One to one and Onto functions

one-to-one function

one-to-one function or injective function is one of the most common functions used. 

One to One Function

One-to-One functions define that each element of one set say Set (A) is mapped with a unique element of another set, say, Set (B).

To understand this, let us consider ‘f’ is a function whose domain is set A. The function is said to be one to one if for all x and y in A,

x=y if whenever f(x)=f(y)

In the same manner if x ≠ y, then f(x) ≠ f(y)

Formally, it is stated as, if f(x)=f(y)  implies x=y, then f is one-to-one mapped or f is 1-1.

if f(x) = y then only f-1(y) = x

Example 1: Show that f: R → R defined as f(a) = 3a3 – 4 is one to one function?


Let f (a1) = f ( a2 ) for all a1 , a2 ∈ R

So 3a13 – 4 = 3a23 – 4

a13 = a23

a13 – a23 = 0

(a1 – a2) (a1 + a1a2 + a22) = 0

a1 = a2 and (a12 + a1a2 + a22) = 0

(a12 + a1a2 + a22) = 0 is not considered because there are no real values of a1 and a2.

Therefore, the given function f is one-one.

Onto Function

To explain Onto function consider two sets, Set A and Set B which consist of elements. Ifthere is at least one or more than one element matching with A for every element of B, then the function is said to be onto function or surjective function.

Onto Funtion

You can see in the figure that for each element of B there is a pre-image or a matching element in Set A. Hence it is called an onto function. In the second figure it can be seen that one element in Set B is not mapped with any of the element of set A. Because of this it’s not an onto function

  • Every onto function has a right inverse and every function with a right inverse is an onto function. When we compose onto functions, the result will be onto function only.

Example: Let A={1,5,8,9) and B{2,4} And f={(1,2),(5,4),(8,2),(9,4)}. Then prove f is a onto function.

Solution: From the question itself we get,



& f={(1,2),(5,4),(8,2),(9,4)}

Here, the entire element on B has a domain element on A or we can say element 1 and 8 & 5 and 9 has same range 2 & 4 respectively.

Therefore, f: A → B is ansurjective function.

Hence, the onto function proof is explained.


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