Group of Rotations about Fixed Point is not Abelian
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Theorem
Let $\SS$ be a rigid body in space.
Let $O$ be a fixed point in space.
Let $\GG$ be the group of all rotations of $\SS$ around $O$.
Then $\GG$ is not an abelian group.
Proof
Let $O$ be the center of $\SS$.
Recall the definition of the symmetry group of the square $D_4$:
Let $\SS = ABCD$ be a square.
The various symmetry mappings of $\SS$ are:
- the identity mapping $e$
- the rotations $r, r^2, r^3$ of $90^\circ, 180^\circ, 270^\circ$ around the center of $\SS$ anticlockwise respectively
- the reflections $t_x$ and $t_y$ are reflections in the $x$ and $y$ axis respectively
- the reflection $t_{AC}$ in the diagonal through vertices $A$ and $C$
- the reflection $t_{BD}$ in the diagonal through vertices $B$ and $D$.
This group is known as the symmetry group of the square, and can be denoted $D_4$.
We have that:
Thus $D_4$ forms a subgroup of $\GG$.
From Symmetry Group of Square is Group we have that $D_4$ is not abelian.
From Subgroup of Abelian Group is Abelian it follows that $\GG$ is also not abelian.
$\blacksquare$
Sources
- 1964: Walter Ledermann: Introduction to the Theory of Finite Groups (5th ed.) ... (previous) ... (next): Chapter $\text {I}$: The Group Concept: $\S 3$: Examples of Infinite Groups: $\text{(iii)}$