associated rotations and reflections make up the dihedral group D_n. If n is odd each axis of symmetry connects the mid-point of one side to the opposite vertex. If n is even there are n/2 axes of symmetry connecting the mid-points of opposite sides and n/2 axes of symmetry connecting opposite vertices. In either case, there are n axes of symmetry altogether and 2n elements in the symmetry group. Reflecting in one axis of symmetry followed by reflecting in another axis of symmetry produces a rotation through twice the angle between the axes. The following picture shows the effect of the sixteen elements of D₈ on a stop sign:

The first row shows the effect of the eight rotations, and the second row shows the effect of the eight reflections.

Group structure

As with any geometric object, the composition of two symmetries of a regular polygon is again a symmetry. This operation gives the symmetries of a polygon the algebraic structure of a finite group.

The composition of these two reflections is a rotation.

The following Cayley table shows the effect of composition in the group D₃ (the symmetries of an equilateral triangle). R₀ denotes the identity; R₁ and R₂ denote counterclockwise rotations by 120 and 240 degrees; and S₀, S₁, and S₂ denote reflections across the three lines shown in the picture to the right.

	R0	R1	R2	S0	S1	S2
R0	R0	R1	R2	S0	S1	S2
R1	R1	R2	R0	S1	S2	S0
R2	R2	R0	R1	S2	S0	S1
S0	S0	S2	S1	R0	R2	R1
S1	S1	S0	S2	R1	R0	R2
S2	S2	S1	S0	R2	R1	R0

For example, S₂S₁ = R₁ because the reflection S₁ followed by the reflection S₂ results in a 120-degree rotation. (This is the normal backwards order for composition.) Note that the composition operation is not commutative.

In general, the group D_n has elements R₀,...,R_n−1 and S₀,...,S_n−1, with composition given by the following formulae:

$R_i\,R_j = R_{i+j},\;\;\;\;R_i\,S_j = S_{i+j},\;\;\;\;S_i\,R_j = S_{i-j},\;\;\;\;S_i\,S_j = R_{i-j}.$

In all cases, addition and subtraction of subscripts should be performed using modular arithmetic with modulus n.

Matrix representation

The symmetries of this pentagon are linear transformations.

If we center the regular polygon at the origin, then elements of the dihedral group act as linear transformations of the plane. This lets us represent elements of D_n as matrices, with composition being matrix multiplication. This is an example of a (2-dimensional) group representation.

For example, the elements of the group D₄ can be represented by the following eight matrices:

$\begin{matrix} R_0=\bigl(\begin{smallmatrix}1&0\\[0.2em]0&1\end{smallmatrix}\bigr), & R_1=\bigl(\begin{smallmatrix}0&-1\\[0.2em]1&0\end{smallmatrix}\bigr), & R_2=\bigl(\begin{smallmatrix}-1&0\\[0.2em]0&-1\end{smallmatrix}\bigr), & R_3=\bigl(\begin{smallmatrix}0&1\\[0.2em]-1&0\end{smallmatrix}\bigr), \\[1em] S_0=\bigl(\begin{smallmatrix}1&0\\[0.2em]0&-1\end{smallmatrix}\bigr), & S_1=\bigl(\begin{smallmatrix}0&1\\[0.2em]1&0\end{smallmatrix}\bigr), & S_2=\bigl(\begin{smallmatrix}-1&0\\[0.2em]0&1\end{smallmatrix}\bigr), & S_3=\bigl(\begin{smallmatrix}0&-1\\[0.2em]-1&0\end{smallmatrix}\bigr). \end{matrix}$

In general, the matrices for elements of D_n have the following form:

$\begin{align} R_k & = \begin{pmatrix} \cos \frac{2\pi k}{n} & -\sin \frac{2\pi k}{n} \\ \sin \frac{2\pi k}{n} & \cos \frac{2\pi k}{n} \end{pmatrix} \ \ \text{and} \\ S_k & = \begin{pmatrix} \cos \frac{2\pi k}{n} & \sin \frac{2\pi k}{n} \\ \sin \frac{2\pi k}{n} & -\cos \frac{2\pi k}{n} \end{pmatrix} . \end{align}$

R_k is a rotation matrix, expressing a counterclockwise rotation through an angle of 2πk ⁄ n. S_k is a reflection across a line that makes an angle of πk ⁄ n with the x-axis.

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 2n > 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 s of order 2 such that

$srs = r^{-1} \,$

In geometric terms: in the mirror a rotation looks like an inverse rotation.

In terms of complex numbers: multiplication by $e^{2\pi i \over n}$ and complex conjugation.

In matrix form, by setting

$r_1 = \begin{bmatrix}\cos{2\pi \over n} & -\sin{2\pi \over n} \\[8pt] \sin{2\pi \over n} & \cos{2\pi \over n}\end{bmatrix} \qquad s_0 = \begin{bmatrix}1 & 0 \\ 0 & -1\end{bmatrix}$

and defining $r_j = r_1^j$ and $s_j = r_j \, s_0$ for $j \in \{1,\ldots,n-1\}$ we can write the product rules for  D_n as

$r_j \, r_k = r_{(j+k) \text{ mod }n}$

$r_j \, s_k = s_{(j+k) \text{ mod }n}$

$s_j \, r_k =s_{(j-k) \text{ mod }n}$

$s_j \, s_k = r_{(j-k) \text{ mod }n}.$

(Compare coordinate rotations and reflections.)

The dihedral group D₂ is generated by the rotation r of 180 degrees, and the reflection s across the x-axis. The elements of D₂ can then be represented as {e, r, s, rs}, where e is the identity or null transformation and rs is the reflection across the y-axis.

The four elements of D₂ (x-axis is vertical here)

D₂ is isomorphic to the Klein four-group.

For n>2 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.

D₄ is nonabelian (x-axis is vertical here).

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 2n elements of D_n can be written as e, r, r², ..., rⁿ⁻¹, s, r s, r² s, ..., rⁿ⁻¹ s. 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).

Examples of 2D dihedral symmetry

• 

  2D D₆ symmetry – The Red Star of David

• 

  2D D₂₄ symmetry – Ashoka Chakra, as depicted on the National flag of the Republic of India.

Equivalent definitions

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

$D_n=\langle r, s \mid r^n = 1, s^2 = 1, s^{-1}rs = r^{-1} \rangle$

or

$D_n=\langle x, y \mid x^2 = y^2 = (xy)^n = 1 \rangle.$

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 the group Z_n

$\mathbb{Z}_n \rtimes_\varphi \mathbb{Z}_2$ is isomorphic to Dih_n if φ(0) is the identity and φ(1) is inversion.

Properties

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 via this permutation representation.

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_2n 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.

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.

Conjugacy classes of reflections

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. Geometrically, in an odd polygon every axis of symmetry passes through a vertex and a side, while in an even polygon half the axes pass through two vertices, and half pass through two sides.

Algebraically, this is an instance of the conjugate Sylow theorem (for n odd): for n odd, each reflection, together with the identity, form a subgroup of order 2, which is a Sylow 2-subgroup (2 = 2¹ is the maximum power of 2 dividing 2n = 2(2k + 1)), while for n even, these order 2 subgroups are not Sylow subgroups because 4 (a higher power of 2) divides the order of the group.

For n even there is instead an outer automorphism interchanging the two types of reflections (properly, a class of outer automorphisms, which are all conjugate by an inner automorphism).

Automorphism group

The automorphism group of Dih_n is isomorphic to the affine group Aff(Z/nZ) $=\{ax + b \mid (a,n) = 1\}$ and has order nϕ(n), where ϕ is Euler's totient function, the number of k in $1,\dots,n-1$ coprime to n.

It can be understood in terms of the generators of a reflection and an elementary rotation (rotation by k(2π / n), for k coprime to n); which automorphisms are inner and outer depends on the parity of n.

• For n odd, the dihedral group is centerless, so any element defines a non-trivial inner automorphism; for n even, the rotation by 180° (reflection through the origin) is the non-trivial element of the center.
• Thus for n odd, the inner automorphism group has order 2n, and for n even the inner automorphism group has order n.
• For n odd, all reflections are conjugate; for n even, they fall into two classes (those through two vertices and those through two faces), related by an outer automorphism, which can be represented by rotation by π / n (half the minimal rotation).
• The rotations are a normal subgroup; conjugation by a reflection changes the sign (direction) of the rotation, but otherwise leaves them unchanged. Thus automorphisms that multiply angles by k (coprime to n) are outer unless $k=\pm 1.$

Examples of automorphism groups

Dih₉ has 18 inner automorphisms. As 2D isometry group D₉, 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 D₁₀, 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).

Generalizations

There are several important generalizations of the dihedral groups:

• The infinite dihedral group is an infinite group with algebraic structure similar to the finite dihedral groups. It can be viewed as the group of symmetries of the integers.
• The orthogonal group O(2), i.e. the symmetry group of the circle, also has similar properties to the dihedral groups.
• The family of generalized dihedral groups includes both of the examples above, as well as many other groups.
• The quasidihedral groups are family of finite groups with similar properties to the dihedral groups.

See also

• 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
• Cycle index of the dihedral group

References

1. ^ Dummit, David S.; Foote, Richard M. (2004). Abstract Algebra (3rd ed.). John Wiley & Sons. ISBN 0-471-43334-9. 

External links

• Dihedral Group n of Order 2n by Shawn Dudzik, Wolfram Demonstrations Project.