DE-BUNDESWETTBEWERB 2015 Problem 4
Many people use the social network "BWM". It is known that: By choosing any four people of that network there always is one that is a friend of the other three. Is it then true that by choosing any four people there always is one that is a friend of everyone in "BWM"?Note: If member is a friend of member , then member also is a friend of member .
Dataset text AI-MO AoPS. Quoted with source.
Official text: Bundeswettbewerb Mathematik 2015 official paper, page 1.
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