| dbp:mathStatement
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- Every proper filter on a set is contained in some ultrafilter on (en)
- If is an ultrafilter on then the following are equivalent:
is fixed, or equivalently, not free.
is principal.
Some element of is a finite set.
Some element of is a singleton set.
is principal at some point of which means for some
does contain the Fréchet filter on as a subset.
is sequential. (en)
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