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Summary
DescriptionFilter vs ultrafilter.svg
English: The Hasse diagram shows the set of all subsets of {1,2,3,4}, partially ordered by set inclusion (⊆). The upper set ↑{1,4} is highlighted in dark green, it is a filter. However, it is not an ultrafilter, since it can still be extended to the properly larger filter ↑{1}, shown in light green. The latter cannot be extended to any nontrivial larger filter in turn, it is hence an ultrafilter.
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