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Subtract-a-square (also referred to as take-a-square) is a two-player mathematical game of strategy starting with a positive integer and both players taking turns subtracting a non-zero square number not larger than the current value. The game is usually played as a ''normal play'' game, which means that the last person who can make a subtraction wins. == Illustration == A normal play game starting with the number 13 is a win for the first player provided he does start with a subtraction of 1: player 1: 13 - 1 *1 = 12 Player 2 now has three choices: subtract 1, 4 or 9. In each of these cases, player 1 can ensure that within a few moves the number 2 gets passed on to player 2: player 2: 12 - 1 *1 = 11 player 2: 12 - 2 *2 = 8 player 2: 12 - 3 *3 = 3 player 1: 11 - 3 *3 = 2 player 1: 8 - 1 *1 = 7 player 1: 3 - 1 *1 = 2 player 2: 7 - 1 *1 = 6 or: 7 - 2 *2 = 3 player 1: 6 - 2 *2 = 2 3 - 1 *1 = 2 Now player 2 has to subtract 1, and player 1 subsequently does the same: player 2: 2 - 1 *1 = 1 player 1: 1 - 1 *1 = 0 player 2 loses 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Subtract a square」の詳細全文を読む スポンサード リンク
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