In Group theory, we analyze the algebraic structures of a set with a binary operation given. In this article, we will learn the definition of a group (in Abstract Algebra) with their properties, examples, and applications.

Table of Contents

## Definition of a Group

Let G be a set and o be a binary operation acting on it. Then the pair (G, o) is called a group if the following axioms are satisfied.

**[Closure]**G is closed under the composition o. That is, for every a, b ∈ G, we have that aob ∈ G.**[Associativity]**o is associative. That is, for all a,b,c ∈ G, we have (aob)oc=ao(boc) ∈ G.**[Identity]**There exists an element e in G such that aoe=eoa=a, for all a ∈ G.**[Inverse]**For every a ∈ G, there exists a^{-1}∈ G such that aoa^{-1}= a^{-1}oa = e, for all a ∈ G.

In addition to the above four axioms, if (G, o) satisfies the commutative property, that is, aob=boa for all a,b ∈ G, then the group G is called a commutative group or an abelian group (after the name of Norwegian mathematician Niels Henrik Abel).

For example, (Z, +) is an abelian group as we have m+n=n+m for all m,n ∈ Z.

## Examples of Groups

**Example 1:** (Z, +) is a group, where Z is the set of all integers.

*Proof:*

1. | We know that for any two integers a and b, we always have a+b∈Z. Thus, the set Z is closed under the operation +. |

2. | For any three integers a, b, and c, we have (a+b)+c=a+(b+c). So + is associative on Z. |

3. | We have a+0=a for all a∈Z. Thus, 0 is the identity element in Z. In other words, the identity element exists in Z. |

4. | For any a∈Z, a+(-a)=0. So -a is the inverse of a in Z. That is, the inverse exists for every element in Z. |

**Conclusion:**

As the above four conditions are satisfied for (Z, +), we can say that the pair (Z, +) is a group. It is called the additive group Z.

**More Examples of Groups:**

- (R, +) is a group, where R is the set of all real numbers.
- (C, +) is a group, where R is the set of all complex numbers.

## Non-Examples of Groups

1 | Z is not a group under multiplication, that is, (Z, ×) is not a group. This is because 2 does not have an inverse in Z as 1/2 ∉ Z. |

2 | The set of real numbers R is not a group under multiplication, that is, (R, ×) is not a group. This is because 0 does not have an inverse in R. Note that (R-{0}, ×) is a group with the identity 1. |

## Order of Groups

Let (G, o) be a group. The order of the group G is the cardinality of G, denoted by |G|. If |G| is finite, we say that (G, o) is a finite group. Otherwise, it is called an infinite group.

- (Z, +) is an infinite group as the number of elements of Z is not finite.
- (Z/2Z, +) is a finite group of order 2.

## Types of Groups

There are many types of groups. For example,

- Abelian Group: commutative groups are known as abelian groups
- Cyclic Group: If a group is generated by a single element, then it is called a cyclic group. For example, (Z, +) is a cyclic group generated by 1.
- Prime order groups: If a group has prime order, then it is called a group of prime order. These groups are always cyclic.

## Properties of Groups

Every group (G, o) satisfies the following properties:

- The composition of two elements always belongs to G. That is, aob∈G for all a,b in G.
- Every group (G, 0) contains only one identity element.
- Each element in a group (G, o) has only one inverse.
- In a group (G, o), the cancellation law holds.
- aob=aoc ⇒b=c (left cancellation law)
- boa=coa ⇒b=c (right cancellation law)

- We have (aob)
^{-1}= b^{-1}oa^{-1}for all a,b ∈G. That is, the inverse of ab is equal to b^{-1}a^{-1}.

## Applications of Group Theory

Group theory has many applications in Physics, Chemistry, Mathematics, and many other areas. It plays a crucial role in public-key cryptography.

## FAQs

**Q1: What is the inverse of the identity in a group?**

Answer: The inverse of identity is the identity itself. That is, e^{-1}=e.

**Q2: Is (2Z, +) a group?**

Answer: Yes, (2Z, +) is a commutative group.