Difference in Naturally Ordered Semigroup is Unique
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Theorem
Let $\struct {S, \circ, \preceq}$ be a naturally ordered semigroup.
Let $n, m \in S$ such that $m \preceq n$.
Then there exists a unique difference $n - m$ of $m$ and $n$.
Proof
Since $m \preceq n$, by Naturally Ordered Semigroup Axiom $\text {NO} 3$: Existence of Product:
- $\exists p \in S: m \circ p = n$
Now suppose that $p, q \in S$ are such that:
- $m \circ p = m \circ q = n$
Then it follows from Naturally Ordered Semigroup Axiom $\text {NO} 2$: Cancellability that:
- $p = q$
Hence the result.
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {III}$: The Natural Numbers: $\S 16$: The Natural Numbers