Binary cyclic group

In mathematics, the binary cyclic group of the n-gon is the cyclic group of order 2n, $C_{2n}$, thought of as an extension of the cyclic group $C_n$ by a cyclic group of order 2. Coxeter writes the binary cyclic group with angle-brackets, ⟨n⟩, and the index 2 subgroup as (n) or [n]⁺.

It is the binary polyhedral group corresponding to the cyclic group.

In terms of binary polyhedral groups, the binary cyclic group is the preimage of the cyclic group of rotations ($C_n<\mathrm{SO}(3)$) under the 2:1 covering homomorphism

$\mathrm{Spin}(3)\to \mathrm{SO}(3)$

of the special orthogonal group by the spin group.

As a subgroup of the spin group, the binary cyclic group can be described concretely as a discrete subgroup of the unit quaternions, under the isomorphism $\mathrm{Spin}(3)\cong \mathrm{Sp}(1)$ where Sp(1) is the multiplicative group of unit quaternions. (For a description of this homomorphism see the article on quaternions and spatial rotations.)

Presentation

The binary cyclic group can be defined as the set of $2n$^th roots of unity—that is, the set $\{{\omega }_n^k|k\in \{0,1,2,\mathrm{...},2n-1\}\}$, where

${\omega }_n=e^{i\pi /n}=\cos \frac{\pi }{n}+i\sin \frac{\pi }{n},$

using multiplication as the group operation.

See also

• binary dihedral group, ⟨2,2,n⟩, order 4n
• binary tetrahedral group, ⟨2,3,3⟩, order 24
• binary octahedral group, ⟨2,3,4⟩, order 48
• binary icosahedral group, ⟨2,3,5⟩, order 120