Uniformly Cauchy sequence

From Wikipedia, the free encyclopedia
Jump to: navigation, search

In mathematics, a sequence of functions from a set S to a metric space M is said to be uniformly Cauchy if:

  • For all , there exists such that for all : whenever .

Another way of saying this is that as , where the uniform distance between two functions is defined by

Convergence criteria[edit]

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[edit]

A sequence of functions from a set S to a metric space U is said to be uniformly Cauchy if:

  • For all and for any entourage , there exists such that whenever .

See also[edit]