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MathNet 2016-part-06 — 184 problems (EN–VI) Question 92

Math Olympiad Vault · MathNet 2016-part-06 — 184 problems (EN–VI)

Question 92

Let $n$ be a positive integer, and write down $1, 2, \dots, n$ on the blackboard. Alpha and Lee take turn choosing a number from the board according to the following rules:
(i) You cannot choose any number that was previously selected by either player.
(ii) If you have chosen $k$, you cannot choose $k-1$ or $k+1$.
(iii) The game is a draw if all numbers are chosen. Otherwise, the player who cannot choose any number first loses the game.
Suppose Alpha chooses first. Determine all $n$ such that Lee has a winning strategy.