# Bezout's Lemma

**Bezout's Lemma** states that if and are nonzero integers and , then there exist integers and such that . In other words, there exists a linear combination of and equal to .

Furthermore, is the smallest positive integer that can be expressed in this form, i.e. .

In particular, if and are relatively prime then there are integers and for which .

## Proof

Let , , and notice that .

Since , . So is smallest positive for which . Now if for all integers , we have that , then one of those integers must be 1 from the Pigeonhole Principle. Assume for contradiction that , and WLOG let . Then, , and so as we saw above this means but this is impossible since . Thus there exists an such that .

Therefore , and so there exists an integer such that , and so . Now multiplying through by gives, , or .

Thus there does exist integers and such that .

Now to prove is minimum, consider any positive integer . As we get , and as and are both positive integers this gives . So is indeed the minimum.

## Generalization to Principal Ideal Domains

Bezout's Lemma can be generalized to principal ideal domains.

Let be a principal ideal domain, and consider any . Let . Then there exist elements for which . Furthermore, is the minimal such element (under divisibility), i.e. if then .

Note that this statement is indeed a generalization of the previous statement, as the ring of integers, is a principal ideal domain.

## Proof

Consider the ideal . As is a principal ideal domain, must be principle, that is it must be generated by a single element, say . Now from the definition of , there must exist such that . We now claim that .

First we prove the following simple fact: if , then . To see this, note that if , then there must be some such that . But now by definition we have .

Now from this, as , we get that . Furthermore, consider any with . We clearly have that . So indeed .

Now we shall prove minimality. Let . Then as , we have , as desired.

## See also

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