# Cauchy's Convergence Criterion/Real Numbers

## Theorem

Let $\sequence {x_n}$ be a sequence in $\R$.

Then $\sequence {x_n}$ is a Cauchy sequence if and only if $\sequence {x_n}$ is convergent.

## Proof

### Necessary Condition

Let $\sequence {x_n}$ be a sequence in $\R$.

Let $\sequence {x_n}$ be convergent.

Then $\sequence {x_n}$ is a Cauchy sequence.

$\Box$

### Sufficient Condition

Let $\sequence {x_n}$ be a sequence in $\R$.

Let $\sequence {x_n}$ be a Cauchy sequence.

Then $\sequence {x_n}$ is convergent.

$\Box$

The conditions are shown to be equivalent.

Hence the result.

$\blacksquare$

## Also known as

Cauchy's Convergence Criterion is also known as the Cauchy convergence condition.

## Source of Name

This entry was named for Augustin Louis Cauchy.