Problem

Source: BWM Germany Round 1, 2015 - Problem 4

Tags: combinatorics, graph theory, Germany



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 $A$ is a friend of member $B$, then member $B$ also is a friend of member $A$.