If is an infinite set, then the collection is a filter called the cofinite (or Fréchet) filter on .
Cofinite Filter
See also
Filter, UltrafilterThis entry contributed by Viktor Bengtsson
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Cite this as:
Bengtsson, Viktor. "Cofinite Filter." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/CofiniteFilter.html