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April 2002's Go Answer of the Month

Two corner situations this month that are worth knowing.

Dia A1 - Status.

Dia A1

This is from a game in Breakthrough to Shodan .

Dia B1 - Status.

Dia B1

For what it is worth, this is from a game played at a West Surrey club night.

It is vaguely similar in size and shape to Dia A1.

(Since problem was first posed, I have altered the page layout and changed the colours round in Dia B1.)

When looking for reference books to help me get these answers right, I made the discovery that both these shapes are well covered in James Davies' Life and Death. Dia A1 in Chapter 31 and Dia B1 in Chapter 28.

To start with Dia A1...

Dia A2 - How the corner developed.

Dia A2

In Breakthru to Shodan there is a thick black group in the middle of the upper side. They recommend that Black's next move, A, attaching beside 7 was wrong and should have been elsewhere.

Having reached the situation of Dia A1, white can live easily. The move I have seen mentioned most frequently is to just connect the two parts at the 4-3 point to make the "Long L group with one leg".

Black to play is more interesting:

Dia A3 - Black can force a ko.

Dia A3

The key move here is 3.

Note that if the liberty at the 5-4 point had been filled, Black could kill absolutely.

So the status of Dia A1 is "White can live, Black can kill in ko".

The status of Dia B1 is "White can live, Black can make a seki" (unless White insists on a ko).

To live white can play at the 2-2 point which is also the key move for Black.

Dia B2 & B3 - Two sekis.

Dia B2   Dia B3

In the two Dias (and other similar sequences), Black gets a seki due to White's shortage of liberties. This prevents white from making an atari on the stones in what ought to be his second eye!

Note that the throw in of 5 in Dia B2 is essential to eliminate a real eye and to maintain the liberty shortage.

Dia B4 - If White must fight a ko...

Dia B4

If White plays 1 here instead of 4 in Dia B3...

White has to win the ko. The books suggest that this is usually disadvantageous for White and he would be better off with the seki.

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