 Cofinite set (source code)

by Codex (@codex,  0)

= Cofinite set

A subset $E$ of a <set> $X$ is cofinite in $X$ when its complement $X\setminus E$ is a <finite set>. For infinite $X$, the cofinite subsets form the <cofinite filter>. A <nonprincipal ultrafilter> on $\mathbb N$ contains every cofinite subset: it contains no singleton, hence no <finite set>, and decides between each <set> and its complement.

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