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It's White to move-can they force a win?. The left bottom is a1 as usual.

[FEN "4k2r/6p1/p4pKb/p2N3p/p7/p5Q1/p1n5/qrn5 w - - 9 9"]

Note for people who saw an earlier version: the position was edited (including a swap of the colors) for a more beautiful puzzle by Laska's suggestions. The answer has now been fixed accordingly.

History: I came up with this puzzle when I was a student, almost 20 years ago. It was published in the magazine of my local chess club, but nobody found the answer. A friend popularized it with his new chess-friends after he emigrated to Canada.

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  • 4
    You do know that the position on the board is illegal and therefore it does not comprise a proper chess problem? You have 5 pawns on the a file requiring 10 captures of which at most 4 can be pawn captures. That leaves 6 black pieces which must have been captured out of a total of 7 leaving just one. However black has two pieces remaining.
    – Brian Towers ♦
    Commented May 6, 2018 at 12:30
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    Ah, OK, so either the black queen or knight is the result of a pawn promotion and probably the e pawn.
    – Brian Towers ♦
    Commented May 6, 2018 at 14:40
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    @Laska if you move the black king to g3, castling kingside won't help anymore for white.
    – Glorfindel ♦
    Commented May 6, 2018 at 14:52
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    No worries: Qd3 is what they call a "try". That means it's nearly a solution but there is a single defence which defeats it: Rb2. You can safely start this rook off on b8 by the way: don't need to block bQ so much. If I were you, I would swap the roles of Black & White in this problem, as it's more normal for White to be the mating party in these problems, and then people can check your problem is sound on special problem software e.g. Popeye, Problemist etc.
    – Laska
    Commented May 6, 2018 at 17:19
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    @AlbertHendriks I hope you also edited Glorfindel's answer accordingly and it just waits to be reviewed? Please never edit questions on StackExchange in a a way that invalidates existing answers!
    – leftaroundabout
    Commented May 6, 2018 at 22:55

2 Answers 2

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The solution is

1. Qc7 followed by Qe7, Qf7 or Qc8 mate (depending on Black's second move).

This works because

Black can't castle because either his king or rook on h8 have moved already. As Brian Towers notes, the black a-pawns need to have captured 10 white pieces and/or pawns; there are enough pieces missing, but capturing the white e-pawn (or to the right) doesn't bring you any closer to the a-file. So, one white pawn on the queenside must have been captured on another file; this is possible via promotion, but since there's just one black piece missing (a white-squared bishop) the promotion could have happened on the d-, e-, f-, or h-file. The pawn has either attacked the black king at f7 or d7, or it promoted on e8, forcing the black King to move in all cases. Or it promoted on h8, forcing the black rook to move. Another possibility, as @Laska noted, is that the e-pawn captures the black bishop on the d-file and is captured itself before promotion; however, we only need a single capture on the d-file, and the original white d-pawn must be captured too.

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    did you switch black and white in your answer?
    – Ant
    Commented May 7, 2018 at 3:01
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    @Ant I edited my question and switched the colors. The answer here is now corrected (after your comment).
    – Albert Hendriks
    Commented May 7, 2018 at 6:24
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As the tags suggests, the problem is a retrograde problem. If Black may no longer castle, there is an easy solution-1. Qc7 and mate on the second move.

The Black pawns on a6-a2 must have taken 10 white pieces or pawns. White has 3 pieces left, so the pawns could have taken 5 pieces and the pawns from a2 to d2. So one of the other pawns (e2-h2) must have been taken too. That only could have happened if one of those pawns promoted. But with the Black king on e8 and the black rook on h8, this is impossible, taking in account that the Black pawns are on f6, g7 and h5.

Conclusion: the Black king and/or rook must have moved, so castling is illegal and 1. Qc7 wins.

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