VincentL: "Very Difficult".
I will be one of the last posters here.
White effectively has a bishop for a pawn.
But how to win with that?
The first question is: which of white´s pawns is going to promote? Surely it has to be the KRP.
The most obvious first move is 46. Bd1. I hesitate with this, as it does not seem to me to be
a "puzzle move".
With the B on the d1 - h5 diagonal, white can pick off black´s f pawn if it advances to f3,
and also take black´s h pawn, enabling his own h pawn forward.
Black´s best chance is to force white to trade the B for the f pawn (Ke4 and f3 will accomplish that)
and then advance the Q side pawns.
Effectively we have a pawn race here.
Let´s look at an example line to see how this goes.
46. Bd1 Ke4 47. Kxh5 f3 48. Bxf3 Kxf3 49. Kg5. Now, white needs four moves for his h pawn
to reach h8; also when it queens there, the new queen will protect a1m but not b1.
Black has to move both his a and b pawns (obviously the king will not be used). Quickest is
a4, b4 and b3. It will not help white to move his a pawn to any square, since this will lose a tempi. Now. best for black is b2, b1=Q. (not bxa2, a1=Q), since white´s new queen
will not protect the b1 square.
This process takes two moves, so a total of 5 in all for black.
Now... black effectively "moves first" in this sequence. So there is a problem.
White will play h8=Q (not check) and black´s next move will be b1=Q.
Now we see why this is a Saturday puzzle.
OK - so what trick can we conjure to circumvent this problem?
The answer may be to "make black´s f pawn move further". It could well be captured by white´s B on f1, and not f3.
In this case, white´s first move must be 46. Kxh5. Now what happens?
Black can then try 3 strategies. (a) Move his K back and try to stop white´s h pawn from
queening. (b) Move the f pawn forward; force white to trade the B for it, and then push
the Q side pawns. (c) Push the Q side pawns first.
Let´s examine each of these.
(a) Black would play 46......Kf6. Then 47. Kh6 and now black cannot play Kf7 (due to white´s B on B3),
so white follows with 48. Kg7 and then h5, h6, h7 h8=Q.
So this loses simply.
(b) 46.....f3 47. Bd1. Now black has a choice.He can play (i) 47....f2, and then 48. Be2 b4,
Now white needs 5 moves to promote the h pawn, and black requires 5 also. But this is of no
use to white, since black´s b pawn can promote, and white´s new queen on h8 will not protect
b1.