Construction of Inverse Completion/Cartesian Product with Cancellable Elements

Theorem
Let $\left({S, \circ}\right)$ be a commutative semigroup which has cancellable elements.

Let $\left({C, \circ_{\restriction_C}}\right) \subseteq \left({S, \circ}\right)$ be the subsemigroup of cancellable elements of $\left({S, \circ}\right)$, where $\circ_{\restriction_C}$ denotes the restriction of $\circ$ to $C$.

Let $\left({S \times C, \oplus}\right)$ be the external direct product of $\left({S, \circ}\right)$ and $\left({C, \circ_{\restriction_C}}\right)$, where $\oplus$ is the operation on $S \times C$ induced by $\circ$ on $S$ and $\circ_{\restriction_C}$ on $C$.

That is:
 * $\forall \left({x, y}\right), \left({u, v}\right) \in S \times C: \left({x, y}\right) \oplus \left({u, v}\right) = \left({x \circ u, y \mathop{\circ_{\restriction_C}} v}\right)$

Then $\left({S \times C, \oplus}\right)$ is a commutative semigroup.

Proof
By Cancellable Elements of a Semigroup form Subsemigroup, $\left({C, \circ_{\restriction_C}}\right)$ is a subsemigroup of $\left({S, \circ}\right)$, where $\circ_{\restriction_C}$ is the restriction of $\circ$ to $C$.

By Restriction of Operation Commutativity, as $\left({C, \circ_{\restriction_C}}\right)$ is a substructure of a commutative structure, it is also commutative.

From:


 * the external direct product preserves the nature of semigroups
 * the external direct product preserves commutativity

we see that $\left({S \times C, \oplus}\right)$ is a commutative semigroup.