Union of Subgroups
Theorem
Let $\struct {G, \circ}$ be a group.
Let $H, K \le G$ be subgroups of $G$.
Let neither $H \subseteq K$ nor $K \subseteq H$.
Then $H \cup K$ is not a subgroup of $G$.
Corollary 1
Let $H \cup K$ be a subgroup of $G$.
Then either $H \subseteq K$ or $K \subseteq H$.
Corollary 2
Let $H \vee K$ be the join of $H$ and $K$.
Then $H \vee K = H \cup K$ if and only if $H \subseteq K$ or $K \subseteq H$.
Proof
As neither $H \subseteq K$ nor $K \subseteq H$, it follows from Set Difference with Superset is Empty Set that neither $H \setminus K = \O$ nor $K \setminus H = \O$.
So, let $h \in H \setminus K, k \in K \setminus H$.
Thus, $h \notin K, k \notin H$.
If $\struct {H \cup K, \circ}$ is a group, then it must be closed.
If $\struct {H \cup K, \circ}$ is closed, then $h \circ k \in H \cup K \implies h \circ k \in H \lor h \circ k \in K$.
If $h \circ k \in H$ then $h^{-1} \circ h \circ k \in H \implies k \in H$.
If $h \circ k \in K$ then $h \circ k \circ k^{-1} \in K \implies h \in K$.
So $h \circ k$ can be in neither $H$ nor $K$.
Therefore $\struct {H \cup K, \circ}$ is not closed.
Therefore $H \cup K$ is not a subgroup of $G$.
$\blacksquare$
Examples
Subgroups of $S_3$
Let $S_3$ denote the Symmetric Group on $3$ Letters, whose Cayley table is given as:
- $\begin{array}{c|cccccc}
\circ & e & (123) & (132) & (23) & (13) & (12) \\ \hline e & e & (123) & (132) & (23) & (13) & (12) \\ (123) & (123) & (132) & e & (13) & (12) & (23) \\ (132) & (132) & e & (123) & (12) & (23) & (13) \\ (23) & (23) & (12) & (13) & e & (132) & (123) \\ (13) & (13) & (23) & (12) & (123) & e & (132) \\ (12) & (12) & (13) & (23) & (132) & (123) & e \\ \end{array}$
Consider the subgroups $H, K \le G$:
- $H = \set {e, \tuple {12} }$
- $K = \set {e, \tuple {13} }$
We have that:
- $H \cup K = \set {e, \tuple {12}, \tuple {13} }$
and:
- $\tuple {12} \circ \tuple {13} = \tuple {123}$
But $\tuple {123} \notin H \cup K$.
Hence $H \cup K$ is not closed and so is not a group.
The result follows by definition of subgroup.
$\blacksquare$
Sources
- 1965: J.A. Green: Sets and Groups ... (previous) ... (next): $\S 5.2$. Subgroups: Example $94$
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {II}$: New Structures from Old: $\S 8$: Compositions Induced on Subsets: Exercise $8.13$
- 1966: Richard A. Dean: Elements of Abstract Algebra ... (previous) ... (next): $\S 1.9$
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): Chapter $6$: An Introduction to Groups: Exercise $7$