Difference between revisions of "Compactification"

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In [[general topology]], a '''compactification''' of a [[topological space]] is a [[compact space]] in which the original space can be embedded, allowing the space to be studied using the properties of compactness.
 
In [[general topology]], a '''compactification''' of a [[topological space]] is a [[compact space]] in which the original space can be embedded, allowing the space to be studied using the properties of compactness.
  
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:<math>e : x \mapsto (f \mapsto f(x)) . \,</math>
 
:<math>e : x \mapsto (f \mapsto f(x)) . \,</math>
  
The evaluation map ''e'' is a continuous map from ''X'' to the cube and we let β(''X'') denote the [[closure (mathematics)|closure]] of the image of ''e''.  The Stone-Čech compactification is then the pair (''e'',β(''X'')).
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The evaluation map ''e'' is a continuous map from ''X'' to the cube and we let β(''X'') denote the [[closure (topology)|closure]] of the image of ''e''.  The Stone-Čech compactification is then the pair (''e'',β(''X'')).
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If we restrict attention to the [[partial order]] of [[Hausdorff space|Hausdorff]] compactifications, then the one-point compactification is the minimum and the Stone-Čech compactification is the maximum element for this order.  The latter states that if ''X'' is a [[Tychonoff space]] then any continuous map from ''X'' to a compact space can be extended to a map from β(''X'') compatible with ''e''.  This extension property characterises the Stone-Čech compactification.
  
 
==References==
 
==References==
 
* {{cite book | author=J.L. Kelley | authorlink=John L. Kelley | title=General topology | publisher=van Nostrand | year= 1955 | pages=149-156 }}
 
* {{cite book | author=J.L. Kelley | authorlink=John L. Kelley | title=General topology | publisher=van Nostrand | year= 1955 | pages=149-156 }}

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In general topology, a compactification of a topological space is a compact space in which the original space can be embedded, allowing the space to be studied using the properties of compactness.

Formally, a compactification of a topological space X is a pair (f,Y) where Y is a compact topological space and f:XY is a homeomorphism from X to a dense subset of Y.

Compactifications of X may be ordered: we say that if there is a continuous map h of Y onto Z such that h.f = g.

The one-point compactification of X is the disjoint union where the neighbourhoods of ω are of the form for K a closed compact subset of X.

The Stone-Čech compactification of X is constructed from the unit interval I. Let F(X) be the family of continuous maps from X to I and let the "cube" IF(X) be the Cartesian power with the product topology. The evaluation map e maps X to IF(X),regarded as the set of functions from F(X) to I, by

The evaluation map e is a continuous map from X to the cube and we let β(X) denote the closure of the image of e. The Stone-Čech compactification is then the pair (e,β(X)).

If we restrict attention to the partial order of Hausdorff compactifications, then the one-point compactification is the minimum and the Stone-Čech compactification is the maximum element for this order. The latter states that if X is a Tychonoff space then any continuous map from X to a compact space can be extended to a map from β(X) compatible with e. This extension property characterises the Stone-Čech compactification.

References

  • J.L. Kelley (1955). General topology. van Nostrand, 149-156.