Symmetry Group of Regular Hexagon/Examples/Normalizer of Subgroup of Rotations
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Examples of Operations on Symmetry Group of Regular Hexagon
Let $\HH = ABCDEF$ be a regular hexagon.
Let $D_6$ denote the symmetry group of $\HH$.
Let $e$ denote the identity mapping
Let $\alpha$ denote rotation of $\HH$ anticlockwise through $\dfrac \pi 3$ radians ($60 \degrees$)
Let $\beta$ denote reflection of $\HH$ in the $AD$ axis.
Let $\gen \alpha$ denote the subgroup generated by $\alpha$.
The normalizer of $\gen \alpha$ is $D_6$ itself:
- $\map {N_{D_6} } {\gen \alpha} = D_6$
Proof
We have that:
- $\order {\gen \alpha} = 6 = \dfrac {\order {D_6} } 2$
and so:
- $\index {D_6} {\gen \alpha} = 2$
From Subgroup of Index 2 is Normal, $\gen \alpha$ is normal in $D_6$.
The result follows from Normal Subgroup iff Normalizer is Group.
$\blacksquare$
Sources
- 1971: Allan Clark: Elements of Abstract Algebra ... (previous) ... (next): Chapter $2$: Conjugacy, Normal Subgroups, and Quotient Groups: $\S 48 \alpha$