Dihedral Group D4 is not Internal Group Product/Proof 2
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Theorem
The dihedral group $D_4$ is not an internal group product of any $2$ of its proper subgroups.
Proof
The proper subgroups of $D_4$ are of order no greater than $4$.
From Group of Order less than 6 is Abelian, all such proper subgroups are abelian.
From External Direct Product of Abelian Groups is Abelian Group, the group direct product of $2$ of these proper subgroups is in turn abelian.
From Internal and External Group Direct Products are Isomorphic, the internal group product of $2$ of these proper subgroups is in turn abelian.
But $D_4$ is not abelian.
Hence it cannot be the internal group product of any $2$ of these proper subgroups.
$\blacksquare$