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Difference Between Injective and Surjective Function

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Last updated date: 13th Jul 2024
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What are Injective Vs Surjective: An Introduction

To explain injective and surjective functions: functions are fundamental tools used to describe relationships between sets. Injective and surjective functions are two important properties that characterize the behaviour and mappings of these functions. .Understanding what is injective and surjective functions and the characteristics of injective and surjective functions is a big part of mathematics and especially important for students studying for tests like NEET and JEE. In this article, we'll look at some of the most important ways in which the characteristics of injective and surjective functions are the same and different.


Category:

JEE Main Difference Between

Content-Type:

Text, Images, Videos and PDF

Exam:

JEE Main

Topic Name:

Difference Between Injective and Surjective Function

Academic Session:

2025

Medium:

English Medium

Subject:

Mathematics

Available Material:

Chapter-wise Difference Between Topics


Defining Injective:

An injective function, also known as a one-to-one function, is a type of function that ensures each element in the domain maps to a unique element in the codomain. In other words, for every input value, there is a distinct output value. Formally, a function f: A → B is injective if for every pair of distinct elements a, a' ∈ A, f(a) ≠ f(a'). Alternatively, we can say that if f(a) = f(a'), then a = a'.


The key characteristic of an injective function lies in the absence of "collisions" or multiple inputs mapping to the same output. This property guarantees that each element in the codomain can be traced back to a unique element in the domain, making it a useful property in various mathematical and computational contexts.

Injective functions have several important properties:

  1. One-to-One Mapping: Each element in the domain maps to a unique element in the codomain.

  2. No Collisions: No two distinct elements in the domain map to the same element in the codomain.

  3. Inverse Function: An inverse function exists for every element in the codomain, allowing for the reversal of the mapping.

  4. Range and Codomain: The range of an injective function may or may not be equal to the codomain. In some cases, the range may be a proper subset of the codomain.


Defining Surjective:

A surjective function, also known as an onto function, is a type of function that ensures that every element in the codomain has at least one corresponding element in the domain. In simpler terms, the range of the function is equal to its codomain. Formally, a function f: A → B is surjective if for every element b ∈ B, there exists an element a ∈ A such that f(a) = b.


The main focus of a surjective function is to ensure that the function "covers" or spans the entire codomain, leaving no gaps or missing elements. Unlike injectivity, surjectivity does not require uniqueness in the mapping of inputs to outputs.


Surjective functions have the following important properties:

  1. One-to-Many Mapping: Multiple elements in the domain may map to the same element in the codomain.

  2. Full Coverage: Every element in the codomain has at least one corresponding element in the domain.

  3. Inverse Function: An inverse function may not exist due to potential "missing" elements in the domain.

  4. Range and Codomain: The range of a surjective function is equal to the entire codomain, ensuring full coverage.


Injective And Surjective Function Difference

S.No

Category

Injective

surjective

1

One-to-One Mapping

Each element in the domain maps to a unique element in the codomain.

Every element in the codomain has at least one corresponding element in the domain.

2

Collisions

No two distinct elements in the domain map to the same element in the codomain.

Multiple elements in the domain may map to the same element in the codomain.

3

Inverse Function

An inverse function exists for every element in the codomain

An inverse function may not exist due to potential "missing" elements in the domain.

4

Range and Codomain

The range may or may not be equal to the codomain.

The range is equal to the entire codomain.

5

Example

$f(x) = x^2$

$g(x) = e^x$

6

Symbolic Notation

$f: A \to B$

$g: A \to B$


So, based on the above description and table, we understand what is injective and surjective function, the difference between injective and surjective function, and their characteristics.


Summary

Injective and surjective functions are important properties used to describe the behaviour of mathematical functions. An injective function guarantees a one-to-one mapping between elements in the domain and the codomain without any collisions. It ensures the uniqueness of the mapping and allows for the existence of an inverse function. On the other hand, a surjective function ensures that every element in the codomain has at least one corresponding element in the domain, effectively covering the entire codomain. Surjectivity does not require uniqueness and may or may not have an inverse function. Understanding these distinctions between injectivity and surjectivity is crucial in various mathematical applications, where functions serve as essential tools for modelling and analyzing real-world phenomena.

FAQs on Difference Between Injective and Surjective Function

1. Are all one-to-one functions also onto (surjective)?

No, not all one-to-one functions are surjective. While injective functions guarantee uniqueness in mappings, surjective functions focus on covering the entire codomain. A function can be injective without being surjective if the range is not equal to the entire codomain.

2. Can a function be both injective and surjective?

Yes, a function can be both injective and surjective. When a function satisfies both properties, it is called a bijective function.


A bijective function is a special type of function where each element in the domain maps to a unique element in the codomain, and every element in the codomain has exactly one corresponding element in the domain. In other words, a bijective function establishes a one-to-one correspondence between the domain and the codomain.


The injective property ensures that no two distinct elements in the domain map to the same element in the codomain, guaranteeing uniqueness. The surjective property ensures that every element in the codomain is covered, meaning that there are no missing outputs.


The combination of injectivity and surjectivity in a bijective function allows for a complete and reversible mapping between the domain and the codomain. Consequently, a bijective function has a well-defined inverse function that can precisely reverse the mapping.


Examples of bijective functions include simple linear functions like f(x) = x and trigonometric functions like g(x) = sin(x) or h(x) = arctan(x).

3. How can I determine if a function is surjective?

To determine if a function is surjective, you can check if every element in the codomain has at least one pre-image in the domain. In other words, for every element b in the codomain, there exists an element an in the domain such that f(a) = b.

4. Explain injective and surjective functions in brief.

To explain injective and surjective function differences, in brief, First, we have to know what is injective and surjective functions.


An injective function is a type of function where each element in the domain maps to a unique element in the codomain. In other words, no two distinct inputs in the domain produce the same output. This property is often referred to as "one-to-one mapping." Injective functions ensure the uniqueness of the mapping, as each input has a unique output. A surjective function is a type of function where every element in the codomain has at least one corresponding element in the domain. In simpler terms, the function "covers" or spans the entire codomain. This property is often referred to as "onto mapping." Surjective functions ensure that there are no "missing" elements in the mapping, as every element in the codomain is reached by at least one input in the domain.

5. Concisely describe the injective and surjective function difference with help of an example.

An injective function, also known as a one-to-one function, ensures that each element in the domain maps to a unique element in the codomain. In other words, no two distinct inputs in the domain can produce the same output. For example, consider the function $f(x) = x^2$. In this case, every input value has a unique square value as the output. For instance, f(2) = 4 and f(−2) = 4, but since 2 and -2 are distinct inputs, we see that this function is not injective.


On the other hand, a surjective function, also known as an onto function, guarantees that every element in the codomain has at least one corresponding element in the domain. In simpler terms, the function "covers" or spans the entire codomain. For example, consider the function $g(x) = e^x$, where e is Euler's number. In this case, every positive real number has a corresponding input that produces that value. Thus, the function $g(x) = e^x$ is surjective.