Recalling Modular Arithmetic

Oct 13, 2025 by pinkandtrash.com 127 views

Introduction

In number theory, the greatest common divisor (GCD) of two numbers is a fundamental concept that plays a crucial role in various mathematical operations and theorems. The GCD of two numbers is the largest positive integer that divides both numbers without leaving a remainder. In this article, we will explore a specific property related to the GCD of two numbers and another number, and its implications on modular arithmetic.

Understanding the Problem

The problem statement asks us to prove that if the GCD of two numbers $a$ and $n$ is equal to the GCD of another number $b$ and $n$ , then there exist integers $x$ and $y$ that are coprime to $n$ such that $ax=b$ and $by=a$ modulo $n$ . In other words, we need to show that there exist integers $x$ and $y$ that are relatively prime to $n$ and satisfy the given modular equations.

Recalling Modular Arithmetic

Before we dive into the proof, let's recall some basic concepts related to modular arithmetic. The modulo operation is denoted by the symbol $\mod$ . For example, $a \equiv b \mod n$ means that $a$ and $b$ leave the same remainder when divided by $n$ . In other words, $a$ and $b$ are congruent modulo $n$ . We can also write $a \equiv b \mod n$ as $a-b$ is divisible by $n$ .

Recalling Coprime Numbers

Two numbers are said to be coprime if their GCD is 1. In other words, two numbers are coprime if they have no common factors other than 1. For example, 5 and 7 are coprime because their GCD is 1.

Proof

To prove the statement, we will use the following steps:

Step 1: Expressing a and b in Terms of d

Since $d=\gcd(a,n)=\gcd(b,n)$ , we can express $a$ and $b$ in terms of $d$ as follows:

$a = da_1$ and $b = db_1$ , where $a_1$ and $b_1$ are integers.

Step 2: Expressing x and y in Terms of a_1 and b_1

We need to find integers $x$ and $y$ that are coprime to $n$ such that $ax=b$ and $by=a$ modulo $n$ . Let's express $x$ and $y$ in terms of $a_1$ and $b_1$ as follows:

$x = a_1 + kn$ and $y = b_1 + ln$ , where $k$ and $l$ are integers.

Step 3: Substituting x and y into the Modular Equations

Substituting $x$ and $y$ into the modular equations, we get:

$a(a_1 + kn) \equiv b \mod n$ and $b(b_1 + ln) \equiv a \mod n$ .

Step 4: Simplifying the Modular Equations

Simplifying the modular equations, we get:

$da_1 + dkn \equiv db_1 \mod n$ and $db_1 + dln \equiv da_1 \mod n$ .

Step 5: Canceling Out d

Since $d$ is a common factor of both sides of the modular equations, we can cancel it out:

$a_1 + kn \equiv b_1 \mod n$ and $b_1 + ln \equiv a_1 \mod n$ .

Step 6: Showing that x and y are Coprime to n

Since $a_1$ and $b_1$ are integers, we can see that $x = a_1 + kn$ and $y = b_1 + ln$ are also integers. Moreover, since $a_1$ and $b_1$ are relatively prime to $n$ , we can conclude that $x$ and $y$ are also relatively prime to $n$ .

Step 7: Conclusion

Therefore, we have shown that there exist integers $x$ and $y$ that are coprime to $n$ such that $ax=b$ and $by=a$ modulo $n$ .

Conclusion

In this article, we have proved that if the GCD of two numbers $a$ and $n$ is equal to the GCD of another number $b$ and $n$ , then there exist integers $x$ and $y$ that are coprime to $n$ such that $ax=b$ and $by=a$ modulo $n$ . This result has important implications in number theory and modular arithmetic.

References

[1] "Elementary Number Theory" by David Burton
[2] "Modular Arithmetic" by Keith Conrad
[3] "Coprime Numbers" by MathWorld

Introduction

In our previous article, we proved that if the GCD of two numbers $a$ and $n$ is equal to the GCD of another number $b$ and $n$ , then there exist integers $x$ and $y$ that are coprime to $n$ such that $ax=b$ and $by=a$ modulo $n$ . In this article, we will answer some frequently asked questions related to this result.

Q: What is the significance of the GCD of two numbers being equal to the GCD of one number and another number?

A: The significance of the GCD of two numbers being equal to the GCD of one number and another number lies in the fact that it implies a certain relationship between the two numbers. Specifically, it implies that the two numbers have a common factor that is also a factor of the third number.

Q: What is the relationship between the GCD of two numbers and the GCD of one number and another number?

A: The GCD of two numbers $a$ and $n$ is equal to the GCD of another number $b$ and $n$ if and only if there exist integers $x$ and $y$ that are coprime to $n$ such that $ax=b$ and $by=a$ modulo $n$ .

Q: What is the significance of the integers $x$ and $y$ being coprime to $n$ ?

A: The significance of the integers $x$ and $y$ being coprime to $n$ lies in the fact that it implies that $x$ and $y$ have no common factors with $n$ other than 1. This is important because it means that $x$ and $y$ can be used to construct a solution to the modular equations $ax=b$ and $by=a$ modulo $n$ .

Q: How can we use the result to solve modular equations?

A: We can use the result to solve modular equations by finding integers $x$ and $y$ that are coprime to $n$ such that $ax=b$ and $by=a$ modulo $n$ . This can be done by using the Euclidean algorithm to find the GCD of $a$ and $n$ , and then using the result to construct a solution to the modular equations.

Q: What are some applications of the result?

A: The result has many applications in number theory and modular arithmetic. Some examples include:

Cryptography: The result can be used to construct secure cryptographic protocols that rely on the difficulty of factoring large numbers.
Coding Theory: The result can be used to construct error-correcting codes that rely on the properties of modular arithmetic.
Number Theory: The result can be used to study the properties of modular forms and elliptic curves.

Q: What are some limitations of the result?

A: The result has some limitations. For example:

Assumes coprime integers: The result assumes that the integers $x$ and $y$ are coprime to $n$ . This may not always be the case.
Requires GCD to be equal: The result requires the GCD of $a$ and $n$ to be equal to the GCD of $b$ and $n$ . This may not always be the case.

Conclusion

In this article, we have answered some frequently asked questions related to the result that if the GCD of two numbers is equal to the GCD of one number and another number, then there exist coprime numbers satisfying certain modular equations. We hope that this article has been helpful in clarifying the significance and applications of the result.

References

[1] "Elementary Number Theory" by David Burton
[2] "Modular Arithmetic" by Keith Conrad
[3] "Coprime Numbers" by MathWorld