A semigroup in mathematics is a set equipped with a binary operation that is associative. In this article, we will study the definition of semigroups together with examples, and properties.

Table of Contents

## Definition of a Semigroup

Let G be a non-empty set and o be an algebraic operation acting on it. Then the pair (G, o) is called a semigroup if the following are satisfied:

- G is closed under o. In other words, aob ∈ G for all a, b ∈ G.
- o is associative, that is, (aob)oc = ao(boc) for all a,b,c ∈ G.

## Examples of Semigroup

(R, +) is a semigroup where R is the set of real numbers.

*Proof:*

1 | For any two real numbers a, b we have that a+b ∈ R. Thus, R is closed under +. |

2 | As (a+b)+c=a+(b+c) for any real numbers a,b,c we have that + is associative on R. |

**Conclusion:** Thus (R, +) is a semigroup and it follows from the above definition.

## Non-Examples of Semigroups

(Z, -) is not a semigroup where Z denotes the set of all integers. This is because `-` is not associative on Z. See that (1-1)-2 ≠ 1-(1-2).

## Properties of Semigroups

The list of properties satisfied by semigroups is given below.

- If a semigroup (G, o) is commutative, that is, aob=boa for all a,b ∈ G, then it is called a commutative semigroup or an abelian semigroup (named after the mathematician N. Abel). For example, (Z,+), (Q,+), (R,+) all are commutative semigroups.
- Let (G, o) be a semigroup and a ∈ G. Then a
^{m+n}= a^{m}oa^{n}for all natural numbers m,n. This is because o is associative on G. - A semigroup is called a finite order semigroup if it contains a finite number of elements. For example, (Z
_{n, }.) is a semigroup of order n.

**Related Topics:**

**Group Theory: Definition, Examples, Orders, Types, Properties, Applications**