Alice and Bob play a game in which they take turns choosing integers from to . Before any integers are chosen, Bob selects a goal of "odd" or "even". On the first turn, Alice chooses one of the 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 th turn, which is forced and ends the game. Bob wins if the parity of \{k\colon \mbox{the number kkth turn}\} matches his goal. For which values of does Bob have a winning strategy?
Solution
(Communicated by Kai Wang) For all , Bob has a winning strategy. Note that we can interpret the game play as building a permutation of , and the number of times an integer is chosen on the -th turn is exactly the number of fixed points of this permutation. For even, Bob selects the goal "even". Divide into the pairs ; each time Alice chooses an integer, Bob follows suit with the other integer in the same pair. For each pair , we see that is a fixed point if and only if is, so the number of fixed points is even. For 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 (with no moves made). We may thus assume hereafter that Alice's first move is some , which Bob counters with 2; at this point there is exactly one fixed point. Thereafter, as long as Alice chooses on the -th turn (for odd), either , in which case Bob can choose to keep the number of fixed points odd; or , in which case is even and Bob can choose 1 to transpose into the strategy for (with no moves made). Otherwise, at some odd turn , Alice does not choose . 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 for Bob, if is available Bob chooses it; otherwise, Bob has at least two choices available, so he can choose a value other than .