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I have a fun-ky puzzle for everyone today.

Here is the position.

enter image description here

FEN-rnbq1rkb/ppp1pp2/5p2/8/8/4K3/PPPPNP1P/RNBQ1R2

It can be reached in one way in exactly 9.5 moves, or 10 White and 9 Black moves. However, there is a catch. A rule of chess must be broken in order for there to be a solution in time.

Task: Find the unique sequence of moves and state the broken rule.

Good luck and have fun!

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2 Answers 2

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5
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We can reach the position in the allotted number of moves by either

temporarily ignoring check

or

by underpromoting a pawn

to a pawn

Sequence of moves is essentially the same in both cases:

[FEN ""] 1.g4 d5 2.g5 d4 3.g6 d3 4.gxh7 dxe2 5.hxg8=N exf1=P 6.Nf6+ gxf6 7.Ke2 Bg7 8.Ke3 O-O 9.Ne2 Bh8 10.Rxf1 *

Replay

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0
6
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This may not be the intended solution, but it does follow all the instructions:

1. e4 d5 2. g4 Nf6 3. g5 dxe4 4. gxf6 gxf6 5. Bd3 Bg7 6. Bxe4 0-0 7. Bxh7 Kxh7 8. Ke3 Kg8 9. Ne2 Bh8 10. Rf1

In this solution, the broken rule is that the king can only move one square, since it moves two squares in 8. Ke3. I recognize this may be a liberal interpretation of "broken rule", since I would normally interpret a broken rule as, for example, castling through check or castling after king/rook has moved, but this was the best I could do.

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  • 1
    $\begingroup$ But the problem says the position "can be reached in one way". If the king can move two squares, is this solution unique? (Maybe it is. I haven't tried. But it's up to the person proposing a solution to show it is.) $\endgroup$
    – msh210
    Commented Jun 29, 2020 at 7:42
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    $\begingroup$ @JohnBrookfields 0-0 is the official notation, cf. handbook.fide.com/chapter/E012018 $\endgroup$
    – Glorfindel
    Commented Jun 29, 2020 at 9:24

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