Ordered Semigroup Isomorphism is Surjective Monomorphism
Theorem
Let $\struct {S, \circ, \preceq}$ and $\struct {T, *, \preccurlyeq}$ be ordered semigroups.
Let $\phi: \struct {S, \circ, \preceq} \to \struct {T, *, \preccurlyeq}$ be a mapping.
Then $\phi$ is an ordered semigroup isomorphism if and only if:
- $(1): \quad \phi$ is an ordered semigroup monomorphism
- $(2): \quad \phi$ is a surjection.
Proof
Necessary Condition
Let $\phi: \struct {S, \circ, \preceq} \to \struct {T, *, \preccurlyeq}$ be an ordered semigroup isomorphism.
Then by definition:
- $\phi$ is a semigroup isomorphism from the semigroup $\struct {S, \circ}$ to the semigroup $\struct {T, *}$
- $\phi$ is an order isomorphism from the ordered set $\struct {S, \preceq}$ to the ordered set $\struct {T, \preccurlyeq}$.
A semigroup isomorphism is by definition:
which is:
- A monomorphism and an epimorphism.
From Order Isomorphism is Surjective Order Embedding, an order isomorphism is an order embedding which is also a surjection.
Putting this all together, we see that an ordered semigroup isomorphism is:
- A monomorphism
- An order embedding
- A surjection.
An ordered semigroup monomorphism is by definition:
which is also
Hence $\phi$ is:
$\Box$
Sufficient Condition
Let $\phi$ be:
By definition, that means $\phi$ be:
- A monomorphism
- An order embedding
- A surjection.
From Order Isomorphism is Surjective Order Embedding, an order isomorphism is an order embedding which is also a surjection.
A semigroup isomorphism is by definition:
which is:
- A semigroup monomorphism and an semigroup epimorphism.
Thus a semigroup monomorphism which is also a surjection is a semigroup isomorphism.
So $\phi$ is:
- A semigroup isomorphism from the semigroup $\struct {S, \circ}$ to the semigroup $\struct {T, *}$
- An order isomorphism from the ordered set $\struct {S, \preceq}$ to the ordered set $\struct {T, \preccurlyeq}$.
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {III}$: The Natural Numbers: $\S 15$: Ordered Semigroups