# Cauchy filter

A filter $\mathfrak{F}$ on a uniform space $X$ such that for any entourage $V$ of the uniform structure of $X$ there exists a set which is $V$-small and belongs to $\mathfrak{F}$. In other words, a Cauchy filter is a filter which contains arbitrarily small sets in a uniform space $X$. The concept is a generalization of the concept of a Cauchy sequence in metric spaces.

Every convergent filter (cf. Limit) is a Cauchy filter. Every filter which is finer than a Cauchy filter is also a Cauchy filter. The image of a Cauchy filterbase under a uniformly-continuous mapping is again a Cauchy filterbase. A uniform space in which every Cauchy filter is convergent is a complete space.

#### References

[1] | N. Bourbaki, "Elements of mathematics. General topology" , Addison-Wesley (1966) pp. Chapt. II: Uniform structures (Translated from French) |

#### Comments

A Cauchy filterbase (or Cauchy $d$-filterbase) is a filterbase $\mathfrak{A} = \{ A_\alpha : \alpha \in \mathcal{A} \}$ in a metric space $(X,d)$ such that for every $\epsilon > 0$ there is some $\alpha \in \mathcal{A}$ for which $\text{diam}\,A_\alpha < \epsilon$ (cf. [a1]).

A filterbase in a space $X$ is a family $\{ A_\alpha : \alpha \in \mathcal{A} \}$ of subsets of $X$ with the properties: 1) $A_\alpha \neq \emptyset$ for all $\alpha \in \mathcal{A}$; and 2) for all $\alpha,\beta \in \mathcal{A}$ there is a $\gamma \in \mathcal{A}$ such that $A_\gamma \subseteq A_\alpha \cap A_\beta$ (see also Directed set).

#### References

[a1] | J. Dugundji, "Topology" , Allyn & Bacon (1978) |

**How to Cite This Entry:**

Cauchy filter.

*Encyclopedia of Mathematics.*URL: http://www.encyclopediaofmath.org/index.php?title=Cauchy_filter&oldid=38771