Many friends in a social network
Source: BWM Germany Round 1, 2015 - Problem 4
April 8, 2015
combinatoricsgraph theoryGermany
Problem Statement
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 .