View source for Cofinite subset

''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'''.

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