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April 1998's Go Answer of the Month

Dia 1 & 2 - Status.

Dia 1   Dia 2

Two related corner positions. Both are from the answer diagrams to a book problem!
(Do you ever get the situation where you have difficulty solving the solution, let alone the problem?)

In both diagrams, the question is "What is the status of the corner?"

The original problem is Puzzle 1 in Yang Yilun's "Ingenious Life and Death Puzzles, Volume 1" from Yutopian.

Dia 1 is three moves into the failure diagram. Dia 2 is the solution diagram.

Note the use on this page of the "single quote mark" to indicate a "what-if" move compared to the unmarked "as-the- diagram" moves.

Let's look at Dia 1 first.

Dia 3 - White can live.

Dia 3

No matter what Black does, White can guarantee an eye at each of the 1-3 points by playing at one of the 1-2 points and closing his shape to the left.

Dia 4 - Black can kill.

Dia 4

This is the rest of the failure diagram which shows Black making a bent four in the corner.
The crucial thing here is for Black to get to play at both 1-2 points whilst forcing White to narrow his space with a play at the 1-4 point. Thus 4 & 5 are miai. While 2' at 4 gets 3' at 5. And 4' at 7 gets 5' at 6.

Hands up all those who can honestly say they saw the bent four from Dia 1.

Now on to Dia 2. There is a ko involved here!

Dia 5 - Black can kill.

Dia 5

After Black files the ko, White has insufficient space to live.

Dia 6 - White can live in ko.

Dia 6 2 is a ko threat, 3 a ko answer. 6 at 1.

5 is a local ko threat and I reckon that Black is best to answer this by filling the ko, transferring the fight to a slightly smaller version right in the corner.
If Black wins this next ko fight, he fills at 7. If White wins, he captures the right side 1-2 stone for two eyes.

Dia 7 - How Black can get the ko wrong.

Dia 7 7 at 1.

Having read Dia 6, if Black tries to be clever and plays 2 as a local ko threat, White ends up absolutely alive through a double ko (1,4 and 5,6).

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