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Real Functions

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Introduction to Real Numbers

From the cartesian point of view, here, X is a function of Y because the elements of X are directly related to the elements of Y. Here, 1 directly maps with D; 2 and 3 are directly related to C. As a result of this, we can understand that a function is a process that connects each element of set X to a single element of a set Y. The process for reading this is Y= f(x). These are the simplest operations of function. Next, we will look into what is a real function?

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What is a Real Function?

A function whose range lies within the real numbers i.e., non-root numbers and non-complex numbers, is said to be a real function, also called a real-valued function.

A real function is an entity that assigns values to arguments. The notation P = f (x) means that to the value x of the argument, the function f assigns the value P. Sometimes, we also use the notation f: x ↦ P, in words, the function f sends x to P. The most usual way of specifying this assignment is by some formula, that is, the function value P can be obtained by substituting x to a specific formula that identifies the given function.


Any function in the form of F(x) is called a positive real function, if it falls under these four critical categories:

  1. F(v) should have real values for all real values of x.

  2. F(v) must be a Hurwitz polynomial.

  3. If we substitute v = j*ω then on splitting up the real and imaginary parts, the real part of the function must be more than or equivalent to zero, which means it should not be negative. This is the most critical condition, and we frequently use this theory to clear doubts regarding the fact that the function is a positive real function or not.

  4. On substituting v = go, F(x) should own simple poles, and the residues must be real and positive.

Properties of Positive Real Function

There are some important properties of a positive real function, which are listed below:

  1. The numerator and denominator of F(v) must be Hurwitz polynomials.

  2. The degree of the numerator of F(v) must not be more than the degree of the denominator by more than 1. In other words, (N-n) must be lesser than or equal to one.

  3. If F(v) is a positive real function, then the reciprocal of F(v) must also be a positive real function.

  4. Do not forget that the addition of two or more positive real functions is also a positive real function, but in the case of the subtraction, either it will be a positive real function or a negative real function.

Operations on Real Functions

Now, we have to pay attention to the following procedures in order to understand the basic problems of real functions.

Adding Two Real Functions: The process of summation of two real functions can be done after defining the functions j and k as j: Y ⟶R and k: Y ⟶R is two real functions, such that Y is a subset of R. Then (j + k): Y ⟶R can be defined as (j + k)(y) = j(y) + k(y), for all y ϵ Y.

Subtracting Two Real Functions: The process of finding out the difference of two real functions can be done after defining the functions j and k as j: Y ⟶R and k: Y ⟶R are two real functions, such that Y is a subset of R. Then (j - k): Y ⟶R can be defined as (j – k)(y) = j(y) – k(y), for all y ϵ Y.

Multiplication of Real Function: The process of finding out the product of two real-life examples of rational functions can be done after defining the functions j and k as j: Y ⟶R and k: Y ⟶R are two real functions, such that Y is a subset of R. Then jk: Y ⟶R can be defined as (jk)(y) = j(y)k(y), for all y ϵ Y.

The quotient of Two Real Functions: The process of finding out the quotient or division of two real functions can be done after defining the functions j and k as j: Y ⟶R and k: Y ⟶R are two real functions, such that Y is a subset of R. Then (j/k): Y ⟶R can be defined as (j/k)(y)=j(y) / k(y), for all y ϵ Y.

Solved Example

If function ‘h’ is defined by

l(x) = 3x2 - 7x - 5,

find l(x - 2).


By the theory and concept of function,

Substitute x by x -2 in the formula of function written below,

l(x - 2) = 3 (x - 2)2 - 7 (x - 2) - 5

Expand and group the like terms for your convenience. For expansions, use the basic algebraic theorems on polynomial multiplications and additions. Do not forget to look upon the degree of the polynomials for the accuracy of results. 

l (x - 2) = 3 ( x² - 4 x + 4 ) - 7 x + 14 - 5

After the expansion and grouping of like terms, our job is to simplify the terms and make a compact polynomial after making the required summations and subtractions.

= 3 x² - 19 x + 7.

Last updated date: 05th Jun 2023
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FAQs on Real Functions

1. What are the Functions of a Quadratic Equation?

The quadratic equations are the equations that have the degree 2. The algebraic degree of the polynomial should be 2. It is any equation having the form where y represents an unknown, and a, b, and c represent known numbers, which is not equal to 0. If a = 0, then the equation is linear, not quadratic, as there is no term. Functions are frequently categorized by the characteristics of formulas that can define them: A quadratic function is a function that may be written f ( x ) = r x2+ k x + c  where r, k, c are constants, and we are supposed to find the value of x i.e. the roots of the equation.

2. What is Real Function or Real - Valued Mathematical Functions?

As in mathematics, a real-valued function is a function whose values of outcome are real numbers. It is a function that relates a real number to each member of its domain. We can say that a real-valued function is a function whose outputs, which are calculated by generating certain formulas, are real numbers.


For example, we can assume a case where Riya, a working woman whose monthly income is Rs. 35,000, spends Rs. 20,000. So, what will be the savings of her per month? It would be Rs. 15,000. 

So this case generates the formula, Saving = Income-expenses. We can see here that both the result and the data used for calculation are real numbers. The above scenario is a representation of a real function definition and a case of subtraction in the algebraic operation of real functions.

3. Can you give a real-life example where discontinuous functions are used?

When you have checked out, you can spot as many discontinuous functions from real life. A few of them are pointed below:

  • When it comes to labour cost, it is said to be discontinuous to be with the total hours calculated. Because when someone took over time and you are charging based on the hours they worked it is difficult.

  • The vendors charging for the product also come in discontinuous functions. 

  • When you are using a container-load one, Shipping costs compared with the volume of the loads come under discontinuous functions.

4. What is the meaning of mode in Mathematics when related to Real Functions?

If you are asking what mode is, it is best to define accordingly with the statistics subject. Since if you are given a series and you identify a number that is repeated a lot of times. So that number will be regarded as the mode of that number series. And for the best way to express it, we can start with an example.

Eg: 1, 11, 2, 3, 1 , 5, 6, 1, 4, 8, 1

Here in this number series, you can see that the number 1 is repeated often. And hence, the mode of the series is 1. 

5. Give some Examples of the Step function in Mathematics.

Students can be given an activity to find the number of step functions that are happening in real life. That is what Vedantu websites and apps are trying to do. By giving students a visual exploration along with learning. So the examples you can identify in real life with step functions are pointed below:

  • Closing and opening of the door

  • When a switch is flipped

  • Walking up or walking down a stair