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| In [[topology]] and related branches of [[mathematics]], a [[topological space]] is called '''locally compact''' if, roughly speaking, each small portion of the space looks like a small portion of a [[compact space]].
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| ==Formal definition==
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| Let ''X'' be a [[topological space]]. Most commonly ''X'' is called ''locally compact'', if every point of ''X'' has a compact [[neighbourhood (topology)|neighbourhood]].
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| There are other common definitions: They are all '''equivalent if ''X'' is a [[Hausdorff space]]''' (or preregular). But they are '''not equivalent''' in general:
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| :1. every point of ''X'' has a compact [[neighbourhood (topology)|neighbourhood]].
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| :2. every point of ''X'' has a [[closed set|closed]] compact neighbourhood.
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| :2′. every point of ''X'' has a [[relatively compact]] neighbourhood.
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| :2″. every point of ''X'' has a [[local base]] of [[relatively compact]] neighbourhoods.
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| :3. every point of ''X'' has a [[local base]] of compact neighbourhoods.
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| Logical relations among the conditions:
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| *Conditions (2), (2′), (2″) are equivalent.
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| *Neither of conditions (2), (3) implies the other.
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| *Each condition implies (1).
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| *Compactness implies conditions (1) and (2), but not (3).
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| Condition (1) is probably the most commonly used definition, since it is the least restrictive and the others are equivalent to it when ''X'' is a [[Hausdorff space|Hausdorff]]. This equivalence is a consequence of the facts that compact subsets of Hausdorff spaces are closed, and closed subsets of compact spaces are compact.
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| Authors such as Munkres and Kelley use the first definition. Willard uses the third. In Steen and Seebach, a space which satisfies (1) is said to be ''locally compact'', while a space satisfying (2) is said to be ''strongly locally compact''.
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| In almost all applications, locally compact spaces are also Hausdorff, and this article is thus primarily concerned with locally compact Hausdorff (LCH) spaces.
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| '''Warning: Allen Hatcher uses another definition!''' On page 62 of his ''Algebraic Topology'', he explains that if he says locally [something] then he means that every open neighbourhood of any point contains a [something] type of neighbourhood for the point. Thus, for Hatcher, locally compact has a stronger meaning. Interestingly, this stronger definition allows for us to omit the Hausdorff condition in many situations. For example, all of Hatcher's locally compact spaces have the property that the functor: <math>(-)\times X</math> has the right adjoint :<math>(-)^X</math>.
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| By definition of a local base, this property is identical to
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| :3'. for every point ''x'' of ''X'' the compact neighbourhoods of ''x'' form a [[local base]].
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| and therefore equivalent to (3).
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| == Examples and counterexamples ==
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| === Compact Hausdorff spaces ===
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| Every compact Hausdorff space is also locally compact, and many examples of compact spaces may be found in the article [[compact space]].
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| Here we mention only:
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| * the [[unit interval]] [0,1];
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| * any closed [[topological manifold]];
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| * the [[Cantor set]];
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| * the [[Hilbert cube]].
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| === Locally compact Hausdorff spaces that are not compact ===
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| *The [[Euclidean space]]s '''R'''<sup><var>n</var></sup> (and in particular the [[real line]] '''R''') are locally compact as a consequence of the [[Heine–Borel theorem]].
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| *[[Topological manifold]]s share the local properties of Euclidean spaces and are therefore also all locally compact. This even includes [[paracompact|nonparacompact]] manifolds such as the [[long line (topology)|long line]].
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| *All [[discrete space]]s are locally compact and Hausdorff (they are just the [[0 (number)|zero]]-dimensional manifolds). These are compact only if they are finite.
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| *All [[open subset|open]] or [[closed subset]]s of a locally compact Hausdorff space are locally compact in the [[subspace topology]]. This provides several examples of locally compact subsets of Euclidean spaces, such as the [[unit disc]] (either the open or closed version).
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| *The space '''Q'''<sub>''p''</sub> of [[p-adic number|''p''-adic numbers]] is locally compact, because it is [[homeomorphic]] to the [[Cantor set]] minus one point. Thus locally compact spaces are as useful in [[p-adic analysis|''p''-adic analysis]] as in classical [[mathematical analysis|analysis]].
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| === Hausdorff spaces that are not locally compact ===
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| As mentioned in the following section, no Hausdorff space can possibly be locally compact if it is not also a [[Tychonoff space]]; there are some examples of Hausdorff spaces that are not Tychonoff spaces in that article.
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| But there are also examples of Tychonoff spaces that fail to be locally compact, such as:
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| * the space '''Q''' of [[rational number]]s (endowed with the topology from '''R'''), since its compact [[subset]]s all have empty [[interior (topology)|interior]] and therefore are not neighborhoods;
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| * the subspace {(0,0)} [[union (set theory)|union]] {(''x'',''y'') : ''x'' > 0} of '''R'''<sup>2</sup>, since the origin does not have a compact neighborhood;
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| * the [[lower limit topology]] or [[upper limit topology]] on the set '''R''' of real numbers (useful in the study of [[one-sided limit]]s);
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| * any [[T0 space|T<sub>0</sub>]], hence Hausdorff, [[topological vector space]] that is [[Infinity|infinite]]-[[dimension]]al, such as an infinite-dimensional [[Hilbert space]].
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| The first two examples show that a subset of a locally compact space need not be locally compact, which contrasts with the open and closed subsets in the previous section.
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| The last example contrasts with the Euclidean spaces in the previous section; to be more specific, a Hausdorff topological vector space is locally compact if and only if it is finite-dimensional (in which case it is a Euclidean space).
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| This example also contrasts with the [[Hilbert cube]] as an example of a compact space; there is no contradiction because the cube cannot be a neighbourhood of any point in Hilbert space.
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| ===Non-Hausdorff examples=== | |
| * The [[one-point compactification]] of the [[rational number]]s '''Q''' is compact and therefore locally compact in senses (1) and (2) but it is not locally compact in sense (3).
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| * The [[particular point topology]] on any infinite set is locally compact in sense (3) but not in sense (2), because it has no nonempty closed compact subspaces containing the particular point. The same holds for the real line with the upper topology.
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| == Properties ==
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| Every locally compact [[preregular space]] is, in fact, [[completely regular space|completely regular]]. It follows that every locally compact Hausdorff space is a [[Tychonoff space]]. Since straight regularity is a more familiar condition than either preregularity (which is usually weaker) or complete regularity (which is usually stronger), locally compact preregular spaces are normally referred to in the mathematical literature as ''locally compact regular spaces''. Similarly locally compact Tychonoff spaces are usually just referred to as ''locally compact Hausdorff spaces''.
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| Every locally compact Hausdorff space is a [[Baire space]].
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| That is, the conclusion of the [[Baire category theorem]] holds: the [[interior (topology)|interior]] of every [[union (set theory)|union]] of [[countable|countably many]] [[nowhere dense]] [[subset]]s is [[empty set|empty]].
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| A [[topological subspace|subspace]] ''X'' of a locally compact Hausdorff space ''Y'' is locally compact [[if and only if]] ''X'' can be written as the [[complement (set theory)|set-theoretic difference]] of two [[closed set|closed]] [[subset]]s of ''Y''.
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| As a corollary, a [[dense (topology)|dense]] subspace ''X'' of a locally compact Hausdorff space ''Y'' is locally compact if and only if ''X'' is an [[open subset]] of ''Y''.
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| Furthermore, if a subspace ''X'' of ''any'' Hausdorff space ''Y'' is locally compact, then ''X'' still must be the difference of two closed subsets of ''Y'', although the [[converse (logic)|converse]] needn't hold in this case.
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| [[Quotient space]]s of locally compact Hausdorff spaces are [[compactly generated space|compactly generated]]. | |
| Conversely, every compactly generated Hausdorff space is a quotient of some locally compact Hausdorff space.
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| For locally compact spaces [[local uniform convergence]] is the same as [[compact convergence]].
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| === The point at infinity === | |
| Since every locally compact Hausdorff space ''X'' is Tychonoff, it can be [[embedding (topology)|embedded]] in a compact Hausdorff space b(''X'') using the [[Stone–Čech compactification]].
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| But in fact, there is a simpler method available in the locally compact case; the [[one-point compactification]] will embed ''X'' in a compact Hausdorff space a(''X'') with just one extra point.
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| (The one-point compactification can be applied to other spaces, but a(''X'') will be Hausdorff [[if and only if]] ''X'' is locally compact and Hausdorff.)
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| The locally compact Hausdorff spaces can thus be characterised as the [[open subset]]s of compact Hausdorff spaces.
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| Intuitively, the extra point in a(''X'') can be thought of as a '''point at infinity'''.
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| The point at infinity should be thought of as lying outside every compact subset of ''X''.
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| Many intuitive notions about tendency towards infinity can be formulated in locally compact Hausdorff spaces using this idea.
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| For example, a [[continuous function (topology)|continuous]] [[real number|real]] or [[complex number|complex]] valued [[function (mathematics)|function]] ''f'' with [[domain (function)|domain]] ''X'' is said to ''[[vanish at infinity]]'' if, given any [[positive number]] ''e'', there is a compact subset ''K'' of ''X'' such that |''f''(''x'')| < ''e'' whenever the [[Point (geometry)|point]] ''x'' lies outside of ''K''. This definition makes sense for any topological space ''X''. If ''X'' is locally compact and Hausdorff, such functions are precisely those extendable to a continuous function ''g'' on its one-point compactification a(''X'') = ''X'' ∪ {∞} where ''g''(∞) = 0.
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| The set C<sub>0</sub>(''X'') of all continuous complex-valued functions that vanish at infinity is a [[C-star algebra|C* algebra]]. In fact, every [[commutative]] C* algebra is [[isomorphic]] to C<sub>0</sub>(''X'') for some [[unique]] ([[up to]] [[homeomorphism]]) locally compact Hausdorff space ''X''. More precisely, the [[category theory|categories]] of locally compact Hausdorff spaces and of commutative C* algebras are [[duality (category theory)|dual]]; this is shown using the [[Gelfand representation]]. Forming the one-point compactification a(''X'') of ''X'' corresponds under this duality to adjoining an [[identity element]] to C<sub>0</sub>(''X'').
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| === Locally compact groups === | |
| The notion of local compactness is important in the study of [[topological group]]s mainly because every Hausdorff [[locally compact group]] ''G'' carries natural [[measure theory|measures]] called the [[Haar measure]]s which allow one to [[integral|integrate]] [[measurable function]]s defined on ''G''.
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| The [[Lebesgue measure]] on the [[real line]] '''R''' is a special case of this.
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| The [[Pontryagin dual]] of a [[topological abelian group]] ''A'' is locally compact [[if and only if]] ''A'' is locally compact.
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| More precisely, Pontryagin duality defines a self-[[duality (category theory)|duality]] of the [[category theory|category]] of locally compact abelian groups.
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| The study of locally compact abelian groups is the foundation of [[harmonic analysis]], a field that has since spread to non-abelian locally compact groups.
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| ==References==
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| *{{cite book |last = Kelley |first = John | title = General Topology |year= 1975 | publisher = Springer | isbn = 0-387-90125-6}}
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| *{{cite book | last = Munkres | first = James | year = 1999 | title = Topology | edition = 2nd | publisher = Prentice Hall | isbn = 0-13-181629-2}}
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| *{{Cite book | last1=Steen | first1=Lynn Arthur | author1-link=Lynn Arthur Steen | last2=Seebach | first2=J. Arthur Jr. | author2-link=J. Arthur Seebach, Jr. | title=[[Counterexamples in Topology]] | origyear=1978 | publisher=[[Springer-Verlag]] | location=Berlin, New York | edition=[[Dover Publications|Dover]] reprint of 1978 | isbn=978-0-486-68735-3 | mr=507446 | year=1995 | postscript=<!--None-->}}
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| *{{cite book | last = Willard | first = Stephen | title = General Topology | publisher = Addison-Wesley | year = 1970 | isbn = 0-486-43479-6 (Dover edition)}}
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| [[Category:Compactness (mathematics)]]
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| [[Category:General topology]]
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| [[Category:Properties of topological spaces]]
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