Two squares on an chessboard are called touching if they have at least one common vertex. Determine if it is possible for a king to begin in some square and visit all the squares exactly once in such a way that all moves except the first are made into squares touching an even number of squares already visited.
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See this reference . It is a simple parity argument that tours of this type are not possible on nontrivial m × n boards if m and n have the same parity. Tours are not possible on other sized boards (except 1 × 1 or 1 × 2 ones), but the proof is harder.