Identity of Algebraic Structure is Preserved in Substructure
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Theorem
Let $\struct {S, \circ}$ be an algebraic structure with identity $e$.
Let $\struct {T, \circ}$ be a algebraic substructure of $\struct {S, \circ}$.
That is, let $T \subseteq S$.
Let $e \in T$.
Then $e$ is an identity of $\struct {T, \circ}$.
Proof
Let $x \in T$.
By the definition of subset, $x \in S$.
Since $e$ is an identity of $\struct {S, \circ}$:
- $e \circ x = x \circ e = x$
Since this holds for all $x \in T$, $e$ is an identity of $\struct {T, \circ}$.
$\blacksquare$