Example of Two Groups and a Subgroup of the Direct Product that is Not of the Form of Direct Product

Group Theory Problems and Solutions in Mathematics

Problem 467

Give an example of two groups $G$ and $H$ and a subgroup $K$ of the direct product $G\times H$ such that $K$ cannot be written as $K=G_1\times H_1$, where $G_1$ and $H_1$ are subgroups of $G$ and $H$, respectively.

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Solution.

Let $G$ be any nontrivial group, and let $G=H$.
(For example, you may take $G=H=\Zmod{2}$.)

Then consider the subset $K$ in the direct product given by
\[K:=\{(g,g) \mid g\in G\} \subset G\times G.\]

We claim that $K$ is a subgroup of $G\times G$.
In fact, we have
\begin{align*}
(g,g)(h,h)=(gh,gh)\in K \text{ and }\\
(g,g)^{-1}=(g^{-1}, g^{-1})\in K
\end{align*}
for any $g, h\in G$.
Thus, $K$ is closed under multiplications and inverses, and hence $K$ is a subgroup of $G\times G$.

Now we show that $K$ is not of the form $G_1\times H_1$ for some subgroups $G_1, H_1$ of $G$.
Assume on the contrary $K=G_1\times H_1$ for some subgroups $G_1, H_1$ of $G$.

Since $G$ is a nontrivial group, there is a nonidentity element $x\in G$.
So $(x,x)\in K$ and $K$ is not the trivial group.
Thus, both $G_1$ and $H_1$ cannot be the trivial group.

Without loss of generality, assume that $G_1$ is nontrivial.
Then $G_1$ contains a nonidentity element $y$.

Since the identity element $e$ is contained in all subgroups, we have
\[(y,e)\in G_1\times H_1.\] However, this element cannot be in $K$ since $y\neq e$, a contradiction.

Hence $K$ is not of the form $G_1\times H_1$.

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