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# Discrete functions

Discrete functions are mathematical functions defined only for specific, separate values, often consisting of integers or whole numbers. These functions do not form a continuous line but are represented by distinct, individual points on a graph. They play a crucial role in computer science, digital signal processing, and various fields of discrete mathematics.

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## Definition of Discrete Functions

A discrete function is a function that is defined only for a set of discrete points. Unlike continuous functions that have outputs for every input within a certain range, discrete functions are defined at specific and separate values.

### Characteristics of Discrete Functions

Discrete functions have several key characteristics:

• Defined only at specific, isolated points.
• The graph consists of isolated points, not a continuous line.
• Examples include functions defined on integers or specific subsets of real numbers.

### Examples of Discrete Functions

Consider the function that maps each integer to its square:

$f(x) = x^2$

This function is defined only for integer values of x. Therefore, f is a discrete function.

Example:

If you take the input values x = -2, -1, 0, 1, 2, the corresponding output values are:

 x f(x) -2 4 -1 1 0 0 1 1 2 4

A function defined only for distinct and separate points is a discrete function.

Remember, not all functions defined on integers are discrete. The key is that the function is only defined at isolated points.

When dealing with discrete functions, you might encounter sequences. A sequence is a type of function whose domain is the set of natural numbers. Sequences can be finite or infinite. One of the most famous is the Fibonacci sequence defined as:

$F(n) = F(n-1) + F(n-2)$

with the initial conditions:

$F(0) = 0,$ $F(1) = 1$

This sequence is defined only for non-negative integers, making it a discrete function.

## Functions in Discrete Mathematics

In discrete mathematics, functions are a fundamental concept used to map elements from one set to another. Discrete functions are a special kind of function where the input values are distinct and separate points.

### Characteristics of Discrete Functions

There are some unique characteristics that differentiate discrete functions from continuous functions:

• Isolation: The function is defined only at distinct, separate points.
• Graph: The graph of a discrete function consists of isolated points rather than a continuous line.
• Examples: Functions defined on integers or specific subsets of real numbers.

### Examples of Discrete Functions

To better understand discrete functions, let's consider some examples:

Example:

Consider the function that maps each integer to its square:

$f(x) = x^2$

This function is defined only for integer values of x. For instance:

 x f(x) -2 4 -1 1 0 0 1 1 2 4

A deeper look into sequences is essential when discussing discrete functions. A sequence is a function where the domain is the set of natural numbers. This means that each natural number maps to a specific element in the sequence.

One of the most famous sequences is the Fibonacci sequence, defined as:

$F(n) = F(n-1) + F(n-2)$

with initial conditions:

$F(0) = 0$
$F(1) = 1$

The Fibonacci sequence is defined only for non-negative integers, making it a discrete function.

Remember, not all functions defined on integers are discrete. Isolated definition is the key.

A function defined only for distinct and separate points is a discrete function.

### Real-world Applications of Discrete Functions

Discrete functions have numerous applications in real-world scenarios:

• Computer Science: Algorithms often use discrete functions to process data structured in lists or arrays.
• Cryptography: Functions defined on discrete points are crucial for encryption and decryption processes.
• Inventory Management: Businesses use discrete functions to manage stock levels, tracking items in integer quantities.

Example:

In computer science, a hash function is a discrete function used for efficiently finding data. The function maps input data (keys) to specific locations in a hash table:

$h(k) = k \, \text{mod} \, N$

where k is the key, and N is the number of slots in the hash table.

Discrete functions are foundational in graph theory, where edges and vertices are analysed using such functions.

## Examples of Discrete Functions in Mathematics

In mathematics, discrete functions play a critical role, especially in areas like computer science, cryptography, and operations research. These functions are defined at distinct, isolated points.

### Understanding Sequences as Discrete Functions

A sequence is a type of discrete function where the domain is the set of natural numbers. For example, the arithmetic sequence can be defined by:

$a_n = a + (n-1)d$

where a is the first term, d is the common difference, and n is the position of the term in the sequence.

Example:

Consider the arithmetic sequence $$a_n = 2 + (n-1)3$$. Here, $$a = 2$$ and $$d = 3$$.

 n an 1 2 2 5 3 8 4 11 5 14

### The Use of Discrete Functions in Graph Theory

In graph theory, discrete functions are used to define the relationships between vertices and edges in a graph. Consider a graph representing a network:

• Vertices (Nodes): Represent the individual entities.
• Edges (Links): Represent the connections between entities.

Example:

A simple undirected graph can be defined by the adjacency function:

$f(u, v) = \begin{cases}1, & \text{if } u \text{ and } v \text{ are connected}\0, & \text{otherwise} \end{cases}$

Where u and v are vertices in the graph.

In computer science, the concept of a hash function is crucial. A hash function is a discrete function used to efficiently map data to specific locations in a hash table. An example of a simple hash function could be:

$h(k) = k \% N$

where k is the key and N is the number of slots in the hash table. This function ensures data is stored in an organised manner for quick access.

### Discrete Probability Distributions

Discrete functions also play a significant role in probability theory. A discrete probability distribution describes the probabilities of the outcomes of a discrete random variable. For example, the probability mass function (PMF) of a discrete random variable X is:

$P(X = x)$

where x is a possible value of X.

Example:

For a fair six-sided die, the PMF is:

$P(X = x) = \frac{1}{6}, \text{ for } x = 1, 2, 3, 4, 5, 6$

Remember, in a discrete probability distribution, all probabilities must sum to one.

### Real-world Applications of Discrete Functions

Discrete functions have numerous real-world applications:

• Computer Science: Algorithms often use discrete functions to process data structured in lists or arrays.
• Cryptography: Functions defined on discrete points are crucial for encryption and decryption processes.
• Economics: Discrete functions are used in modelling economic decisions based on discrete time intervals.

Example: In computer science, a hash function is used for efficiently finding data. The function maps input data (keys) to specific locations in a hash table:

$h(k) = k \% N$

where k is the key, and N is the number of slots in the hash table.

Discrete functions are foundational in graph theory, where edges and vertices are analysed using such functions.

## Discrete Functions Exercises

Practising with discrete functions helps you grasp essential mathematical concepts and improves problem-solving skills. Engaging with exercises can consolidate your understanding of how these functions work and their applications.

### Discrete Function Example

Consider an example where a discrete function maps each integer to its factorial value:

$f(x) = x!$

This function is only defined for non-negative integer values of x.

Example:

For input values $$x = 0, 1, 2, 3, 4$$, the corresponding output values are:

xf(x)
01
11
22
36
424

Factorial of a number $$n$$ is the product of all positive integers up to $$n$$: $$n! = n \cdot (n-1) \cdot (n-2) \cdot \, \ldots \cdot 1$$.

In some examples, you can encounter the recurrence relation. For instance, the factorial function can be recursively defined as:

$f(x) = x \cdot f(x-1)$

with the base case $$f(0) = 1$$.

### Discrete Function Graph

Graphs of discrete functions help visualise the relationship between the input and output values. Unlike continuous functions, the graph of a discrete function consists of isolated points.

Example:

Consider the discrete function $$f(x) = x^2$$, only defined for integer values of x. The graph will show a series of isolated points where each point corresponds to a square of an integer.

To plot a discrete function, use a scatter plot, where each point represents a value of the function at a specific input.

Graphs of discrete functions can also reveal properties such as periodicity and symmetry. For example, the function $$f(x) = (-1)^x$$ alternates between 1 and -1 for integer values of x, showing a clear periodic pattern with a period of 2.

## Discrete functions - Key takeaways

• Definition of Discrete Functions: A function defined only for distinct, separate points.
• Characteristics: Defined at isolated points; graph consists of isolated points, not a continuous line.
• Example: The function mapping each integer to its square (\f(x) = x^2\for integer values).
• Applications: Used in computer science algorithms, cryptography, inventory management, and graph theory.
• Discrete Function Graph: A graph consisting of isolated points rather than a continuous line, can use a scatter plot to visualise.

#### Flashcards in Discrete functions 12

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What is the difference between discrete and continuous functions?
Discrete functions are defined only at specific, distinct points, often resulting from countable sets, while continuous functions are defined over an entire interval, forming an unbroken curve without gaps. Discrete functions typically involve sequences or isolated points, whereas continuous functions are characterised by smooth, uninterrupted paths.
What are some common examples of discrete functions?
Common examples of discrete functions include the floor function, the ceiling function, the greatest common divisor function, and the factorial function. These functions operate over discrete sets, such as the integers.
How do you calculate the value of a discrete function?
To calculate the value of a discrete function, identify the domain (specific inputs) and the corresponding range (outputs). Substitute the input value into the function's rule or equation. Evaluate according to mathematical operations specified. For tabulated discrete functions, locate the input in the table and read the corresponding output.
What are the applications of discrete functions in real-world scenarios?
Discrete functions are used in computer science for algorithms, in economics for modelling different market scenarios, in operations research for optimising logistics and supply chains, and in digital signal processing for analysing and manipulating discrete data points. They are essential in areas involving finite processes and digital representations.
What are the key properties of discrete functions?
Key properties of discrete functions include: they map inputs from a discrete set to outputs, often integers or rational numbers; they can be represented by sequences or tables; they do not vary continuously and are defined at specific, separated points; and they can exhibit jumps or gaps in their output values.

## Test your knowledge with multiple choice flashcards

What is a sequence in the context of discrete functions?

What is a discrete function?

What is an example of a discrete function?

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