n guests at a party, any 2 guests are either friends or not friends
Source: Dutch NMO 2019 p2
January 9, 2020
combinatorics
Problem Statement
There are guests at a party. Any two guests are either friends or not friends. Every guest is friends with exactly four of the other guests. Whenever a guest is not friends with two other guests, those two other guests cannot be friends with each other either.
What are the possible values of ?