# Fermat's Little Theorem

Delving into the world of advanced mathematics, you may come across Fermat's Little Theorem, a remarkable mathematical statement that has wide-ranging applications in number theory and cryptography. This theorem, proposed by Pierre de Fermat in the 17th century, is a stepping stone to understanding more complex concepts within mathematics. In this article, you will explore the fundamentals of Fermat's Little Theorem, its underlying principles, and how it can be demonstrated through proofs and detailed examples. Furthermore, you will learn about the practical applications of this theorem and how its formula can be utilised effectively in various mathematical scenarios. Prepare to enhance your knowledge of further mathematics and discover the fascinating intricacies of Fermat's Little Theorem.

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## Fermat's Little Theorem Explanation

Fermat's Little Theorem is an essential concept in number theory, particularly in the field of modular arithmetic. It was first formulated by the French mathematician Pierre de Fermat in 1640. This theorem establishes a crucial relationship between prime numbers and modular arithmetic. It is a powerful tool for simplifying certain mathematical calculations and it is widely used in cryptography and computer science.

Fermat's Little Theorem states that if $$p$$ is a prime number and $$a$$ is an integer not divisible by $$p$$, then $$a^{p-1} \equiv 1 \pmod{p}$$.

This theorem can be better understood by taking a closer look at its proof and applications. To prove Fermat's Little Theorem, mathematicians rely on the concept of modular congruence and the properties of prime numbers. The theorem has numerous proofs, but one of the most popular approaches is Euler's proof using the Euler function.

For example, let's consider the prime number $$p = 5$$ and the integer $$a = 2$$. According to Fermat's Little Theorem, $$a^{p-1} \equiv 1 \pmod{p}$$, which translates to $$2^{5-1} \equiv 1 \pmod{5}$$. Upon calculating, we get $$2^4 \equiv 1 \pmod{5}$$, which is indeed true because $$2^4 = 16$$ and $$16 \equiv 1 \pmod{5}$$.

## The Principle Behind Fermat's Little Theorem

The underlying principle of Fermat's Little Theorem revolves around the properties of prime numbers and modular arithmetic. Prime numbers have unique properties that set them apart from composite numbers. Modular arithmetic allows us to study the remainders of integer division without focusing on the actual quotients, which is particularly helpful in dealing with large numbers.

Deep dive: Modular arithmetic is based on the congruence relation, which is denoted by the symbol $$\equiv$$. Two integers, $$a$$ and $$b$$, are said to be congruent modulo $$m$$ if their difference, $$a - b$$, is divisible by $$m$$. In other words, $$a \equiv b \pmod m$$ if and only if $$m$$ divides $$a - b$$. This allows us to greatly simplify arithmetic operations when working with large or cumbersome numbers.

In order to better comprehend the principle behind Fermat's Little Theorem, it is essential to understand the following aspects:

• Modular arithmetic and congruence
• Properties of prime numbers
• Proofs of Fermat's Little Theorem
• Applications of Fermat's Little Theorem

A deeper understanding of these aspects will provide the necessary foundation to appreciate the theorem and its uses.

It should be noted that Fermat's Little Theorem doesn't apply to composite numbers (non-prime numbers). In certain cases, an extension of Fermat's Little Theorem, known as Euler's Totient Theorem, is used for composite numbers. Euler's theorem can be stated as follows: If $$a$$ and $$m$$ are coprime integers, then $$a^{\phi(m)} \equiv 1 \pmod m$$, where $$\phi(m)$$ is the Euler totient function. This function counts the number of positive integers less than $$m$$ that are coprime to $$m$$.

## Demonstrating Fermat's Little Theorem

To fully appreciate the power and versatility of Fermat's Little Theorem, it is necessary to delve into a proof of the theorem and also explore an in-depth example that demonstrates its application to problem-solving.

### Fermat's Little Theorem Proof

There are various proofs for Fermat's Little Theorem, but one of the most common and accessible methods is Euler's proof using the Euler's Totient Function ($$\phi$$). This proof relies on properties of prime numbers, modular arithmetic, and Euler's Totient Function.

We'll present Euler's proof of Fermat's Little Theorem step by step:

1. Let $$p$$ be a prime number and $$a$$ be any integer not divisible by $$p$$.
2. Consider the set $$A$$ of integers $$1$$ to $$p-1$$, which are all the numbers less than $$p$$. Since $$p$$ is prime, these integers are all relatively prime to $$p$$.
3. Multiply each element of $$A$$ by $$a$$ and reduce modulo $$p$$. This results in a new set $$B$$.
4. Set $$B$$ contains the same elements as $$A$$ and has the same size. Therefore, the product of the elements of $$A$$ and $$B$$ are congruent modulo $$p$$. This is because each element in $$B$$ is a unique multiple of $$a$$ modulo $$p$$, and no two elements of $$A$$ produce the same result.
5. So, the product of the elements of $$A$$ is given by $$(1)(2)(3)\cdots(p-1)$$. The product of the elements of $$B$$ is given by $$(a)(2a)(3a)\cdots((p-1)a)$$.
6. Using the fact that the product of the elements of $$A$$ is congruent to the product of the elements of $$B$$ modulo $$p$$, we can write:
7. $$(1)(2)(3)\cdots(p-1) \equiv (a)(2a)(3a)\cdots((p-1)a) \pmod p$$.
8. Now, divide both sides of the congruence by $$(1)(2)(3)\cdots(p-1)$$ modulo $$p$$. As $$p$$ is prime, the integers from $$1$$ to $$p-1$$ have multiplicative inverses modulo $$p$$. Therefore, we get:
9. $$1 \equiv a^{p-1} \pmod p$$.

The proof is now complete, and this confirms Fermat's Little Theorem: $$a^{p-1} \equiv 1 \pmod{p}$$ for any prime number $$p$$ and an integer $$a$$ not divisible by $$p$$.

### Detailed Fermat's Little Theorem Example

With a better understanding of Fermat's Little Theorem and its proof, let us now delve into a detailed example that demonstrates its application in solving a problem.

Problem: Compute the remainder when dividing $$3^{100}$$ by $$11$$.

Using Fermat's Little Theorem, we know that $$a^{p-1} \equiv 1 \pmod{p}$$ for $$p = 11$$ and $$a = 3$$. Thus, we can rewrite the problem as follows:

Find $$3^{100} \pmod{11}$$.

Applying Fermat's Little Theorem, we can write:

$3^{(11-1)} \equiv 1 \pmod{11}$ $3^{10} \equiv 1 \pmod{11}$

Now divide the exponent of $$3$$ in the problem by $$10$$ (the $$p-1$$ term):

$$100 = (10)(10)$$.

We can exploit Fermat's Little Theorem to simplify the problem:

Since $$3^{10} \equiv 1 \pmod{11}$$, we can write:

$$3^{100} \equiv (3^{10})^{10} \equiv 1^{10} \equiv 1 \pmod{11}$$.

Therefore, the remainder when dividing $$3^{100}$$ by $$11$$ is $$1$$.

This detailed example illustrates the power of Fermat's Little Theorem in simplifying and solving problems, particularly in modular arithmetic and number theory. Understanding the proof and applying the theorem to various problems is key to unlocking its potential and appreciating its significance.

## Applying Fermat's Little Theorem

Fermat's Little Theorem is an essential mathematical tool that has numerous applications in various fields. Its simplicity and power enable problem-solving and provide insights into the properties of prime numbers, as well as modular arithmetic. In this section, we will delve into practical applications of Fermat's Little Theorem and explore how to utilise its formula effectively.

### Practical Fermat's Little Theorem Applications

While Fermat's Little Theorem is rooted in number theory, its applications extend beyond mathematics and into the realms of computer science, cryptography, and engineering. Some practical applications include:

• Primality testing: Fermat's Little Theorem can be employed to test whether a number is prime or composite. Using this theorem, one can quickly identify if a number is possibly prime by performing a modular arithmetic calculation. However, it should be noted that the test is not foolproof, as there exist certain composite numbers called Carmichael numbers that also satisfy the theorem's condition.
• Cryptography: One of the most prominent uses of Fermat's Little Theorem is in the field of cryptography, specifically within the context of the RSA cryptosystem. By exploiting the properties of prime numbers and modular arithmetic, the RSA algorithm relies on this theorem to create secure communication channels that protect sensitive information from unauthorized access.
• Random number generation: Fermat's Little Theorem can be used to generate random numbers in a specific range, especially for applications that require large prime numbers. These random numbers can then be utilised in cryptographic algorithms or simulations that require random input data.
• Computer science algorithms:Algorithms involving modular arithmetic, such as the Fast Exponentiation Algorithm, can benefit from Fermat's Little Theorem to decrease the complexity and runtime of the computation by simplifying the input parameters. This is particularly useful when dealing with large numbers or computationally intensive tasks.

These applications demonstrate the versatility of Fermat's Little Theorem and its usefulness in various domains.

### Utilising the Fermat's Little Theorem Formula

By effectively utilising the Fermat's Little Theorem formula ($$a^{p-1} \equiv 1 \pmod p$$), one can solve problems and gain insights into relationships between numbers and prime factors. Here are some guidelines to help you exploit the theorem:

1. Identify the prime number: Look for a prime number ($$p$$) in the given problem, as Fermat's Little Theorem holds only for prime numbers.
2. Find the modular base: Determine the integer ($$a$$) that is not divisible by the prime number, which will serve as the base of the exponentiation operation in the theorem.
3. Apply the theorem: Use the theorem to calculate modular congruence. Often, the application of the theorem will lead to a simplified expression that makes it easier to solve the problem at hand.
4. Consider related problems: Evaluate whether the problem at hand can be reduced or transformed into a format or domain where the Fermat's Little Theorem formula can be directly applied.
5. Combine with other techniques: Fermat's Little Theorem is not a standalone tool. In many cases, integrating it with other mathematical techniques, such as Euler's Totient Theorem or the Chinese Remainder Theorem, can lead to a more comprehensive and efficient solution.

By understanding the underlying principles of Fermat's Little Theorem and implementing these guidelines, one can effectively apply the theorem to solve problems and derive unique insights into mathematical relationships. Always remember that, like any mathematical tool, the value of Fermat's Little Theorem is proportional to its proper understanding, effective utilisation, and integration with other related theories and techniques.

## Fermat's Little Theorem - Key takeaways

• Fermat's Little Theorem: If p is a prime number and a is an integer not divisible by p, then $$a^{p-1} \equiv 1 \pmod{p}$$.

• Fermat's Little Theorem proof: Euler's proof uses the Euler function and properties of prime numbers.

• Fermat's Little Theorem example: For p=5 and a=2, $$2^{5-1} \equiv 1 \pmod{5}$$, since $$2^4 = 16$$ and $$16 \equiv 1 \pmod{5}$$.

• Fermat's Little Theorem application: Widely used in primality testing, cryptography, random number generation, and computer science algorithms.

• Fermat's Little Theorem formula: Utilize the theorem effectively by identifying prime numbers, finding the modular base, applying the theorem, considering related problems, and combining with other techniques.

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Why is Fermat's Little Theorem important?
Fermat's Little Theorem is important because it is a fundamental theorem in number theory, providing a quick and effective way to check for prime numbers. Additionally, it forms the basis for various algorithms in number theory, cryptography, and computer science, making it a crucial concept in these fields.
How does Fermat's theorem work?
Fermat's Little Theorem states that if p is a prime number and a is an integer not divisible by p, then a^(p-1) is congruent to 1 modulo p (written a^(p-1) ≡ 1 mod p). The theorem works by establishing that the remainders of the powers of a when divided by p, cycle in a regular pattern that repeats after every p-1 steps. Hence, it helps identify prime numbers and is useful for modular arithmetic-related computations.
Who first proved Fermat's Little Theorem?
Fermat's Little Theorem was first proved by Swiss mathematician Leonhard Euler in 1736, even though it was initially proposed by French mathematician Pierre de Fermat in 1640.
What are some examples of Fermat's Little Theorem in use?
Some examples of Fermat's Little Theorem in use include primality testing algorithms, such as the Miller-Rabin test, calculating modular inverses, and cryptography applications, particularly in the RSA encryption. Additionally, it plays a key role in understanding the properties of cyclic groups in number theory.
How does one utilise Fermat's Little Theorem?
Fermat's Little Theorem states that if p is a prime number, then for any integer a, a^p is congruent to a (mod p). To use this theorem, first ensure that p is prime, then raise the chosen integer a to the power of p, and finally, calculate the remainder when dividing the result by p. This remainder should equal the initial integer a.

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