Originally posted by Zeddicus
Ok, apologies I guess. I've discussed this with a lecturer who's a big chess fanatic, and now accept that Chess doesn't follow the same rules as pure mathematics - that a disproof of something (like I've done above), doesn't necessarily mean that you shouldn't also be able to prove it is true.
I must admit I'm still confused as to why, but never mind.
I'd just like to see the full, official solution to this problem. 🙁
Refer to the last post on page 2 of this thread for the necessary retroanalysis. It proves that there are two possible realities for the position:
A) White can't castle, nor can he capture en passant.
B) White can capture en passant, and he still has castling rights.
The solution is:
1.dxc6ep! b5!
White has not yet proven that he has the right to play en passant on move 1, so it is an open question whether 2.d7 is checkmate or not - obviously, if an illegal move has been played in the past, all checkmates are null and void.
2.0-0-0!
The proof! White has just established that we're in reality B) above. This is in line with the FIDE Codex, which tells us that we may assume castling is legal unless provable otherwise.
2...Bb6 3.Nd7+ Ka7 4.b8Q#
I'm sure a lot of chess players won't be happy with the idea of
A Posteriori proof - proof offered
after the fact. However, there is no good alternative - too many nice chess problems would be ruined if castling could not be assumed legal. Same goes for retrograde problems involving a lengthy proof of the legality of en passant.