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IMO Shortlist 2012, Combinatorics 4

Source: IMO Shortlist 2012, Combinatorics 4

July 29, 2013
combinatoricsgameinvariantIMO Shortlist

Problem Statement

Players AA and BB play a game with N2012N \geq 2012 coins and 20122012 boxes arranged around a circle. Initially AA distributes the coins among the boxes so that there is at least 11 coin in each box. Then the two of them make moves in the order B,A,B,A,B,A,B,A,\ldots by the following rules: (a) On every move of his BB passes 11 coin from every box to an adjacent box. (b) On every move of hers AA chooses several coins that were not involved in BB's previous move and are in different boxes. She passes every coin to an adjacent box. Player AA's goal is to ensure at least 11 coin in each box after every move of hers, regardless of how BB plays and how many moves are made. Find the least NN that enables her to succeed.