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