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What known position has one side with a forced mate in N moves, with N the largest? I came across one with 517 moves using only 7 pieces, so there should be much be larger. (Ignoring the 50-move rule).

Dont know if this is on-topic here, - but how large can we make N (asymptotically) on an nxn chess-board with any number of pieces and placements?

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Duplicate maybe ? – Nikana Reklawyks Jan 9 '13 at 1:43
Note: the 517 move 7-piecer mentioned in the OP is actually longer than that to mate. The 517 moves are how long it takes, with best play, to reduce the 7-piece position to a won 6-piece ending (which then requires some further moves to mate). See also – ETD Jan 9 '13 at 10:32
Given your interest in the second, asymptotic question, you might find this MathOverflow question about play on an infinite board to be interesting: – ETD Jan 9 '13 at 10:36

I think answers to this question fall into 2 categories : problem composition and computer-driven few-pieced endgames' thorough analysis. The second one — to which your discovery belongs — being a tad bit more efficient than the first, that's probably all there is to say about it, if you're about sheer numbers. Art still has a long way to go on this road.

You'll probably be interested in this answer, pointing to Wikipedia suggesting 545 as the last found maximum (details there on rybkaforum).

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