After a break they sit again around the circular table
Source: IMO ShortList 1988, Problem 31, USS 2, Problem 85 of ILL
November 9, 2005
modular arithmeticcombinatoricsinvariantIMO Shortlist
Problem Statement
Around a circular table an even number of persons have a discussion. After a break they sit again around the circular table in a different order. Prove that there are at least two people such that the number of participants sitting between them before and after a break is the same.