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Difference between revisions of "Cofinite subset"

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(Start article: Cofinite subset)
 
(Anchor Fréchet filter, cofinite topology)
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A subset $A$ of $X$ for which the [[relative complement]] $X \setminus A$ is finite.
 
A subset $A$ of $X$ for which the [[relative complement]] $X \setminus A$ is finite.
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The family $\mathcal{F}(X)$ of cofinite subsets of $X$ forms a [[filter]], known as the '''Fréchet filter''' on $X$.  It is contained in any non-principal ultrafilter on $X$.
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The cofinite subsets of $X$, together with the empty set, constitute the open sets of a [[topology]] on $X$, known as the '''cofinite topology'''.
  
 
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Revision as of 10:06, 22 October 2016

of a set $X$

A subset $A$ of $X$ for which the relative complement $X \setminus A$ is finite.

The family $\mathcal{F}(X)$ of cofinite subsets of $X$ forms a filter, known as the Fréchet filter on $X$. It is contained in any non-principal ultrafilter on $X$.

The cofinite subsets of $X$, together with the empty set, constitute the open sets of a topology on $X$, known as the cofinite topology.

How to Cite This Entry:
Cofinite subset. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Cofinite_subset&oldid=39477