Epimorphism Preserves Identity
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Theorem
Let $\struct {S, \circ}$ and $\struct {T, *}$ be algebraic structures.
Let $\phi: \struct {S, \circ} \to \struct {T, *}$ be an epimorphism.
Let $\struct {S, \circ}$ have an identity element $e_S$.
Then $\struct {T, *}$ has the identity element $\map \phi {e_S}$.
Proof
Let $\struct {S, \circ}$ be an algebraic structure in which $\circ$ has an identity $e_S$.
Then:
- $\forall x \in S: x \circ e_S = x = e_S \circ x$
Thus:
- $\forall x \in S: x \circ e_S, e_S \circ x \in \Dom \phi$
Hence:
\(\ds \map \phi x\) | \(=\) | \(\ds \map \phi {x \circ e_S}\) | Definition of Identity Element | |||||||||||
\(\ds \) | \(=\) | \(\ds \map \phi x * \map \phi {e_S}\) | Definition of Morphism Property |
and:
\(\ds \map \phi x\) | \(=\) | \(\ds \map \phi {e_S \circ x}\) | Definition of Identity Element | |||||||||||
\(\ds \) | \(=\) | \(\ds \map \phi {e_S} * \map \phi x\) | Definition of Morphism Property |
The result follows because every element $y \in T$ is of the form $\map \phi x$ with $x \in S$.
$\blacksquare$
Warning
Note that this result is applied to epimorphisms.
For a general homomorphism which is not surjective, nothing definite can be said about the behaviour of the elements of its codomain which are not part of its image.
Also see
- Epimorphism Preserves Associativity
- Epimorphism Preserves Commutativity
- Epimorphism Preserves Inverses
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {II}$: New Structures from Old: $\S 12$: Homomorphisms: Theorem $12.2$
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {II}$: New Structures from Old: $\S 12$: Homomorphisms: Exercise $12.1$