The dihedral group, $D_{2n}$, is a finite group of order $2n$. It may be defined as the symmetry group of a regular $n$-gon.
For instance $D_6$ is the symmetry group of the equilateral triangle and is isomorphic to the symmetric group, $S_3$.
For $n \in \mathbb{N}$, $n \geq 1$, the dihedral group $D_{2n}$ is thus the subgroup of the orthogonal group $O(2)$ which is generated from the finite cyclic subgroup $C_n \,\coloneqq\, \mathbb{Z}/n$ of $SO(2)$ and the reflection at the $x$-axis (say). It is a semi-direct product of $C_n$ and a $C_2 \,\coloneqq\, \mathbb{Z}/2$ corresponding to that reflection, hence fitting into a short exact sequence as follows:
Under the further embedding $O(2)\hookrightarrow SO(3)$ the cyclic and dihedral groups are precisely those finite subgroups of SO(3) that, among their ADE classification, are not in the exceptional series.
Warning on notation
There are two different conventions for numbering the dihedral groups.
The above is the algebraic convention in which the suffix gives the order of the group: ${\vert D_{2 n}\vert} = 2 n$.
In the geometric convention one writes “$D_n$” instead of “$D_{2n}$”, recording rather the geometric nature of the object of which it is the symmetry group.
Also beware that there is yet another group denoted $D_n$ mentioned at Coxeter group.
Under the further lift through the spin group-double cover map $SU(2) \simeq Spin(3) \to SO(3)$ of the special orthogonal group, the dihedral group $D_{2n}$ is covered by the binary dihedral group, also known as the dicyclic group and denoted
Equivalently, this is the lift of the dihedral group $D_{2n}$ (above) through the pin group double cover of the orthogonal group O(2) to Pin(2)
Explicity, let $\mathbb{H} \simeq \mathbb{C} \oplus \mathrm{j} \mathbb{C}$ be the quaternions realized as the Cayley-Dickson double of the complex numbers, and identify the circle group
with the unit circle in $\mathbb{C} \hookrightarrow \mathbb{H}$ this way, with group structure given by multiplication of quaternions. Then the Pin group Pin(2) is isomorphic to the subgroup of the group of units $\mathbb{H}^\times$ of the quaternions which consists of this copy of SO(2) together with the multiples of the imaginary quaternion $\mathrm{j}$ with this copy:
The binary dihedral group $2 D_{2n}$ is the subgroup of that generated from
$a \coloneqq \exp\left( \pi \mathrm{i} \tfrac{1}{n} \right) \in S(\mathbb{C}) \subset Pin_-(2) \subset Spin(3)$
$x \coloneqq \mathrm{j} \in Pin_-(2) \subset Spin(3)$.
It is manifest that these two generators satisfy the relations
and in fact these generators and relations fully determine $2 D_{2n}$, up to isomorphism.
The group cohomology of the dihedral group is discussed for instance at Groupprops.
ADE classification and McKay correspondence
The dihedral group $D_{2n}$ has a group presentation
From this it is easy to see that it is a semi-direct product of the $C_n$ generated by $x$ and the $C_2$ generated by $y$. The action of $y$ on $x$ is given by $\,^y x= x^{-1}$.
It is a standard example considered in elementary combinatorial group theory.
The first binary dihedral group $2 D_4$ is isomorphic to the quaternion group of order 8:
In the ADE-classification this is the entry D4.
linear representation theory of binary dihedral group $2 D_4$
$=$ dicyclic group $Dic_2$ $=$ quaternion group $Q_8$
$\,$
group order: ${\vert 2D_4\vert} = 8$
conjugacy classes: | 1 | 2 | 4A | 4B | 4C |
---|---|---|---|---|---|
their cardinality: | 1 | 1 | 2 | 2 | 2 |
$\,$
splitting field | $\mathbb{Q}(\alpha, \beta)$ with $\alpha^2 + \beta^2 = -1$ |
field generated by characters | $\mathbb{Q}$ |
character table over splitting field $\mathbb{Q}(\alpha,\beta)$/complex numbers $\mathbb{C}$
irrep | 1 | 2 | 4A | 4B | 4C | Schur index |
---|---|---|---|---|---|---|
$\rho_1$ | 1 | 1 | 1 | 1 | 1 | 1 |
$\rho_2$ | 1 | 1 | -1 | 1 | -1 | 1 |
$\rho_3$ | 1 | 1 | 1 | -1 | -1 | 1 |
$\rho_4$ | 1 | 1 | -1 | -1 | 1 | 1 |
$\rho_5$ | 2 | -2 | 0 | 0 | 0 | 2 |
character table over rational numbers $\mathbb{Q}$/real numbers $\mathbb{R}$
irrep | 1 | 2 | 4A | 4B | 4C |
---|---|---|---|---|---|
$\rho_1$ | 1 | 1 | 1 | 1 | 1 |
$\rho_2$ | 1 | 1 | -1 | 1 | -1 |
$\rho_3$ | 1 | 1 | 1 | -1 | -1 |
$\rho_4$ | 1 | 1 | -1 | -1 | 1 |
$\rho_5 \oplus \rho_5$ | 4 | -4 | 0 | 0 | 0 |
References
GroupNames, Q8,
Discussion in the context of the classification of finite rotation groups goes back to
See also
Wikipedia, Dihedral group
Wikipedia, Binary dihedral group
Wikipedia, Dicyclic group
Discussion as the equivariance group in equivariant cohomology theory:
and specifically in equivariant K-theory and KR-theory:
Discussion of equivariant ordinary cohomology (Bredon cohomology) over the point but in arbitrary RO(G)-degree, for equivariance group a dihedral group of order $2p$:
see also
Last revised on December 22, 2021 at 15:51:35. See the history of this page for a list of all contributions to it.