uniform space
A uniform structure (or uniformity) on a set is a non empty set of subsets of which satisfies the following axioms:
- 1.
Every subset of which contains a set of belongs to .
- 2.
Every finite intersection of sets of belongs to .
- 3.
Every set of is a reflexive relation on (i.e. contains the diagonal).
- 4.
If belongs to , then belongs to .
- 5.
If belongs to , then exists in such that, whenever , then (i.e. ).
The sets of are called entourages or vicinities. The set together with the uniform structure is called a uniform space.
If is an entourage, then for any we say that and are -close.
Every uniform space can be considered a topological space with a natural topology induced by uniform structure. The uniformity, however, provides in general a richer structure
, which formalize the concept of relative closeness: in a uniform space we can say that is close to as is to , which makes no sense in a topological space. It follows that uniform spaces are the most natural environment for uniformly continuous functions and Cauchy sequences
, in which these concepts are naturally involved.
Examples of uniform spaces are metric spaces, topological groups, and topological vector spaces
.