Find a position with 7 pieces or less, where no king is in check, it is not given who moved last, but it is still possible to conclude what was the exact position before the last ply.
Originally posted by David113 Find a position with 7 pieces or less, where no king is in check, it is not given who moved last, but it is still possible to conclude what was the exact position before the last ply.
Originally posted by David113 Find a position with 7 pieces or less, where no king is in check, it is not given who moved last, but it is still possible to conclude what was the exact position before the last ply.
That position cant be legal in my humble opinion. The only way for the King to get to a2 is via the back rank, but then the rook cant get to b1, since the last move must have been Kc1. Where is my thinking error?
Originally posted by Darrie That position cant be legal in my humble opinion. The only way for the King to get to a2 is via the back rank, but then the rook cant get to b1, since the last move must have been Kc1. Where is my thinking error?
I think the answer is correct!
My first solution was this:
The problem here is, that I cannot tell what the position before was. I only know the king moved from d8 to c8 but I don't know which piece it took there (or if he took a piece at all). This is covered in BigDoggs answer: There must have been a knight on c1, because that is the only piece that can get there in the move before White's last move. It could not have been unoccupied because then it really seems to be an illegal position as you pointed out.
Originally posted by David113 BDP's solution is not correct. Do you see why?
Edit: David113 cooked my first try by -1.Kd1-c1 Ka1-a2 -2.Rc1x~b1+ etc.
Good spot. I noticed that Pa3 is serving no useful purpose in my first try. I can make better use of that 7th unit to jam the a1 square, which (I believe!) saves my idea:
Originally posted by BigDoggProblem Good spot. I noticed that Pa3 is serving no useful purpose in my solution. I can make better use of that 7th unit to jam the a1 square, which (I believe!) saves my idea:
[fen]8/8/8/8/8/8/kPPP4/bRK5 w - - 0 1[/fen]
The bishop can only have got there through promotion, and the king cannot have moved to a2 afterwards so that position is impossible.
I think BDP's second solution is fine. The bishop promoted on a1. Then the black king came to a2 from b1. then the white rook came to b1, the white king came to d1 and the black knight came to c1. Anyway, if you don't like the black bishop on a1 you can replace it by a white knight.
Construct your own retro #2
13 Nov '06 06:12
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