MathDB
everyone has d friends, every 2 strangers have exactly one common friend

Source: Czech-Polish-Slovak Match Junior 2019, individual p5 CPSJ

January 20, 2020
combinatorics

Problem Statement

Given is a group in which everyone has exactly dd friends and every two strangers have exactly one common friend. Prove that there are at most d2+1d^2 + 1 people in this group.
Back to ProblemsView on AoPS