View source for Finite intersection property

In [[General topology|general topology]], a branch of [[Mathematics|mathematics]], a non-empty family ''A'' of [[Subset|subset]]s of a [[Set (mathematics)|set]] <math>X</math> is said to have the '''finite intersection property''' (FIP) if the [[Intersection (set theory)|intersection]] over any finite subcollection of <math>A</math> is [[Empty set|non-empty]]. It has the '''strong finite intersection property''' (SFIP) if the intersection over any finite subcollection of <math>A</math> is infinite.  Sets with the finite intersection property are also called '''centered systems''' and '''filter subbases'''.{{sfn|Joshi|1983|pp=242−248}}  
The finite intersection property can be used to reformulate topological compactness in terms of [[Closed set|closed sets]]; this is its most prominent application.  Other applications include proving that certain [[Perfect set|perfect sets]] are uncountable, and the construction of [[Ultrafilter (set theory)|ultrafilters]].  

==Definition==

Let <math display="inline">X</math> be a set and <math display="inline">\mathcal{A}</math> a nonempty [[Family of sets|family of subsets]] of {{Nowrap|<math display=inline>X</math>;}} that is, <math display="inline">\mathcal{A}</math> is a [[Subset|subset]] of the [[Power set|power set]] of {{Nowrap|<math display=inline>X</math>.}}  Then <math display="inline">\mathcal{A}</math> is said to have the finite intersection property if every nonempty finite subfamily has nonempty intersection; it is said to have the strong finite intersection property if that intersection is always infinite.{{sfn|Joshi|1983|pp=242−248}}  

In symbols, <math display="inline">\mathcal{A}</math> has the FIP if, for any choice of a finite nonempty subset <math display="inline">\mathcal{B}</math> of {{Nowrap|<math display=inline>\mathcal{A}</math>,}} there must exist a point <math display="block">x\in\bigcap_{B\in \mathcal{B}}{B}\text{.}</math>  Likewise, <math display="inline">\mathcal{A}</math> has the SFIP if, for every choice of such {{Nowrap|<math display=inline>\mathcal{B}</math>,}} there are infinitely many such {{Nowrap|<math display=inline>x</math>.}}{{sfn|Joshi|1983|pp=242−248}}  

In the study of [[Filter (set theory)|filters]], the common intersection of a family of sets is called a kernel, from much the same etymology as the [[Sunflower (mathematics)|sunflower]].  Families with empty kernel are called free; those with nonempty kernel, fixed.{{sfn|Dolecki|Mynard|2016|pp=27–29, 33–35}}

==Families of examples and non-examples==
The empty set cannot belong to any collection with the finite intersection property. 

A sufficient condition for the FIP intersection property is a nonempty kernel.  The converse is generally false, but holds for finite families; that is, if <math>\mathcal{A}</math> is finite, then <math>\mathcal{A}</math> has the finite intersection property if and only if it is fixed. 

=== Pairwise intersection ===
The finite intersection property is ''strictly stronger'' than pairwise intersection; the family <math>\{\{1,2\}, \{2,3\}, \{1,3\}\}</math> has pairwise intersections, but not the FIP.  

More generally, let <math display="inline">n \in \N\setminus\{1\}</math> be a positive integer greater than unity, {{Nowrap|<math display=inline>[n]=\{1,\dots,n\}</math>,}} and {{Nowrap|<math display=inline>\mathcal{A}=\{[n]\setminus\{j\}:j\in[n]\}</math>.}}  Then any subset of <math>\mathcal{A}</math> with fewer than <math display="inline">n</math> elements has nonempty intersection, but <math display="inline">\mathcal{A}</math> lacks the FIP.

=== End-type constructions ===
If <math>A_1 \supseteq A_2 \supseteq A_3 \cdots</math> is a decreasing sequence of non-empty sets, then the family <math display="inline">\mathcal{A} = \left\{A_1, A_2, A_3, \ldots\right\}</math> has the finite intersection property (and is even a [[Pi-system|{{pi}}–system]]).  If the inclusions <math>A_1 \supseteq A_2 \supseteq A_3 \cdots</math> are strict, then <math display="inline">\mathcal{A}</math> admits the strong finite intersection property as well.   

More generally, any <math display="inline">\mathcal{A}</math> that is [[Total order|totally ordered]] by inclusion has the FIP.   

At the same time, the kernel of <math display="inline">\mathcal{A}</math> may be empty: if {{Nowrap|<math display=inline>A_j=\{j,j+1,j+2,\dots\}</math>,}} then the kernel of <math>\mathcal{A}</math> is the [[Empty set|empty set]].  Similarly, the family of intervals <math>\left\{[r, \infty) : r \in \R\right\}</math> also has the (S)FIP, but empty kernel.  

=== "Generic" sets and properties ===
The family of all [[Borel set|Borel subsets]] of <math>[0, 1]</math> with [[Lebesgue measure]] <math display="inline">1</math> has the FIP, as does the family of comeagre sets.  If <math display="inline">X</math> is an infinite set, then the [[Fréchet filter]] (the family {{Nowrap|<math display=inline>\{X\setminus C:C\text{ finite}\}</math>)}} has the FIP.  All of these are free filters; they are upwards-closed and have empty infinitary intersection.{{sfn|Bourbaki|1987|pp=57–68}}{{sfn|Wilansky|2013|pp=44–46}} 

If <math>X = (0, 1)</math> and, for each positive integer <math>i,</math> the subset <math>X_i</math> is precisely all elements of <math>X</math> having digit <math>0</math> in the <math>i</math><sup>th</sup> decimal place, then any finite intersection of <math>X_i</math> is non-empty&nbsp;— just take <math>0</math> in those finitely many places and <math>1</math> in the rest.  But the intersection of <math>X_i</math> for all <math>i \geq 1</math> is empty, since no element of <math>(0, 1)</math> has all zero digits.

=== Extension of the ground set ===
The (strong) finite intersection property is a characteristic of the family {{Nowrap|<math display=inline>\mathcal{A}</math>,}} not the ground set {{Nowrap|<math display=inline>X</math>.}}  If a family <math display="inline">\mathcal{A}</math> on the set <math display="inline">X</math> admits the (S)FIP and {{Nowrap|<math display=inline>X\subseteq Y</math>,}} then <math display="inline">\mathcal{A}</math> is also a family on the set <math display="inline">Y</math> with the FIP (resp.&nbsp;SFIP).  

=== Generated filters and topologies ===
If <math>K \subseteq X</math> are sets with <math>K \neq \varnothing</math> then the family <math>\mathcal{A}=\{S \subseteq X : K \subseteq S\}</math> has the FIP; this family is called the principal filter on <math display="inline">X</math> generated by {{Nowrap|<math display=inline>K</math>.}}  The subset <math>\mathcal{B} = \{I \subseteq \R : K \subseteq I \text{ and } I \text{ an open interval}\}</math> has the FIP for much the same reason: the kernels contain the non-empty set {{Nowrap|<math display=inline>K</math>.}}  If <math display="inline">K</math> is an open interval, then the set <math display="inline">K</math> is in fact equal to the kernels of <math display="inline">\mathcal{A}</math> or {{Nowrap|<math display=inline>\mathcal{B}</math>,}} and so is an element of each filter.   But in general a filter's kernel need not be an element of the filter.   

A proper filter on a set has the finite intersection property. Every neighbourhood subbasis at a point in a [[Topological space|topological space]] has the FIP, and the same is true of every neighbourhood basis and every neighbourhood filter at a point (because each is, in particular, also a neighbourhood subbasis).

== Relationship to {{pi}}-systems and filters ==
A [[Pi-system|{{pi}}–system]] is a non-empty family of sets that is closed under finite intersections. The set <math display="block">\pi(\mathcal{A}) = \left\{A_1 \cap \cdots \cap A_n : 1 \leq n < \infty \text{ and } A_1, \ldots, A_n \in \mathcal{A}\right\}</math>of all finite intersections of one or more sets from <math>\mathcal{A}</math> is called the [[Pi-system|{{pi}}–system]] generated by {{Nowrap|<math display=inline>\mathcal{A}</math>,}} because it is the smallest {{pi}}–system having <math display="inline">\mathcal{A}</math> as a subset.  

The upward closure of <math>\pi(\mathcal{A})</math> in <math display="inline">X</math> is the set <math display="block">\pi(\mathcal{A})^{\uparrow X} = \left\{S \subseteq X : P \subseteq S \text{ for some } P \in \pi(\mathcal{A})\right\}\text{.}</math>

For any family {{Nowrap|<math display=inline>\mathcal{A}</math>,}} the finite intersection property is equivalent to any of the following:

<ul><li>The  [[Pi-system|{{pi}}–system]] generated by <math>\mathcal{A}</math> does not have the [[Empty set|empty set]] as an element; that is, <math>\varnothing \notin \pi(\mathcal{A}).</math>
</li>
<li>The set <math>\pi(\mathcal{A})</math> has the finite intersection property.</li>
<li>The set <math>\pi(\mathcal{A})</math> is a (proper)<ref name="ProperDef" group="note">A filter or prefilter on a set is {{em|{{visible anchor|proper}}}} or {{em|{{visible anchor|non-degenerate}}}} if it does not contain the empty set as an element. Like many − but not all − authors, this article will require non-degeneracy as part of the definitions of "prefilter" and "[[Filter (set theory)|filter]]".</ref> prefilter.</li>
<li>The family <math>\mathcal{A}</math> is a subset of some (proper) prefilter.{{sfn|Joshi|1983|pp=242−248}}</li>
<li>The upward closure <math display="inline">\pi(\mathcal{A})^{\uparrow X}</math> is a (proper) filter on {{Nowrap|<math display=inline>X</math>.}}  In this case, <math>\pi(\mathcal{A})^{\uparrow X}</math> is called the filter on <math display="inline">X</math> generated by {{Nowrap|<math display=inline>\mathcal{A}</math>,}} because it is the minimal (with respect to <math>\,\subseteq\,</math>) filter on <math>X</math> that contains <math>\mathcal{A}</math> as a subset.</li>
<li><math>\mathcal{A}</math> is a subset of some (proper)<ref name="ProperDef" group="note" /> filter.{{sfn|Joshi|1983|pp=242−248}}</li></ul>

==Applications==

=== Compactness ===
The finite intersection property is useful in formulating an alternative definition of [[Compact space|compactness]]: 

{{math theorem | math_statement =A [[Topological space|space]] is compact if and only if every family of [[Closed set|closed subset]]s having the finite intersection property has non-empty intersection.{{sfn|Munkres|2000|p=169}}<ref>{{planetmath| urlname=ASpaceIsCompactIffAnyFamilyOfClosedSetsHavingFipHasNonemptyIntersection| title=A space is compact iff any family of closed sets having fip has non-empty intersection}}</ref>}}

This formulation of compactness is used in some proofs of [[Tychonoff's theorem]].

=== Uncountability of perfect spaces ===
Another common application is to prove that the [[Real number|real numbers]] are [[Uncountable set|uncountable]].  {{math theorem | math_statement = Let <math>X</math> be a non-empty [[Compact space|compact]] [[Hausdorff space]] that  satisfies the property that no one-point set is [[Open set|open]]. Then <math>X</math> is uncountable.}}All the conditions in the statement of the theorem are necessary:
# We cannot eliminate the Hausdorff condition; a countable set (with at least two points) with the indiscrete topology is compact, has more than one point, and satisfies the property that no one point sets are open, but is not uncountable.
# We cannot eliminate the compactness condition, as the set of [[Rational number|rational number]]s shows.
# We cannot eliminate the condition that one point sets cannot be open, as any finite space with the discrete topology shows.

{{math proof | proof =
We will show that if <math>U \subseteq X</math> is non-empty and open, and if <math>x</math> is a point of <math>X,</math> then there is a [[Neighbourhood (mathematics)|neighbourhood]] <math>V \subset U</math> whose [[Closure (topology)|closure]] does not contain <math>x</math> (<math>x</math>' may or may not be in <math>U</math>). Choose <math>y \in U</math> different from <math>x</math> (if <math>x \in U</math> then there must exist such a <math>y</math> for otherwise <math>U</math> would be an open one point set; if <math>x \notin U,</math> this is possible since <math>U</math>is non-empty). Then by the Hausdorff condition, choose disjoint neighbourhoods <math>W</math> and <math>K</math> of <math>x</math> and <math>y</math> respectively. Then <math>K \cap U</math> will be a neighbourhood of <math>y</math> contained in <math>U</math> whose closure doesn't contain <math>x</math> as desired.

Now suppose <math>f : \N \to X</math> is a [[Bijection|bijection]], and let <math>\left\{ x_i : i \in \N \right\}</math> denote the [[Image (mathematics)#Image of a function|image]] of <math>f.</math> Let <math>X</math> be the first open set and choose a neighbourhood <math>U_1 \subset X</math> whose closure does not contain <math>x_1.</math> Secondly, choose a neighbourhood <math>U_2 \subset U_1</math> whose closure does not contain <math>x_2.</math> Continue this process whereby choosing a neighbourhood <math>U_{n+1} \subset U_n</math> whose closure does not contain <math>x_{n+1}.</math> Then the collection <math>\left\{ U_i : i \in \N \right\}</math> satisfies the finite intersection property and hence the intersection of their closures is non-empty by the compactness of <math>X.</math> Therefore, there is a point <math>x</math> in this intersection. No <math>x_i</math> can belong to this intersection because <math>x_i</math> does not belong to the closure of <math>U_i.</math> This means that <math>x</math> is not equal to <math>x_i</math> for all <math>i</math> and <math>f</math> is not surjective; a contradiction. Therefore, <math>X</math> is uncountable.
}}{{math theorem | name=Corollary | math_statement=Every closed interval <math>[a, b]</math> with <math>a < b</math> is uncountable. Therefore, <math>\R</math> is uncountable.}}

{{math theorem | name=Corollary | math_statement=Every [[Perfect set|perfect]], [[Locally compact space|locally compact]] [[Hausdorff space]] is uncountable.}}

{{math proof | proof =
Let <math>X</math> be a perfect, compact, Hausdorff space, then the theorem immediately implies that <math>X</math> is uncountable. If <math>X</math> is a perfect, locally compact Hausdorff space that is not compact, then the one-point compactification of <math>X</math> is a perfect, compact Hausdorff space. Therefore, the one point compactification of <math>X</math> is uncountable. Since removing a point from an uncountable set still leaves an uncountable set, <math>X</math> is uncountable as well.
}}

=== Ultrafilters ===
Let <math>X</math> be non-empty, <math>F \subseteq 2^X.</math> <math>F</math> having the finite intersection property. Then there exists an <math>U</math> [[Ultrafilter (set theory)|ultrafilter]] (in <math>2^X</math>) such that <math>F \subseteq U.</math>  This result is known as the ultrafilter lemma.<ref>{{citation |last1=Csirmaz |first1=László |title=Matematikai logika |url=http://www.renyi.hu/~csirmaz/ |year=1994 |location=Budapest |publisher=Eötvös Loránd University |format=In Hungarian |last2=Hajnal |first2=András |}}.</ref>  

==See also==

* {{annotated link|Filter (set theory)}}
* {{annotated link|Filters in topology}}
* {{annotated link|Neighbourhood system}}
* {{annotated link|Ultrafilter (set theory)}}

==References==
===Notes===
{{reflist|group=note}}

===Citations===
{{reflist}}

===General sources===
* {{Bourbaki General Topology Part I Chapters 1-4}} <!--{{sfn|Bourbaki|1989|p=}}-->
* {{Bourbaki General Topology Part II Chapters 5-10}} <!--{{sfn|Bourbaki|1989|p=}}-->
* {{Bourbaki Topological Vector Spaces Part 1 Chapters 1–5}} <!--{{sfn|Bourbaki|1987|p=}}-->
* {{Comfort Negrepontis The Theory of Ultrafilters 1974}} <!--{{sfn|Comfort|Negrepontis|1974|p=}}-->
* {{Császár General Topology}} <!--{{sfn|Császár|1978|p=}}-->
* {{Dolecki Mynard Convergence Foundations Of Topology}} <!--{{sfn|Dolecki|Mynard|2016|p=}}-->
* {{Dugundji Topology}} <!--{{sfn|Dugundji|1966|p=}}-->
* {{Joshi Introduction to General Topology}} <!--{{sfn|Joshi|1983|p=}}-->
* {{cite journal|last1=Koutras|first1=Costas D.|last2=Moyzes|first2=Christos|last3=Nomikos|first3=Christos|last4=Tsaprounis|first4=Konstantinos|last5=Zikos|first5=Yorgos|title=On Weak Filters and Ultrafilters: Set Theory From (and for) Knowledge Representation|journal=Logic Journal of the IGPL|date=20 October 2021|doi=10.1093/jigpal/jzab030}} <!--{{sfn|Koutras|Moyzes|Nomikos|2021|p=}}-->
* {{cite web|last=MacIver R.|first=David|title=Filters in Analysis and Topology|date=1 July 2004|url=http://www.efnet-math.org/~david/mathematics/filters.pdf|archive-url=https://web.archive.org/web/20071009170540/http://www.efnet-math.org/~david/mathematics/filters.pdf |archive-date=2007-10-09 }} (Provides an introductory review of filters in topology and in metric spaces.)
* {{Munkres Topology|edition=2}} <!--{{sfn|Munkres|2000|p=}}-->
* {{Narici Beckenstein Topological Vector Spaces|edition=2}} <!--{{sfn|Narici|Beckenstein|2011|p=}}-->
* {{Wilansky Modern Methods in Topological Vector Spaces|edition=1}} <!--{{sfn|Wilansky|2013|p=}}-->
* {{Wilansky Topology for Analysis 2008}} <!--{{sfn|Wilansky|2008|p=}}-->

==External links==

* {{planetmathref|urlname=FiniteIntersectionProperty|title=Finite intersection property}}



[[Category:General topology]]
[[Category:Set theory]]

{{Sourceattribution|Finite intersection property}}