The Solution
Such long problems have a peculiar logic all their own. White's ultimate aim is to mate by Nf3 and
Nd4 mate, but at the moment 1 Nf3 Kc6 2 Nd4+ fails to 2...Kd5, so first of all White must advance
his e-pawn to e4 with gain of tempo.
This large problem can be broken down into smaller sub-problems. White would like to maneuver with
his bishop, for example by 1 Bh6 Kc5 2 Be3+, but once again Black's king escapes via d5. So the
first step is to put a temporary guard on d5 to free the bishop.
1 Ke7 Kc5
2 Ke6+ Kb5
Although d5 is now guarded, White cannot yet mate by 3 Nf3 Kc6 4 Nd4+ because he has given up
control of c7. d5 has to be covered by the e-pawn if this mating idea is to work, but that is
still a long way off.
3 Bh6 Kc5
Black's moves are forced throughout the solution.
4 Be3+ Kb5
White is now free to maneuver with his bishop, but it isn't obvious how this helps
to advance the e-pawn. The key idea is to transfer the bishop to f2 with gain of tempo, then e2-e3
will lift the stalemate, forcing ...Kc5, and after e3-e4+ White's wish has been granted.
5 Bg5
The bishop couldn't transfer from f8 to f2 directly, because the f8-h6 diagonal wasn't
long enough. Once again a tricky problem can be broken down into smaller problems, first transfer
the bishop to the d8-h4 diagonal, and then use the h4 square to reach f2.
5...Kc5
6 Be7+ Kb5
7 Bh4 Kc5
8 Bf2+ Kb5
At long last the e-pawn can make its first step forward.
9 e3 Kc5
10 e4+ Kb5
The e-pawn has reached its final destination, but White's king and
bishop are on the wrong squares for the final mating idea. The next step is to execute all the
previous maneuvers in reverse. First of all the bishop has to return to f8.
11 Bh4 Kc5
12 Be7+ Kb5
13 Bg5 Kc5
14 Be3+ Kb5
15 Bh6 Kc5
16 Bf8+ Kb5
Now White's initial king maneuver can be reversed.
17 Ke7 Kc5
18 Kd8+ Kb5
The king controls c7 and the pawn controls d5, so the stage is set for the final act.
19 Nf3 Kc6
20 Nd4 mate.