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''<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609301.png" /> (given on a single set <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609302.png" />)''
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The set-theoretical intersection of this family, that is, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609303.png" />. It is usually denoted by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609304.png" /> and is always a topology on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609305.png" />. If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609306.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609307.png" /> are two topologies on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609308.png" /> and if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l0609309.png" /> is contained (as a set) in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093010.png" />, then one writes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093011.png" />.
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The topology <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093012.png" /> has the following property: If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093013.png" /> is a topology on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093014.png" /> and if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093015.png" /> for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093016.png" />, then <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093017.png" />. The free sum of the spaces that are obtained when all the individual topologies in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093018.png" /> are put on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093019.png" /> can be mapped canonically onto the space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093020.png" />. An important property of this mapping is that it is a [[Quotient mapping|quotient mapping]]. On this basis one proves general theorems on the preservation of a number of properties under the operation of intersecting topologies.
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'' $  F $(
 +
given on a single set  $  X $)''
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The set-theoretical intersection of this family, that is,  $  \cap F $.
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It is usually denoted by  $  \wedge F $
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and is always a topology on  $  X $.  
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If  $  {\mathcal T} _ {1} $
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and  $  {\mathcal T} _ {2} $
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are two topologies on  $  X $
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and if  $  {\mathcal T} _ {1} $
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is contained (as a set) in  $  {\mathcal T} _ {2} $,
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then one writes  $  {\mathcal T} _ {1} \leq  {\mathcal T} _ {2} $.
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The topology  $  \wedge F $
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has the following property: If $  {\mathcal T} _ {1} $
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is a topology on $  X $
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and if $  {\mathcal T} _ {1} \leq  {\mathcal T} $
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for all $  {\mathcal T} \in F $,  
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then $  {\mathcal T} _ {1} \leq  \wedge F $.  
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The free sum of the spaces that are obtained when all the individual topologies in $  F $
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are put on $  X $
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can be mapped canonically onto the space $  ( X , \wedge F ) $.  
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An important property of this mapping is that it is a [[Quotient mapping|quotient mapping]]. On this basis one proves general theorems on the preservation of a number of properties under the operation of intersecting topologies.
  
 
====References====
 
====References====
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  A.V. Arkhangel'skii,  V.I. Ponomarev,  "Fundamentals of general topology: problems and exercises" , Reidel  (1984)  (Translated from Russian)</TD></TR></table>
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  A.V. Arkhangel'skii,  V.I. Ponomarev,  "Fundamentals of general topology: problems and exercises" , Reidel  (1984)  (Translated from Russian)</TD></TR></table>
 
 
  
 
====Comments====
 
====Comments====
The article above actually defines the infimum of the family of topologies, which is a particular (the largest) lower bound for the family; a lower bound being any topology <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/l/l060/l060930/l06093021.png" /> this infimum.
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The article above actually defines the infimum of the family of topologies, which is a particular (the largest) lower bound for the family; a lower bound being any topology $  \leq  $
 +
this infimum.

Latest revision as of 04:11, 6 June 2020


$ F $( given on a single set $ X $)

The set-theoretical intersection of this family, that is, $ \cap F $. It is usually denoted by $ \wedge F $ and is always a topology on $ X $. If $ {\mathcal T} _ {1} $ and $ {\mathcal T} _ {2} $ are two topologies on $ X $ and if $ {\mathcal T} _ {1} $ is contained (as a set) in $ {\mathcal T} _ {2} $, then one writes $ {\mathcal T} _ {1} \leq {\mathcal T} _ {2} $.

The topology $ \wedge F $ has the following property: If $ {\mathcal T} _ {1} $ is a topology on $ X $ and if $ {\mathcal T} _ {1} \leq {\mathcal T} $ for all $ {\mathcal T} \in F $, then $ {\mathcal T} _ {1} \leq \wedge F $. The free sum of the spaces that are obtained when all the individual topologies in $ F $ are put on $ X $ can be mapped canonically onto the space $ ( X , \wedge F ) $. An important property of this mapping is that it is a quotient mapping. On this basis one proves general theorems on the preservation of a number of properties under the operation of intersecting topologies.

References

[1] A.V. Arkhangel'skii, V.I. Ponomarev, "Fundamentals of general topology: problems and exercises" , Reidel (1984) (Translated from Russian)

Comments

The article above actually defines the infimum of the family of topologies, which is a particular (the largest) lower bound for the family; a lower bound being any topology $ \leq $ this infimum.

How to Cite This Entry:
Lower bound of a family of topologies. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Lower_bound_of_a_family_of_topologies&oldid=17432
This article was adapted from an original article by A.V. Arkhangel'skii (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article