# Definition:Cauchy Sequence/Complex Numbers

## Definition

Let $\sequence {z_n}$ be a sequence in $\C$.

Then $\sequence {z_n}$ is a Cauchy sequence if and only if:

$\forall \epsilon \in \R_{>0}: \exists N \in \N: \forall m, n \in \N: m, n \ge N: \size {z_n - z_m} < \epsilon$

where $\size {z_n - z_m}$ denotes the complex modulus of $z_n - z_m$.

Considering the complex plane as a metric space, it is clear that this is a special case of the definition for a metric space.

## Also see

Thus in $\C$ a Cauchy sequence and a convergent sequence are equivalent concepts.

## Source of Name

This entry was named for Augustin Louis Cauchy.