Quaternion group

Cycle diagram of Q. Each color specifies a series of powers of any element connected to the identity element (1). For example, the cycle in red reflects the fact that i ² = −1, i ³ = −i and i ⁴ = 1. The red cycle also reflects the fact that (−i )² = −1, (−i )³ = i and (−i )⁴ = 1.

In group theory, the quaternion group is a non-abelian group of order 8. It is often denoted by Q and written in multiplicative form, with the following 8 elements

Q = {1, −1, i, −i, j, −j, k, −k}

Here 1 is the identity element, (−1)² = 1, and (−1)a = a(−1) = −a for all a in Q. The remaining multiplication rules can be obtained from the following relation:

i^{2}=j^{2}=k^{2}=ijk=-1

The entire Cayley table (multiplication table) for Q is given by:

	1	i	j	k	−1	−i	−j	−k
1	1	i	j	k	−1	−i	−j	−k
i	i	−1	k	−j	−i	1	−k	j
j	j	−k	−1	i	−j	k	1	−i
k	k	j	−i	−1	−k	−j	i	1
−1	−1	−i	−j	−k	1	i	j	k
−i	−i	1	−k	j	i	−1	k	−j
−j	−j	k	1	−i	j	−k	−1	i
−k	−k	−j	i	1	k	j	−i	−1

Note that the resulting group is non-commutative; for example ij = −ji. Q has the unusual property of being Hamiltonian: every subgroup of Q is a normal subgroup, but the group is non-abelian. Every Hamiltonian group contains a copy of Q.

In abstract algebra, one can construct a real 4-dimensional vector space with basis {1, i, j, k} and turn it into an associative algebra by using the above multiplication table and distributivity. The result is a skew field called the quaternions. Note that this is not quite the group algebra on Q (which would be 8-dimensional). Conversely, one can start with the quaternions and define the quaternion group as the multiplicative subgroup consisting of the eight elements {1, −1, i, −i, j, −j, k, −k}.

Note that i, j, and k all have order 4 in Q and any two of them generate the entire group. Q has the presentation

\langle x,y\mid x^{4}=1,x^{2}=y^{2},yxy^{-1}=x^{-1}\rangle

One may take, for instance, x = i and y = j.

The center and the commutator subgroup of Q is the subgroup {±1}. The factor group Q/{±1} is isomorphic to the Klein four-group V. The inner automorphism group of Q is isomorphic to Q modulo its center, and is therefore also isomorphic to the Klein four-group. The full automorphism group of Q is isomorphic to S₄, the symmetric group on four letters. The outer automorphism group of Q is then S₄/V which is isomorphic to S₃.

The quaternion group Q may be regarded as acting on the eight nonzero elements of the 2-dimensional vector space over the finite field GF(3). For a picture, see Visualizing GL(2,p).

Generalized quaternion group

A group is called a generalized quaternion group if it has a presentation

\langle x,y\mid x^{2^{n-1}}=1,x^{2^{n-2}}=y^{2},yxy^{-1}=x^{-1}\rangle

for some integer n ≥ 3. The order of this group is 2ⁿ. The ordinary quaternion group corresponds to the case n = 3. The generalized quaternion group can be realized as the subgroup of unit quaternions generated by

x=e^{2\pi i/2^{n-1}}

y=j\,

The generalized quaternion groups are members of the still larger family of dicyclic groups. The generalized quaternion groups have the property that every abelian subgroup is cyclic. It can be shown that a finite p-group with this property (every abelian subgroup is cyclic) is either cyclic or a generalized quaternion group as defined above.

Generalized quaternion group

See also