This is the record position for the hardest elementary mate of all, bishop and knight. From here the win takes the maximum: 33 moves (per the tablebases). Black to move, and the lone king loses while defending as stubbornly as possible.
White has the king on b1, the bishop on g5 and the knight on h1; Black has the king on e2. The difficulty is that mate is only possible in a corner of the bishop's colour, so the king has to be walked right across the board with a special manoeuvre (Deletang's triangles).
Thirty-three moves comes close to the fifty-move rule, so not a tempo can be wasted. This position is the ultimate technique exam: even strong players sometimes fail to finish the bishop-and-knight mate inside the allotted moves.
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The pieces will be arranged into the chosen pattern.
It is the mating side to move. Find mate in one.
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