$n$ persons meet in a meeting
Source: 4-th Taiwanese Mathematical Olympiad 1995
January 16, 2007
combinatorics proposedcombinatorics
Problem Statement
Suppose that persons meet in a meeting, and that each of the persons is acquainted to exactly others. Any two acquainted persons have exactly common acquaintances, and any two non-acquainted persons have exactly common acquaintances. Find all possible values of .