# Cauchy sequence

Revision as of 12:08, 3 November 2007 by imported>Aleksander Stos (completeness)

In mathematics, a **Cauchy sequence** is a sequence in a metric space with the property that elements in that sequence *cluster* together more and more as the sequence progresses. Another way of thinking of the clustering is that the distance between any two elements diminishes as their indexes grow larger and larger.

Depending on the underlying space, the Cauchy sequences may be convergent or not. This leads to the notion of completeness of the space.

## Formal definition

Let be a metric space. Then a sequence of elements in *X* is a Cauchy sequence if for any real number there exists a positive integer , dependent on , such that for all . In limit notation this is written as .