21/04/20 19:40:19.79 um3o3lUE.net
実は、Wikipediaで、以下を読んで「finite union-closed family of finite sets」というのには空集合も含まれるのかなとふと思ったので質問しました。
In combinatorics, the union-closed sets conjecture is an elementary problem, posed by Peter Frankl in 1979 and still open.
A family of sets is said to be union-closed if the union of any two sets from the family remains in the family.
The conjecture states:
For every finite union-closed family of finite sets, other than the family containing only the empty set, there exists
an element that belongs to at least half of the sets in the family.