5 of n people may sit at a circle on a table among friends / strangers
Source: Czech-Polish-Slovak Junior Match 2013, Team p3 CPSJ
March 21, 2020
combinatorics
Problem Statement
In a certain group there are people, with every two people who do not know each other exactly having one mutual friend and no one knows everyone else. Prove of people, may sit at a circle around the table so that each of them sits between
a) friends,
b) strangers.