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 Post subject: Retrograde Analysis
PostPosted: Sun Jan 13, 2008 3:29 pm 
I'm working through 'The Chess Mysteries of Sherlock Holmes' by Raymond Smullyan and have come acrosss a problem for which I can see no solution although the book, whilst explaining most steps, offers the solution including the part-sentence 'It must have been from a4'. The problem is a question of whether Black can castle, and the problem is entitled 'You really can't, you know!'. If anyone has the book, can they please explain why pawn a3 MUST have come from a4 NOT b4. With this understanding the solution is clear, however I can't see why the pawn couldn't have come from b4 taking a black piece on a3 which would mean that the solution to the problem is different from as published.


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Joined: Sat Aug 13, 2005 3:13 pm
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PostPosted: Sun Jan 13, 2008 6:42 pm 
Sherlock talks about the LAST move done by black. It could not be e6,e5 or fxg6 for the reasons given. But obviously the LAST move could not be b4xa3, because currently there is a white knight on b4. If black did the b4xa3 earlier,then one of the other moves e5,e6,fxg6 or king or rook move was last. In this case casling is not allowed.

Wonderful books the Smullyans, though!


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 Post subject: Re: Retrograde Analysis
PostPosted: Wed Jan 16, 2013 11:10 am 
I just received this book as a gift, and I have the same question as in the original post. If the pawn on a3 was the last black piece to move, why must it have come from a4 instead of b4? The answer, "because there is a white knight on b4", is not satisfactory, because at the very beginning of the chapter, we are told, "Just as we entered, I saw Arthur Palmerston completing a move with a White knight. Then they both saw us and jumped up to greet us." This move of the white knight (to b4) is the move immediately prior to Robert's attempt to castle. Thus, when black moved last, the knight was not on b4. If we are to know that the pawn on a3 came from a4, we need a better explanation for why it could not have been on b4.

I agree with Smullyan (a.k.a. Sherlock) that the pawn on a3 came from d7, but I see nothing that precludes the path d7,d6,c5,b4,a3, which involves the same number of captures as d7,c6,b5,a4,a3. But this ruins the entire analysis, since the analysis depends on the captures occurring on white squares.

Within his analysis, Holmes proves that all of the captured black pieces were captured by white's pawns. This rules out a capture by the white knight. This too is critical to the his argument, but it also means that there was no piece on b4 when black last moved, unless black's last move was to move the pawn from b4 to a3. I still don't see a reason that the pawn could not have come from b4.


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