# Construction of Inverse Completion/Identity of Quotient Structure

## 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 $C$.

Let $\boxtimes$ be the cross-relation on $S \times C$, defined as:

$\left({x_1, y_1}\right) \boxtimes \left({x_2, y_2}\right) \iff x_1 \circ y_2 = x_2 \circ y_1$

This cross-relation is a congruence relation on $S \times C$.

Let the quotient structure defined by $\boxtimes$ be:

$\left({T', \oplus'}\right) := \left({\dfrac {S \times C} \boxtimes, \oplus_\boxtimes}\right)$

where $\oplus_\boxtimes$ is the operation induced on $\dfrac {S \times C} \boxtimes$ by $\oplus$.

Let $c \in C$ be arbitrary.

Then:

$\eqclass {\tuple {c, c} } \boxtimes$

is the identity of $T'$.

## Proof

 $\displaystyle \paren {x \circ c} \circ y$ $=$ $\displaystyle x \circ \paren {y \circ c}$ $\displaystyle \leadsto \ \$ $\displaystyle \eqclass {\tuple {x, y} } \boxtimes \oplus' \eqclass {\tuple {c, c} } \boxtimes$ $=$ $\displaystyle \eqclass {\tuple {x \circ c, y \circ c} } \boxtimes$ Definition of $\oplus'$ $\displaystyle$ $=$ $\displaystyle \eqclass {\tuple {x, y} } \boxtimes$ Cancellability of elements of $C$

Hence the result, by definition of identity element.

$\blacksquare$