# Definition:Semigroup

## Definition

Let $\struct {S, \circ}$ be a magma.

Then $\struct {S, \circ}$ is a semigroup if and only if $\circ$ is associative on $S$.

That is:

A semigroup is an algebraic structure which is closed and whose operation is associative.

### Semigroup Axioms

The properties that define a semigroup can be gathered together as follows:

A semigroup is an algebraic structure $\struct {S, \circ}$ which satisfies the following properties:

 $(\text S 0)$ $:$ Closure $\displaystyle \forall a, b \in S:$ $\displaystyle a \circ b \in S$ $(\text S 1)$ $:$ Associativity $\displaystyle \forall a, b, c \in S:$ $\displaystyle a \circ \paren {b \circ c} = \paren {a \circ b} \circ c$

### Multiplicative Semigroup

Let $\struct {S, \circ}$ be a semigroup whose operation is multiplication.

Then $\struct {S, \circ}$ is a multiplicative semigroup.

Let $\struct {S, \circ}$ be a semigroup whose operation is addition.

Then $\struct {S, \circ}$ is an additive semigroup.

## Also known as

Some older texts have this as semi-group.

A semigroup is also known as an associative algebraic structure.

Some sources call this a monoid, but this term usually has a different meaning.

Make sure you understand which is being used.

## Examples

### Operation Defined as $x + y - x y$ on Integers

Let $\circ: \Z \times \Z$ be the operation defined on the integers $\Z$ as:

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

Then $\struct {\Z, \circ}$ is a semigroup.

### Order $2$ Semigroups

The Cayley tables for the complete set of Semigroups of order $2$ are listed below.

The underlying set in all cases is $\set {a, b}$.

$\begin{array}{r|rr} & a & b \\ \hline a & a & a \\ b & a & a \\ \end{array} \qquad \begin{array}{r|rr} & a & b \\ \hline a & a & a \\ b & a & b \\ \end{array} \qquad \begin{array}{r|rr} & a & b \\ \hline a & a & a \\ b & b & b \\ \end{array}$
$\begin{array}{r|rr} & a & b \\ \hline a & a & b \\ b & a & b \\ \end{array} \qquad \begin{array}{r|rr} & a & b \\ \hline a & a & b \\ b & b & a \\ \end{array} \qquad \begin{array}{r|rr} & a & b \\ \hline a & a & b \\ b & b & b \\ \end{array}$
$\begin{array}{r|rr} & a & b \\ \hline a & b & a \\ b & a & b \\ \end{array}$
$\begin{array}{r|rr} & a & b \\ \hline a & b & b \\ b & b & b \\ \end{array}$

## Also see

• Results about semigroups can be found here.