# Symmetry Group of Line Segment is Group

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## Theorem

The symmetry group of the line segment is a group.

### Definition

Recall the definition of the symmetry group of the line segment:

Let $AB$ be a line segment.

The symmetries of $AB$ are:

- The identity mapping $e$
- The rotation $r$ of $180 \degrees$ about the midpoint of $AB$.

This group is known as the **symmetry group of the line segment**.

## Proof

Let us refer to this group as $D_1$.

Taking the group axioms in turn:

### Group Axiom $\text G 0$: Closure

From the Cayley table it is seen directly that $D_1$ is closed.

$\Box$

### Group Axiom $\text G 1$: Associativity

Composition of Mappings is Associative.

$\Box$

### Group Axiom $\text G 2$: Existence of Identity Element

The identity is $e$.

$\Box$

### Group Axiom $\text G 3$: Existence of Inverse Element

Each element is seen to be self-inverse:

- $r^{-1} = r$

$\Box$

No more need be done. $D_1$ is seen to be a group.

$\blacksquare$

## Sources

- 1971: Allan Clark:
*Elements of Abstract Algebra*... (previous) ... (next): Chapter $2$: The Definition of Group Structure: $\S 26 \eta$