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Anna and Orjan play a game, writine a pos. integer as sum of 2 others

Source: 2008 Swedish Mathematical Competition p5

April 27, 2021
combinatoricsgamegame strategy

Problem Statement

Anna and Orjan play the following game: they start with a positive integer n>1n>1, Anna writes it as the sum of two other positive integers, n=n1+n2n = n_1+n_2. Orjan deletes one of them, n1n_1 or n2n_2. If the remaining number is larger than 11, the process is repeated, i.e. Anna writes it as the sum of two positive integers, n3+n4 n_3+n_4, Orjan deletes one of them etc. The game ends when the last number is 11. Orjan is the winner if there are two equal numbers among the numbers he has deleted, otherwise Anna wins. Who is winning the game if n = 2008 and they both play optimally?