# Bezout's Lemma

Given integers $x$ and $y$, a linear combination of $x$ and $y$ has the form $ax + by$. Bezout's Lemma relates the question of finding the greatest common divisor of $x$ and $y$ to the question of finding linear combinations of $x$ and $y$.

## Technique

Applying the Euclidean Algorithm backwards gives an algorithm to obtain the integer values of $a$ and $b$. We show how to obtain integers $a$ and $b$ such that $16457a + 1638b = 7$. These numbers are calculated in Euclidean Algorithm.

$\begin{array} {llllllllll} 7 & = & 21 &- & 14 \times 1 \\ & = & 21 \times 1 & - & 14 \times 1\\ & = & 21 &- & (77 - 21 \times 3) \times 1\\ & = & - 77 \times 1 &+& 21 \times 4\\ & = & - 77 \times 1 &+ & (1638 - 77 \times 21) \times 4 \\ &= &1638 \times 4 &-& 77 \times 85\\ & = & 1638 \times 4 &- & (16457 - 1638 \times 10) \times 85 \\ & = &16457 \times (-85) &+& 1638 \times 854\\ \end{array}$

## Worked Examples

### 1. Given integers $x, y$, describe the set of all integers $N$ that can be expressed in the form $N=ax+by$, where $a, b$ are integers.

Solution: Let $k = \gcd(x,y)$. Then any integer of the form $kn$, where $n$ is an integer, can be expressed as $ax+by$. We already know that this condition is a necessary condition, so to show that it is sufficient, Bezout's Lemma tells us that there exists integers $a', b'$ such that $k = a' x + b'y$. Therefore,

$kn = (a'n) x + (b'n) y.$

### 2. [Modulo Arithmetic Property I - Multiplicative inverses] Show that if $a, b$ are integers such that $\gcd(a,n)=1$, then there exists an integer $x$ such that $ax \equiv 1 \pmod{n}$.

Solution: Since $\gcd(a,n)=1$, Bezout's Lemma implies there exists integers $x, y$ such that $ax + n y = \gcd (a,n) = 1$. Then

$1 \equiv ax+ny \equiv ax \pmod{n} .$

Note by Calvin Lin
5 years, 7 months ago

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I learned a lot! Thanks :D

- 5 years, 4 months ago

I did not understand what the 1st worked example mean. Any number of form kn can be represented in the form ax+by but that does not mean that all nos of form ax+by will be of form kn. Actually just explain what the question demands?

- 5 years, 3 months ago

The question asks you to classify all numbers that can be written in the form $ax + by$ for given integers $x$ and $y$. The claim is that these numbers are exactly those of the form $k n$, where$k = \gcd (x,y)$ and $n$ is any integer.

As you realized, there are 2 parts to this.
First, we have to show that any number of the form $ax + by$ can be written as $kn$ for some $n$. This is obvious because we can let $x = k x^*, y = ky^*$ then $ax + by = a k x^* + bky^* = k ( ax^* + by^* )$.
Second, we have to show that any number of the form $kn$ can be written as $ax + by$ for some $a$ and $b$. This follows by applying Bezout's lemma, which tells us that there exists integers which satisfy $k = a' x + b' y$, and hence $kn = ( a'n) x + (b'n) y$.

Staff - 5 years, 3 months ago

thanks....

- 5 years, 3 months ago