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In
topology, one defines '''uniform spaces''' in order to study concepts such as
uniform continuity,
completeness and
uniform convergence. Uniform spaces generalize
metric spaces and
topological groups and therefore underlie most of
analysis. They were introduced by
Bourbaki.
If ''X'' is a
set, a nonempty system Φ of
subsets of the
Cartesian product ''X'' × ''X'' is called a '''uniform structure''' on ''X'' if the following axioms are satisfied:
# if ''U'' is in Φ, then ''U'' contains { (''x'', ''x'') : ''x'' in ''X'' }.
# if ''U'' is in Φ, then { (''y'', ''x'') : (''x'', ''y'') in ''U'' } is also in Φ
# if ''U'' is in Φ and ''V'' is a subset of ''X'' × ''X'' which contains ''U'', then ''V'' is in Φ
# if ''U'' and ''V'' are in Φ, then ''U'' ∩ ''V'' is in Φ
# if ''U'' is in Φ, then there exists ''V'' in Φ such that, whenever (''x'', ''y'') and (''y'', ''z'') are in ''V'', then (''x'', ''z'') is in ''U''.
The set ''X'' together with a uniform structure Φ is called a ''uniform space''. The elements of Φ are called ''entourages''.
Intuitively, two points ''x'' and ''y'' are "close together" if the pair (''x'', ''y'') is contained in many entourages. A single entourage captures a particular degree of "closeness". Interpreted as such, the axioms mean the following:
# every point is close to itself
# if ''x'' is close to ''y'', then ''y'' is close to ''x''
# relaxing a degree of closeness yields another degree of closeness
# by combining two degrees of closeness, you get another one
# to every degree of closeness, there exists another one that captures "twice as close".
The essential difference between a
topological space and a uniform space is that
in a uniform space, you can formalize the idea that "''x''
1 is about as far away from ''x''
2 as ''y''
1 is from ''y''
2" while in a topological space you can only formalize "''x''
1 is about as far away from ''x'' as ''x''
2 is from ''x''".
Uniform spaces may be defined alternatively and equivalently using systems of
pseudo-metrics, an approach which is often useful in
functional analysis.
==Uniform spaces as topological spaces==
Every uniform space ''X'' becomes a
topological space by defining a subset ''O'' of ''X'' to be open if and only if for every ''x'' in ''O'' there exists an entourage ''V'' such that { ''y'' in ''X'' : (''x'', ''y'') in ''V'' } is a subset of ''O''. It is possible that two different uniform structures generate the same topology on ''X''.
Every uniform space is a
completely regular topological space, and conversely, every completely regular space can be turned into a uniform space (often in many ways) so that the induced topology coincides with the given one.
A uniform space ''X'' is a
T0-space if and only if the intersection of all the elements of its uniform structure equals the diagonal {(''x'', ''x'') : ''x'' in ''X''}. If this is the case, ''X'' is in fact a
Tychonoff space and in particular
Hausdorff.
==Specific types of uniform spaces, and examples==
Every
metric space (''M'', ''d'') can be considered as a uniform space by defining a subset ''V'' of ''M'' × ''M'' to be an entourage if and only if there exists an ε > 0 such that for all ''x'', ''y'' in ''M'' with ''d''(''x'', ''y'') < ε we have (''x'', ''y'') in ''V''. This uniform structure on ''M'' generates the usual topology on ''M''.
Using metrics, a simple example of distinct uniform structures with coinciding topologies can be constructed. For instance, let ''d''
1(''x'',''y'') = | ''x - y'' | be the usual metric on '''R''' and let ''d''
2(''x'',''y'') = | ''e
x - e
y'' |. Then both metrics induce the usual topology on '''R''', yet the uniform structures are distinct, since { (x,y) : | x - y | < 1 } is an entourage in the uniform structure for ''d''
1 but not for ''d''
2. Informally, this example can be seen as taking the usual uniformity and distorting it through the action of a continuous yet non-uniformly continuous function.
Every
topological group (''G'',*) (in particular, every
topological vector space) becomes a uniform space if we define a subset ''V'' of ''G'' × ''G'' to be an entourage if and only if it contains the set { (''x'', ''y'') : ''x''*''y''
-1 in ''U'' } for some
neighborhood ''U'' of the
identity element of ''G''. This uniform structure on ''G'' is called the ''right uniformity'' on ''G'', because for every ''a'' in ''G'', the right multiplication ''x'' |-> ''x''*''a'' is
uniformly continuous with respect to this uniform structure. One may also define a left uniformity on ''G''; the two need not coincide, but they both generate the given topology on ''G''.
es:Espacio uniforme
