Originally posted by ilywrinKnowing the composer (Gianni Donati), it was probably deliberate. My guess is that it was in honor of Thema Danicum, the publication that it appeared in.
Well it seems I have missed all the fun. I have one question though, BigDogg: do you have any idea if the D-shape of the position is just a coincidence or was this made on purpose?
I think last move was b1Q because...
The black pawns have done major takings. The pawn from...
f7 to c4 (taking 3 pieces, pawns and/or major pieces)
g7 to b2 (taking 5 pieces, pawns and/or major pieces), from b2 it promoted, so we have to figure out if it was possible to take another piece.
h7 to c2 (taking 5 pieces, pawns and/or major pieces)
That makes a total of 13 White pieces been taken. 1 Queen, 1 Rook, 2 Bishops, 2 Knights, 6 Pawns (b- to g-pawn) where on their way. The h-pawn couldnt not have been taken directly, however it can promote on h8 and then sacrifies himself as a major piece to one of the 3 black pawns.
So where is the 14th White piece that we need if Black moved bxa1B?
For that we have to look at the White a-pawn. If it was possible to reach c4, it was taken by the Black pawn (dxc4), which gives us an extra White piece to be taken on a1. How can the White a-pawn get to c4? It has to take 2 pieces of course.
The only missing Black pieces are a Knight and both Black bishops. (because we are talking about the case that bxa1B was the last move, therefore the Black bishop on a1 isn't the original bishop).
However the darksquared Black bishop couldnt have been taken by the a-pawn because a2-b3-c4 are only light squares.
And the lightsquared Black bishop couldnt have been taken either, because it was trapped on his original square c8 by his pawns.
This leaves only one Black knight. Therefore that all above doesn't work and the last move was b1Q.
Of course this means, that the original Queen could have been the 2nd Black piece (taken on c4), making a taking on a1 possible again. But it had to promote into a queen on a1 then, because there is a queen in the diagram. It had to go to b1 afterwards etc., making the current position illegal of course because of the check.
Phew, did I miss anything?
I pm'd a solution which is in line with the one presented above, but still feel unsatisfied with how I formulated it. Let's retry.
Only two options: b2-b1=Q+ or b2x?a1=B+.
If the latter one were true (promoting on a1), then black's three pawns made 14 file displacements = 14 captures. White has lost 14 pawns+pieces. That means all of them taken by a pawn. The white h-pawn could have promoted on h8 (after the black h-pawn left h7), and then the promoted piece got captured on an appropriate square.
The a-pawn, however could have been captured only on b3 or c4 (a2 or higher up on b-file are not reachable for a black pawn). That means it was captured on the diagonal f7-a2, only reachable by one black pawn, which is still on c4. So this is not possible at all.
Leaves the only alternative option: b2-b1=Q+.
edit: typo
I got the same solution by focussing on the white a-pawn.
It must have been captured on c4 and it could only get there by capturing 2 pieces, both of them on white squares. Black's queen's bishop was captured on c8, so the white a-pawn must have captured a knight and a queen on route to c4 ie the queen on b1 is not the original queen, it is the promoted g-pawn.
We know for sure that Black's pawns captured 13 pieces/pawns, so Black's king's bishop must have captured the pawn on b2.
Originally posted by crazyblueThis is the right reasoning to prove the last move was ...b1Q#.
I think last move was b1Q because...
The black pawns have done major takings. The pawn from...
f7 to c4 (taking 3 pieces, pawns and/or major pieces)
g7 to b2 (taking 5 pieces, pawns and/or major pieces), from b2 it promoted, so we have to figure out if it was possible to take another piece.
h7 to c2 (taking 5 pieces, pawns and/or major pieces)
T ...[text shortened]... ing the current position illegal of course because of the check.
Phew, did I miss anything?