Homomorphism Preserves Subsemigroups
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Theorem
Let $\struct {S, \circ}$ and $\struct {T, *}$ be semigroups.
Let $\phi: \struct {S, \circ} \to \struct {T, *}$ be a homomorphism.
Let $S'$ be a subsemigroup of $S$.
Then $\phi \paren {S'}$ is a subsemigroup of $T$.
Proof
By Restriction of Homomorphism to Image is Epimorphism, $\phi$ is an epimorphism onto its image .
Then by Epimorphism Preserves Semigroups, it follows that the image of $S'$ is a semigroup.
The result follows.
$\blacksquare$