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Combinatorics Difficulty 7.5 National olympiad, round 2 Find the answer

Alice and Bob play a game in which they take turns choosing integers from 11 to nn. Before any integers are chosen, Bob selects a goal of "odd" or "even". On the first turn, Alice chooses one of the nn integers. On the second turn, Bob chooses one of the remaining integers. They continue alternately choosing one of the integers that has not yet been chosen, until the nnth turn, which is forced and ends the game. Bob wins if the parity of \{k\colon \mbox{the number kwaschosenonthe was chosen on the kth turn}\} matches his goal. For which values of nn does Bob have a winning strategy?

A number or a short expression. Spacing and $ signs are ignored.

Solution

(Communicated by Kai Wang) For all nn, Bob has a winning strategy. Note that we can interpret the game play as building a permutation of {1,,n}\{1,\dots,n\}, and the number of times an integer kk is chosen on the kk-th turn is exactly the number of fixed points of this permutation. For nn even, Bob selects the goal "even". Divide {1,,n}\{1,\dots,n\} into the pairs {1,2},{3,4},\{1,2\},\{3,4\},\dots; each time Alice chooses an integer, Bob follows suit with the other integer in the same pair. For each pair {2k1,2k}\{2k-1,2k\}, we see that 2k12k-1 is a fixed point if and only if 2k2k is, so the number of fixed points is even. For nn odd, Bob selects the goal "odd". On the first turn, if Alice chooses 1 or 2, then Bob chooses the other one to transpose into the strategy for n2n-2 (with no moves made). We may thus assume hereafter that Alice's first move is some k>2k > 2, which Bob counters with 2; at this point there is exactly one fixed point. Thereafter, as long as Alice chooses jj on the jj-th turn (for j3j \geq 3 odd), either j+1<kj+1 < k, in which case Bob can choose j+1j+1 to keep the number of fixed points odd; or j+1=kj+1=k, in which case kk is even and Bob can choose 1 to transpose into the strategy for nkn-k (with no moves made). Otherwise, at some odd turn jj, Alice does not choose jj. At this point, the number of fixed points is odd, and on each subsequent turn Bob can ensure that neither his own move nor Alice's next move does not create a fixed point: on any turn jj for Bob, if j+1j+1 is available Bob chooses it; otherwise, Bob has at least two choices available, so he can choose a value other than jj.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.