King of ko

In war, diversion is a common tactic. But when a red herring is used in go, it can get out of hand at times, sometimes leading to a fierce battle that in turn affects the overall game. Such a diversionary technique is called ko. Let's recapitulate on the rules concerning ko.

Diagram 1 : Black can capture a stone by playing at 1.

Diagram 2 : This is the new situation. Now, the rule is that White cannot play at A immediately, because a board situation cannot be repeated. So he has to play somewhere else first. This situation is called ko.

Diagram 3 : Now look at this local situation. White has just played the marked stone to start a ko to try and take Black's corner. Black captures with 1. Now white 2 threatens to kill the black group. So black 3 is a natural answer.

Diagram 4 : The board situation has changed, so now white 4 captures the marked black stone. Black 5 is a local threat, meaning it threatens to break up the white group. White must answer with 6, capturing a black stone and removing the threat.

Diagram 5 : Black can now recapture the ko (marked stone) with 7. White 8 needs to be answered by 9. White retakes the ko (to the left of 7) with 10. Black plays another local threat with 11. White answers with 12. Black retakes the ko with 13 (where 7 is). But White has another ko threat at 14 that Black needs to answer with 15 to maintain two eyes.

Diagram 6 : But now when White retakes the ko with 16, Black has no ko threats. His best shot in this situation might be playing at 17. Of course, White ignores this and fills in at 18 (where the marked stone is). Many people would feel inclined to capture at A, but this would only give Black the opportunity to capture that white stone at A some time later. The best Black can do is play at 19.

Solution to last week's problems

Solution 1 : Black 1 is a standard way of settling his shape . Since White ignored black 1 by pretending to attack with 2, Black should ask himself an important question here: If he ignores white 2, will the group on the right die ? If it won't die, Black should seize the opportunity to capture the marked white stone by playing at 3 to (1) settle one of his two weak groups, (2) render White's stones on the upper left eyeless by taking away their base, (3) grab about 20 points of territory (the difference between White making territory there and Black doing so). He then should wait for White to do his worst. If White plays at 4 to seal in the black stones (and connect his own stones), black 5 connects all of Black's stones.

Solution 1B : This is a variation White may choose. Instead of 4 in Solution 1, he could play at 4 and 6, taking away Black's potential eye space and connection. But now Black will not have any trouble escaping into the center. He can also make eyes with the sequence black A, white B, black C, white D, black E, white F and black G, which captures white 4 and makes an eye. Black can also start a ko by playing at H. He can use local threats to escape while playing the ko. Another ploy he may want to use are sente moves against the three marked white stones on the right, e.g. playing at (a), white (b), black (c), White now has to defend at (d). So that is another eye in sente.

Solution 2 : Even after Black wasted his opportunity to deal a decisive blow in Solution 1, he would still have a good game, had he played black 3. White 4 is forced and then black 5 is an excellent move, taking up position in between White's two weak groups (the one on the upper side, and the marked one on the right).

Solution 3A, B, and C: After white 5 in Solution 3C, Black could try to capture at A or B, but his stones on that side would immediately be in atari. This is called a snapback. So Black dies.

Problems

Problem 1 : White to play and make a ko in the corner.

Problem 2 : This is a standard invasion in the corner. How does White proceed ?