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Knight Interchanges: 2

Robert E. Parkin, in “Knight Interchanges” (J. Recreational Math., 7(2), Spring 1974, pp. 108-110) states that 26 is the minimum number of moves in the 6-Knight problem (Figure 3). (The problem is to interchange the positions of the black and the white knights using the usual knight moves and not allowing more than one knight on a square at a time.) I have found an 18-move solution: