Sunday Chess Problem

This week we have a straightforward direct mate problem for you. It was composed by Raffi Rupin in 1961, and calls for mate in four:

A quick look at the diagram shows that white has three potential mates that almost work: Ra3, Nc5, Ra7. Currently, though, none of these work. The rook on b3 is pinned by the black bishop on g8, and therefore cannot move. The c5 square is covered by one of the black rooks, while the a7 square is covered by the other.

So, black currently has everything covered. But he's stretched pretty thin, meaning that white may be able to contrive a decoy.

The key move is 1. Kb1!:

This unpins the white rook, meaning that now white really is threatening to play 2. Ra3 mate.

Now, black has the obvious defense of 1. ... Bxb3. White can deal with this threat with the mundane 2. cxb3+ Kxb3 3. Nd2+ Ka4 4. Bd1 mate, leading to this position:

So far so good. I'm sure you've noticed, though, that black has two other defenses. By moving his king to the back rank, white has exposed himself to checks from the black rooks. For example, black can now play 1. ... Rh1+:

If white now had to move his king right back to a2 then he will have achieved nothing. But this is where the decoy comes in. White plays 2. Bf1! instead. After 2. ... Rxf1+ 3. Ka2

black must retreat his rook back to the seventh rank, to prevent white from playing Ra7 mate on the next move. Sadly, his rook is now on the f-file instead of the h-file, meaning that after 3. ... Rf7, the rook interferes with his own bishop. The white rook on b3 is now unpinned, allowing 4. Ra3 mate:

Let's try black's other defense: 1. ... Rg1+.

This time the trick is to play 2. Bd1!. Now after 2. ... Rxd1+ 3. Ka2,

the black rook has been decoyed to the d-file. So, when the rook races back to the fifth rank to prevent Nc5 mate, he will once more be interfering with his own bishop. We have 3. ... Rd5 4. Ra3 mate.

Pretty nice. This is an illustration of the Roman theme, which we have actually seen once before. The idea is that a black piece prevents a certain mate. That piece is then decoyed to a different square, from which it can prevent the mate in a different way. But this new defense suffers from a critical weakness that white can exploit.

See you next week!