To know about Isomorphism you have to know the definitions of Homomorphism,Epimorphism and Monomorphism.

**Definition of Homomorphism of groups :**

Let (G,*) and (G,o) be two groups and f be a function from G into G1. Then f is called a **homomorphism** of G into G1 if for all a,b ∈ G,

f(a*b) = f(a) o f(b).

**Example**

Consider the group (Z,+).

Define f:Z →Z by f(n) = 3n for all n∈ Z

Here the function f is from the group (Z,+) to (Z,+)

Let n,m ∈ Z then we get n+m ∈ Z

and we have

f(n+m) = 3(n+m) = 3n + 3m = f(n) + f(m)

Hence the function f is a homomorphism.

Definition of **epimorphism** and **monomorphism** of groups

Let (G,*) and (G,o) be two groups and f : G → G1 be a homomorphism of groups then

1) f is called a **monomorphism** if f is an injective function.

2) f is called an **epimorphism **if f is an surjective function.

Example of **Monomorphism**

The example for monomorphism is same as the example given for homomorphism.

The function f: Z → Z defined by f(n) = 3n is an injective function. So, f is a monomorphism.

Example of **Epimorphism**

Consider the function f : Z →{1,-1} by

f(n) = 1, if n is even

= -1 if n is odd

Here the groups are (Z,+) and ({-1,1},*)

(where (Z,+) is a group of the set of integers under addition

and ({-1,1},*) is a group of the set {1,-1} under multiplication.)

Check the above function f is a homomorphism.

Since f is surjective f is an epimorphism.

**Definition of Isomorphism**

Let (G,*) and (G,o) be two groups and f : G → G1 be a homomorphism of groups then

f is called a isomorphism if f is a bijective function.

**Example of Isomorphism**

Consider the function f: Z → Z by f(x) = x

Now I will show that f is a homomorphism.

Take any two elements x,y belongs to Z

Then x + y belongs to Z

Hence f(x+y) = x + y = f(x) + f(y)

Hence f is homomorphism.

Since the function f(x) = x is bijective.

f is an isomorphism.

**Definition of Isomorphic Groups**

**Two groups (G,*) and (G1,o) are isomorphic to each other if there exist a homomorphic bijective function from G to G1.**

**Example for Isomorphic groups **

**Every infinite cyclic group is isomorphic to infinite cyclic group of set of integers under addition**

https://gateoverflow.in/215185/group-theory