## Functional Analysis Definition

**Functional analysis** is a branch of mathematics that studies functions by investigating the behavior of a given function and identifying relationships and hypotheses that may arise.

Throughout this topic, we shall only deal with real functions in one variable. By the end of this article, you should be familiar with the following concepts:

Identifying the domain and range of a function

Recognizing odd and even functions

Finding the x and y-intercepts of a function

### What is a Function?

Before we dive into this topic, let us first recall the definition of a function.

A **function **is an expression, also called a rule, that defines the relationship between one variable (independent input) and another variable (dependent output). This is commonly denoted by y = f(x) where x and y are related such that for every value of x, there is a unique value of y that obeys the rule f.

Below is an example of a function.

Say we have the function $f:\mathrm{\mathbb{R}}\mapsto \mathrm{\mathbb{R}}$ defined by

$f\left(x\right)=x+2$.This describes a function that takes a number, an input, and adds 2. For example, for an input x = 1, we have an output f(1) = 3. Similarly, for an input x = 4, we have an output f(4) = 6.

## The Domain and Range

When plotting a function, it is essential to know the 'size' of the variables. This is known as the domain and range of a function. These two terms are explained below.

The **domain **of a function, f, is the set of all values for which the function is defined. The elements of the domain are represented as an independent variable, or input value, that is not dependent on any other quantity but rather, varies freely. It is often denoted by x.

The **range **of a function is the set of all resulting values that f takes, corresponding to the input values of a function. The elements of the range are dependent on the values within the domain and are sometimes referred to as the output value.

The general notation for the domain and range of a function is

$Domain\left(f\right)=\left\{x:x\in A,A\in \mathrm{\mathbb{R}}\right\}$

and

$Range\left(f\right)=\left\{f\right(x):x\in Domain(f\left)\right\}$

Note that $\mathrm{\mathbb{R}}$ above represents the set of all real values that represents the interval $(-\infty ,\infty )$. Here is a visual representation of a domain and range with regards to a function. Recall the example of the function f we had introduced earlier.

Graphical representation of a domain, range and function, StudySmarter Originals

This representation suggests that a function works like a machine that transforms elements of the domain, the inputs, into elements of the codomain. The actual outputs of this "transformation machine" will be the elements of the range, the outputs.

There are many types of functions to consider in the realm of mathematics. We have polynomial functions, exponential functions, trigonometric functions, etc. In the following sections, we shall summarize the general formulas used to find the associated domain and range of each type of function (usually seen in this syllabus).

### Domain and Range of Polynomial Functions

We shall first note the three types of polynomials we shall often use throughout this topic.

linear functions, $f\left(x\right)=ax+b$

quadratic functions, $f\left(x\right)=a{x}^{2}+bx+c$

cubic functions, $f\left(x\right)=a{x}^{3}+b{x}^{2}+cx+d$

The domain of any polynomial function is the set of all real numbers, IR.

The range of a linear and cubic function is also the set of all real numbers, IR.

The range of a quadratic function of the form$f\left(x\right)=a{(x-h)}^{2}+k$is

$Range\left(f\right)=\left\{f\right(x)\ge k,ifa>0\}$ or $Range\left(f\right)=\left\{f\right(x)\le k,ifa<0\}$.

Find the domain of the function $f\left(x\right)=3x-1$.

**Solution**

This is a linear function. Thus, the domain and range of this function is the set of all real numbers, IR. The graph is shown below.

Example 1, StudySmarter Originals

### Domain and Range of Square Root Functions

For the standard square root function, $f\left(x\right)=\sqrt{x}$, the domain is the set of all real numbers, IR and the range is f(x) ≥ 0.

For a general square root function of the form $f\left(x\right)=\sqrt{g\left(x\right)}$, where g(x) is a function of x, the domain is the set of functions where g(x) ≥ 0 and the range is f(x) ≥ 0.

Determine the domain and range of the function $f\left(x\right)=\sqrt{x-1}$.

**Solution**

The domain is the set of values where the component inside the square root is more than or equal to zero, or in other words,

$x-1\ge 0\Rightarrow x\ge 1$Thus, the domain is the set of values where x is more than or equal to 1. The range is f(x) ≥ 0, for x ≥ 1. The graph is shown below.Example 2, StudySmarter Originals

### Domain and Range of Cube Root Functions

For any function containing a cube root, may it be the standard form $f\left(x\right)=\sqrt[3]{x}$ or the general form $f\left(x\right)=\sqrt[3]{g\left(x\right)}$, the domain and range are both the set of all real numbers, IR.

What is the domain and range of the function $f\left(x\right)=\sqrt[3]{2-x}$.

**Solution**

The domain and range of any cube root function is the set of all real numbers, IR. Graphing this function, we find that the domain and range indeed satisfy the set of all real numbers, IR.

Example 3, StudySmarter Originals

### Domain and Range of Exponential Functions

For an exponential function of the form $f\left(x\right)={a}^{x}$, where a is any real number, the domain is the set of all real numbers, IR.

The range will always yield positive real values, that is, f(x) > 0.

Given the graph of the function $f\left(x\right)={e}^{x}$ below, determine its domain and range.

Example 4, StudySmarter Originals

**Solution**

Observing the graph above, we find that the domain satisfies the set of all real numbers. The range is f(x) > 0.

### Domain and Range of Logarithmic Functions

For a logarithmic function of the form $f\left(x\right)={\mathrm{log}}_{a}x$, where a is any real number, the domain is x > 0 while the range is the set of all real numbers.

The function $f\left(x\right)={\mathrm{log}}_{e}x$ can also be written as $f\left(x\right)=\mathrm{ln}\left(x\right)$. This is also known as the natural logarithm function. What is the domain and range of this function?

**Solution**

The domain here is x > 0. The range on the other hand is the set of all real numbers, IR. The graph is shown below.

Example 5, StudySmarter Originals

### Domain and Range of Rational Functions

**Rational functions** are functions that can be represented by a rational fraction. This is generally denoted by $f\left(x\right)=\frac{p\left(x\right)}{q\left(x\right)}$, where p and q are both polynomial functions of x and q(x) **≠** 0.

The domain is the set of all real numbers except for which the denominator is equal to zero, that is $\{x\in \mathrm{\mathbb{R}}|q\left(x\right)\ne 0\}$.

The range here is the same as the domain of the inverse of this rational function f or in other words, $Range\left(f\right)=Domain\left({f}^{-1}\right)$.

Given the function $f\left(x\right)=\frac{3-x}{x+5}$, find the domain and range.

**Solution**

We shall first attempt to find the domain of this function. To find the excluded value in the domain of the function, equate the denominator to zero and solve for x.

$x+5=0\Rightarrow x=-5$

Thus, the domain is the set of all real numbers except x = –5, $\{x\in \mathrm{\mathbb{R}}|x\ne -5\}$. In other words, the graph is not defined at x = –5. Next, let us find the range by evaluating the inverse of this function. Let y = f(x). Now, interchanging the x and y from our given function, we obtain

$x=\frac{3-y}{y+5}$

Solving for y yields,

$x(y+5)=3-y\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow xy+5x=3-y\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow xy+y=3-5x\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow y(x+1)=3-5x\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow y=\frac{3-5x}{x+1}$

Thus, the inverse of f is

${f}^{-1}\left(x\right)=\frac{3-5x}{x+1}$

The excluded value in the domain of this inverse function can be found by equating the denominator to zero and solving for x. Thus,

$x+1=0\Rightarrow x=-1$

The domain of this inverse function is the set of real numbers except x = –1. However, in this case, this domain is the range of our function, f(x). Thus, the range of the given function is $\{y\in \mathrm{\mathbb{R}}|y\ne -1\}$. The graph is not define at y = –1. The graph is plotted below.

Example 6, StudySmarter Originals

In the graph above, the lines (in red) x = –5 and y = –1 represent the region for which the function is not defined.

### Domain and Range of Trigonometric Functions

Observe the graph of the sine (green line) and cosine (blue line) functions, f(x) = sin(x) and f(x) = cos(x), below.

Sine and cosine graph, StudySmarter Originals

Notice that the value of the functions oscillates between –1 and 1 and it is defined for all real numbers. Thus, for each sine and cosine function, the domain is the set of all real numbers, R and the range is –1 ≤ f(x) ≤ 1. The range here can also be denoted by [–1, 1].

## Even and Odd Functions

Even and odd functions are functions that satisfy a particular rule of symmetry. To check whether a function is even or odd, all we need to do is substitute x into the given function and observe whether it satisfies the condition of an even or odd function, which we shall establish below. We shall look at both of these types of functions and identify their respective properties.

### Even Function

A function, f is **even **when

$f(-x)=f\left(x\right)$,

for all x in the domain of the function.

Geometrically speaking, the graph of an even function is symmetric with respect to the y-axis, in other words, the function remains unchanged when reflected about the y-axis. The properties of even functions include:

The sum of two even functions is even;

The difference between two even functions is even;

The product of two even functions is even;

The quotient of two even functions is even;

The derivative of an even function is odd;

The composition of two even functions is even;

The composition of an even function and odd function is even.

Let us look at an example.

Determine whether the following function is even.

$f\left(x\right)={x}^{2}+2$

**Solution **

Let us substitute –x into our function as below.

$f(-x)={(-x)}^{2}+2\Rightarrow f(-x)={x}^{2}+2\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow f(-x)=f\left(x\right)$

Since $f(-x)=f\left(x\right)$, we conclude that this function is indeed an even function. The graph is shown below.

Example 7, StudySmarter Originals

Notice how the curve is reflected about the y-axis.

### Odd Function

A function, f is **odd **when

$f(-x)=-f\left(x\right)$,

for all x in the domain of the function.

To look at this geometrically, the graph of an odd function has rotational symmetry with respect to the origin. Essentially, the function remains unchanged when rotated 180^{o} about the origin. Below are the properties of odd functions:

The sum of two odd functions is odd;

The difference between two odd functions is odd;

The product of two odd functions is even;

The product of an even function and odd function is odd;

The quotient of two odd functions is even;

The quotient of an even function and odd function is odd;

The derivative of an odd function is even;

The composition of two odd functions is odd.

Below is an example.

Verify if the following function is odd.

$f\left(x\right)=\frac{{x}^{3}-2x}{4}$

**Solution**

Plugging –x into our given function, we have

$f(-x)=\frac{{(-x)}^{3}-2(-x)}{4}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow f(-x)=\frac{{-x}^{3}+2x}{4}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow f(-x)=-\left(\frac{{x}^{3}-2x}{4}\right)\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow f(-x)=-f\left(x\right)$

Since $f(-x)=-f\left(x\right)$, we deduce that this function is odd. Below is a sketch of the graph.

Example 8, StudySmarter Originals

Notice how the curve is reflected about the origin.

### Can a Function be Both Even and Odd?

There is only one function that fulfills this criterion, which is the constant function that is identically zero, f(x) = 0. The domain and range are the set of all real numbers, IR.

Note that the sum of an even function and odd function is neither even nor odd unless one of the functions is equal to zero over a given domain.

It is also possible that we have functions that are neither even nor odd. Here is an example that shows this.

Observe the following function.

$f\left(x\right)=2{x}^{2}+3x$

If we substitute –x into this function we find that we obtain a completely different function since

$f(-x)=2{(-x)}^{2}+3(-x)\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow f(-x)=2{x}^{2}-3x$

Plotting the graph of f(x), observe that the graph is not reflected about the y-axis nor does it have rotational symmetry about the origin. This means that the graph is neither even nor odd.

Example 9, StudySmarter Originals

## Periodic Functions

Periodic functions are used to describe trigonometric functions in particular due to the presence of oscillations and waves in their graphs.

A **periodic function** is a function that repeats itself over regular intervals (or periods). A function, f is periodic, P if

$f(x+P)=f\left(x\right)$,

for all values of x in the domain of the function. Here, $P\ne 0$ is a constant.

A function that is not periodic is termed **aperiodic**. Here is an example of that type of function.

Let us return to our sine function from the previous section.

Example 10, StudySmarter Originals

Now observe the graph above. The function repeats itself over intervals of length the sine function is periodic with period, $P=2\pi $ since

$\mathrm{sin}(x+2\pi )=\mathrm{sin}\left(x\right)$

for all values of x.

## Intercept Points

The points of interception of a function are the points at which the function crosses the axes of the graph. Below is an explicit example of the two points of interception to consider when graphing functions in two dimensions.

The **x-intercept **is a point at which the function, f crosses the x-axis. To find the x-intercept, simply solve for f(x) = 0.

The **y-intercept** is a point at which the function, f crosses the y-axis. To find the y-intercepts, substitute x = 0 into f(x).

Intercept points are important in deducing the change of sign of the curve for a given function. Let us look at an example.

Given the function below, find their x and y-intercepts.

$f\left(x\right)={x}^{2}-7x-8$

**Solution **

We begin by finding the x-intercepts. To do this, we shall equate the function to zero, f(x) = 0.

${x}^{2}-7x-8=0$

Factoring this expression, we obtain

$(x-8)(x+1)=0$

Now using the Zero Product Property and solving for x, we obtain

$x-8=0\Rightarrow x=8\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}x+1=0\Rightarrow x=-1$

Thus, we the x-intercepts are x = –1 and x = 8. Let us now look for the y-intercept. Replacing x = 0 into our function yields

$f\left(0\right)={\left(0\right)}^{2}-7\left(0\right)-8=-8$

Thus, the y-intercept is y = -8. The graph is displayed below.

Example 11, StudySmarter Originals

Notice that between the x-intercepts, x = –1 and x = 8, the function falls below the x-axis meaning that the range in this domain is negative. However, the range before x = –1 and after x = 8 are positive.

## Intersecting Points

Say we are given a pair of functions, f and g. We are told to find the point(s) at which the two functions meet. This is called the intersecting point. This is defined below.

Suppose we have two functions defined by f and g. The **intersection point**(s) of these two graphs is the value(s) of x for which

$f\left(x\right)=g\left(x\right)$.

The exact value(s) of the intersection points can be found by solving the expression above algebraically. Below is an example that shows this.

Given the functions, f (in blue) and g (in red) below.

$f\left(x\right)={x}^{2}-x\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}g\left(x\right)=2x+1$

Deduce their points of intersection. Both functions are plotted in the same graph below.

Example 12, StudySmarter Originals

**Solution**

Looking at the graph above, we see that there are two points of intersection for this pair of functions. We need to equate f(x) = g(x) and solve for x to find the x-coordinates for these points of intersection.

$f\left(x\right)=g\left(x\right)\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow {x}^{2}-x=2x+1\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow {x}^{2}-x-2-1=0\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow {x}^{2}-3x-1=0$

Observe that we cannot factorize the equation in the final line above. In order to solve for x, we need to use the Quadratic Formula.

$x=\frac{-b\pm \sqrt{{b}^{2}-4ac}}{2a}\phantom{\rule{0ex}{0ex}}\Rightarrow x=\frac{-(-3)\pm \sqrt{{(-3)}^{2}-4\left(1\right)(-1)}}{2\left(1\right)}\phantom{\rule{0ex}{0ex}}\Rightarrow x=\frac{3\pm \sqrt{9+4}}{2}\phantom{\rule{0ex}{0ex}}\Rightarrow x=\frac{3\pm \sqrt{13}}{2}$

Thus, we have two values of x, namely $x=\frac{3-\sqrt{13}}{2}andx=\frac{3+\sqrt{13}}{2}$. We shall leave our solution in this radical form.

To find the corresponding y-coordinates, we simply substitute these found x-values into either given function, f or g. For simplicity, we shall use the function g to find our y-values.

$g\left(\frac{3-\sqrt{13}}{2}\right)=\overline{)2}\left(\frac{3-\sqrt{13}}{\overline{)2}}\right)+1=3-\sqrt{13}+1=4-\sqrt{13}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow g\left(\frac{3-\sqrt{13}}{2}\right)=4-\sqrt{13}\phantom{\rule{0ex}{0ex}}$

and

$g\left(\frac{3+\sqrt{13}}{2}\right)=\overline{)2}\left(\frac{3+\sqrt{13}}{\overline{)2}}\right)+1=3+\sqrt{13}+1=4+\sqrt{13}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\Rightarrow g\left(\frac{3+\sqrt{13}}{2}\right)=4+\sqrt{13}$

Thus, the points of intersection are

$\left(\frac{3-\sqrt{13}}{2},4-\sqrt{13}\right)and\left(\frac{3+\sqrt{13}}{2},4+\sqrt{13}\right)$

## Other Components for Graph Sketching

So far, we have looked at the basic information needed for sketching the graph of a given function. In the following topics in this section, we shall be introduced to other fundamental elements that may be helpful when graphing functions. This includes:

Finding limits of a function

Identifying asymptotes. This is explained in the topic of Asymptotes

Locating the maximum and minimum points of a curve. This can be found here: Maxima and Minima

Using derivatives to find critical points and flex points. A detailed overview of this topic can be found here: Finding Maxima and Minima using Derivatives

By familiarizing ourselves with these methods, sketching graphs can be much more straightforward and accurate.

## Functional Analysis - Key takeaways

- The topic of functional analysis examines functions by investigating their behaviors and trends.
- The domain of a function is the set of all values for which the function is defined.
- The range of a function is the set of all resulting values that f takes, based on the domain.
- A function is even when $f(-x)=f\left(x\right)$for all x.
- A function is odd when $f(-x)=-f\left(x\right)$for all x.
- A function is periodic if$f(x+P)=f\left(x\right)$for all x.
- The x-intercept
- The y-intercept
The intersection point of two graphs is the value of x where$f\left(x\right)=g\left(x\right)$.

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##### Frequently Asked Questions about Functional Analysis

What is an example of functional analysis?

Plotting a function on a graph to identify its patterns and behaviours.

What is functional analysis?

Functional analysis studies a function by investigating its behaviour and identifying relationships and hypotheses that may arise.

What are the steps in functional analysis?

Identifying the domain and range of a function

Recognizing odd and even functions

Finding the x and y-intercepts of a function

What is the difference between real analysis and functional analysis?

Real analysis studies the properties of numbers while functional analysis analyses the properties of functions?

Why functional analysis is important?

Functional analysis is important so that we can make predictions and estimations for our given function

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