Filters Part1

[@TheKivifreak]

{{a,b}, {a,b,c}} is not a filter? Otherwise {a,b} intersect {a,b,c} = {a} should be in {{a,b},{a,b,c}} but it is not. 🧐
However {{a},{a,b},{a,c},{a,b,c}} is a filter and also happens to be an ultrafilter.

[@broccoloodle]

{a,b} ∩ {a,b,c} = {a, b}