White mates in 21 moves.
While black moves his king alternatingly a step left and right, white must arrange his pieces such that finally a rook takes the black bishop and mates. Successively, white moves a knight, rook, knight, rook, bishop, rook, knight, rook, knight, bishop, knight, rook, knight, rook, king, knight, king, rook, king, knight, and then the rook takes the bishop: mate. Below, you see the solution as an animated gif.
WWW page created: March 24, 1997.