# Subsemigroup/Examples/x+y-xy on Integers

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## Example of Subsemigroup

Let $\struct {\Z, \circ}$ be the semigroup where $\circ: \Z \times \Z$ is the operation defined on the integers $\Z$ as:

- $\forall x, y \in \Z: x \circ y := x + y - x y$

Let $T$ be the set $\set {x \in \Z: x \le 1}$.

Then $\struct {T, \circ}$ is a subsemigroup of $\struct {\Z, \circ}$.

## Proof

It is established in Operation Defined as $x + y - x y$ on Integers that $\struct {\Z, \circ}$ is a semigroup.

From the Subsemigroup Closure Test it is sufficient to demonstrate that $\struct {T, \circ}$ is closed.

Let $x, y \in \Z_{\le 1}$.

- $\paren {x - 1}\le 0$ and $\paren {y - 1} \le 0$.

\(\displaystyle \paren {x - 1}\) | \(\le\) | \(\displaystyle 0\) | |||||||||||

\(\displaystyle \paren {y - 1}\) | \(\le\) | \(\displaystyle 0\) |

So:

\(\displaystyle \paren {x - 1} \paren {y - 1}\) | \(\ge\) | \(\displaystyle 0\) | |||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle x y - x - y + 1\) | \(\ge\) | \(\displaystyle 0\) | ||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle x y - x - y\) | \(\ge\) | \(\displaystyle -1\) | ||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle x + y - x y\) | \(\le\) | \(\displaystyle 1\) |

So:

- $x \le 1, y \le 1 \implies x \circ y \le 1$

and so $\struct {T, \circ}$ is closed.

The result follows by definition of subsemigroup.

$\blacksquare$

## Sources

- 1978: Thomas A. Whitelaw:
*An Introduction to Abstract Algebra*... (previous) ... (next): Chapter $5$: Semigroups: Exercise $1$