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        riddles >> medium >> Retrograde Chess II
        
(Message started by: THUDandBLUNDER on Apr 16^th, 2003, 12:16pm)

Title: Retrograde Chess II
Post by THUDandBLUNDER on Apr 16^th, 2003, 12:16pm

My niece and nephew came to visit me the other day. After settling down to play chess, and having played quietly for some time, they became involved in a heated argument. One of them had accidentally knocked the White king off the board, and they couldn't agree on where it had been. They asked me to arbitrate.
Luckily, I was able to decide matters to everyone's satisfaction, not least my own.

Where had the White king been before it was knocked off and what were the last three moves played?

#####################################
##+++++-----+++++-----+++++-----+++++------##
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##-----+++++------+++++-----+++++-----+++++##
##-----| r |-----| b |----++++++-----+++++##
##------++++-----+++++-----++++++-----+++++##
##+++++-----+++++-----+++++-----+++++------##
## B |-----+++++-----+++++-----+++++------##
##+++++-----+++++-----+++++-----+++++------##
##-----+++++-----+++++-----+++++-----++++++##
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##+++++-----+++++-----+++++-----+++++------##
##+++++-----+++++-----+++++-----+++++------##
##-----+++++------+++++-----+++++-----+++++##
##-----+++++------| k |-----+++++-----+++++##
##-----+++++------+++++-----+++++-----+++++##
#####################################

WHITE:
King at ?
Bishop at a4

BLACK:
King at d1
Rook at b5
Bishop at d5

Title: Re: Retrograde Chess II
Post by Ulkesh on Apr 16^th, 2003, 1:13pm

I was just about to say that I've found a load of answers, but suddenly the rook changes colour to black! The puzzle is a bit more tricky now...

[hide]

It couldn't have been Black's move last, as the king is in check, and the white king can't be positioned on b3 or c2 (as it would be in check itself).

White couldn't have moved his bishop into the position it's in now from anywhere but another check position, so the only possibility is for the white king to have moved from b3 or c2 causing revealed check. c2 is next to the black king, so the last move was white king from b3 to it's current position. And the only possibilities are a3 and c3.

Now this is the tricky part. The white king is in check on b3, and there are two black pieces attacking it. This means on the move before the white king was protected from both (as he can't move into check). The rook couldn't have moved into an attacking position revealing the bishop, and vice versa. Also no single piece (which may have been captured) could have been shielding the king from both pieces. This means that there were pieces on b4 and c4 which both have suddenly disappeared. The only way this can happen is by en passant. A black pawn on b4 must have captured a white pawn on c4 en passant (ending up on c3). This allows both the rook and bishop to now be attacking the white king. The move prior to that was obviously the white pawn moving from c2 to c4, allowing the black pawn to capture it en passant.

Where has the black pawn gone? The king must have captured it, so the king is on c3.

So, to summarise...

Initally the pieces are set up like this:

white:
K on b3
B on a4
P on c2

black:
K on a4
R on b5
B on d5
P on b4

the moves:
1. c4 bxc3+ (en passant)
2. Kxc3+

The king is now on c3
[/hide]

How's that? ;D

Title: Re: Retrograde Chess II
Post by THUDandBLUNDER on Apr 16^th, 2003, 1:31pm

What took you so long, Ulkesh? I have been timing you. :D Are you good at chess?

Title: Re: Retrograde Chess II
Post by Ulkesh on Apr 16^th, 2003, 1:53pm

on 04/16/03 at 13:31:25, THUDandBLUNDER wrote:

What took you so long, Ulkesh? I have been timing you. :D Are you good at chess?

I'm not bad, I did play for my school a lot when I was between 14-16, but I gave up because I got a bit bored with the game. I was far from the best though -- I was maybe board 5 or 6 in the school for my age group. Now I'm at university, I don't really have the time or the motivation to play much. Too much physics to do unfortunately ;)

I like these puzzles though -- keep them coming!

Title: Re: Retrograde Chess II
Post by Boody on Apr 16^th, 2003, 6:20pm

Blurp !
You can use the tag

[ font=Courier New ]
chessboard here
[ /font ]

(without the space)

to keep your chessboard in the right way.
:)

I post a [Search] post in the Faq section for a chess-editor but no response let me imagine that is not the good place for searching.
???

Title: Re: Retrograde Chess II
Post by Icarus on Apr 16^th, 2003, 8:52pm

The [ pre ] [ /pre ] tags (preformatted text - the button just right of the marque (<-M) button in the tags table) will do the exact same thing. (Completely - as far as I have been able to tell, [ pre ] is equivalent to [ font=Courier New ].)

Title: Re: Retrograde Chess II
Post by wowbagger on Apr 17^th, 2003, 4:59am

Very well done, Ulkesh! :)
It's a pity you don't have the time/motivation to play much, but at least you're pursuing the right studies! ;D
I myself have been playing regularly for about ten years now, but my capabilities of solving such problems are in no way linked to my playing. On the other hand, the last season seems to indicate that my playing skill approaches my problem-solving skill! :-/

on 04/16/03 at 12:16:27, THUDandBLUNDER wrote:

Where had the White king been before it was knocked off and what were the last three moves played?

Technical note (to make future riddles unambiguous):
What you meant to say was "three half-moves", not three complete moves (i.e. three by White and three by Black).
Consider that a mate in three (moves) typically involves five half-moves.

Title: Re: Retrograde Chess II
Post by THUDandBLUNDER on Apr 17^th, 2003, 5:27am

Point taken, wowbagger. Just to be clear, which do you find preferable:

A) What was White's last move?
B) What was White's last half-move?

Title: Re: Retrograde Chess II
Post by wowbagger on Apr 17^th, 2003, 5:55am

If the colour is specified, I think "move" is preferable.