Chess

This problem by Josif Kricheli won first prize in an international composing competition in 1983. It is White to play and mate in seven, and the answer bears more relation to one of those children's block-shifting puzzles than to a normal chess problem.

White has two ways in which he may hope to deliver mate: a safe check with the bishop, or by checking with the spare rook (now on c2) on the second rank.

The trouble is that Black generally has a choice of defences: 1.Bc2 (threatening Bb3) is met by Rh3; 1.Bd3 (threatening Bc4) by b5 or Rc6; 1.Be4 by c6 or Rd6; 1.Rf1 by Bc5; and 1.Re1 by f3.

Ah, but what about 1.Re1 f3 2.Bc2? Surely, with the pawn lured to f3, that forces mate by Bb3 next move since the rook can no longer protect the third rank from h3. What's all this about mate in seven? Isn't that a mate in three?

It isn't, of course, but some ingenuity is required to find Black's correct defence to 1.Re1. It is 1...Rc6! meeting 2.Re2+ with 2...Rc2 3.Bxc2 Bf6! when the bishop is ready to interpose on b2 in reply to a discovered check.

So let's try 1.Be4 c6 2.Re1! Now Black cannot play Rc6, so mate is forced after 2...f3 3.Bc2. But again there's a problem: 1.Be4 is met by 1...Rd6! 2.Bc2 Rd3! 3.Bxd3 b5 4.Bc2 b6+! and Black escapes. The queen on a8 isn't just a pretty face. Okay, let's try another tack: 1.Bc2 Rh3 2.Bg6 looks promising. The threat of Bf7+ forces 2...Rh7! Now, 3.Bxh7 lets Black escape by moving his knight from b8 to defend g8 with the queen. Having lured the rook to h7, White must find something more forceful than capturing it. Here's the answer in full: 1.Bc2 Rh3 2.Bg6 Rh7 3.Be4 c6 (Rd6 is no longer possible) 4.Bf5! Rh6 5.Re1! f3 (Rc6 is impossible) 6.Bc2 and 7 Bb3 mate.