Subgroup of Order 1 is Trivial
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Theorem
Let $\struct {G, \circ}$ be a group.
Then $\struct {G, \circ}$ has exactly $1$ subgroup of order $1$: the trivial subgroup.
Proof
From Trivial Subgroup is Subgroup, $\struct {\set e, \circ}$ is a subgroup of $\struct {G, \circ}$.
Suppose $\struct {\set g, \circ}$ is a subgroup of $\struct {G, \circ}$.
From Group is not Empty, $e \in \set g$.
Thus it follows trivially that $\struct {\set g, \circ} = \struct {\set e, \circ}$.
That is, $\struct {\set e, \circ}$ is the only subgroup of $\struct {G, \circ}$ of order $1$.
$\blacksquare$
Sources
- 1978: John S. Rose: A Course on Group Theory ... (previous) ... (next): $0$: Some Conventions and some Basic Facts