# Uniformly Cauchy sequence

In mathematics, a sequence of functions $\{f_{n}\}$ from a set S to a metric space M is said to be uniformly Cauchy if:

• For all $\varepsilon >0$ , there exists $N>0$ such that for all $x\in S$ : $d(f_{n}(x),f_{m}(x))<\varepsilon$ whenever $m,n>N$ .

Another way of saying this is that $d_{u}(f_{n},f_{m})\to 0$ as $m,n\to \infty$ , where the uniform distance $d_{u}$ between two functions is defined by

$d_{u}(f,g):=\sup _{x\in S}d(f(x),g(x)).$ ## Convergence criteria

A sequence of functions {fn} from S to M is pointwise Cauchy if, for each xS, the sequence {fn(x)} is a Cauchy sequence in M. This is a weaker condition than being uniformly Cauchy.

In general a sequence can be pointwise Cauchy and not pointwise convergent, or it can be uniformly Cauchy and not uniformly convergent. Nevertheless, if the metric space M is complete, then any pointwise Cauchy sequence converges pointwise to a function from S to M. Similarly, any uniformly Cauchy sequence will tend uniformly to such a function.

The uniform Cauchy property is frequently used when the S is not just a set, but a topological space, and M is a complete metric space. The following theorem holds:

• Let S be a topological space and M a complete metric space. Then any uniformly Cauchy sequence of continuous functions fn : SM tends uniformly to a unique continuous function f : SM.

## Generalization to uniform spaces

A sequence of functions $\{f_{n}\}$  from a set S to a metric space U is said to be uniformly Cauchy if:

• For all $x\in S$  and for any entourage $\varepsilon$ , there exists $N>0$  such that $d(f_{n}(x),f_{m}(x))<\varepsilon$  whenever $m,n>N$ .