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Anna and Brian play a game with dominoes

Source: 2007 Swedish Mathematical Competition p5

April 27, 2021
combinatorial geometrycombinatoricsgamegame strategy

Problem Statement

Anna and Brian play a game where they put the domino tiles (of size 2×12 \times 1) in a boards composed of n×1n \times 1 boxes. Tiles must be placed so that they cover exactly two boxes. Players take turnslaying each tile and the one laying last tile wins. They play once for each nn, where n=2,3,,2007n = 2, 3,\dots,2007. Show that Anna wins at least 15051505 of the games if she always starts first and they both always play optimally, ie if they do their best to win in every move.