# Dihedral Group D4 is not Internal Group Product/Proof 1

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## Theorem

The dihedral group $D_4$ is not an internal group product of any $2$ of its proper subgroups.

## Proof

Aiming for a contradiction, suppose $D_4$ is the internal group product of $2$ proper subgroups $H$ and $K$ of $D_4$.

Without loss of generality, let $\order H = 2$ and $\order K = 4$.

$H$ needs to be normal in $D_4$ for the conditions of the internal group product to be satisfied.

So the non-identity element of $H$ needs to be conjugated to itself by every element of $G$.

This means $H$ is a subset of the center of $G$.

From Center of Dihedral Group $D_4$:

- $H = \set {e, a^2}$

But every non-trivial normal subgroup of $D_4$ contains $a^2$.

This makes it impossible for $H \cap K = \set e$.

Thus $D_4$ cannot be an internal group product.

$\blacksquare$

## Sources

- 1996: John F. Humphreys:
*A Course in Group Theory*... (previous) ... (next): Chapter $13$: Direct products: Exercise $1$