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Why is this White 2 incorrect? How to deal with it? [#280]

82.69.58.30: Why is this White 2 incorrect? How to deal with it? (2006-01-30 21:55) [#970]

I have seen that when Black attempts to live by playing the diagonal move of , the placement of is the correct response. But what happens if is played at A instead?

Presumably this is a sub-optimal/incorrect move as I haven't see it discussed in any references to this shape.

My question are:

Is White A incorrect in response to in this position?

If White A is indeed sub-optimal/incorrect, why is this?

How does Black deal with the mistake to punish/take advantage?

66.38.216.89: small differences (2006-01-30 22:51) [#971]

Alex: Looks like it works to me, from the point of view of killing. However, the "correct" move only allows Black one ko threat, while your move allows at least three. Also, if the surrounding White stones are not alive and the Black group will eventually need to be taken off the board, your move may lose a point because it does not fill a Black liberty.

82.6.108.41: Best way to kill gives black less useful sente move (2011-06-18 13:31) [#8613]

Uberdude: The suggested move at A does also kill, but it is not such a good way to kill because the moves it gives black in sente in the failed attempt to live are more useful. To show this, let's slightly weaken the surrounding white stones and add some friendly black stones on the outside:

After black can't connect to the marked stone so is dead in the corner. Make sure you don't play at a or black can cut and fight a ko to live.

This gives black the descent to in sente, which means he can connect out on the first line to safety.

The point about fewer ko threats is also a good one. Another consideration when killing is how many liberties the dead group has, should there be a semeai and you need to actually capture it.

As an aside, because black's descent to the first line is sente, in the starting position is wrong, and he can live as shown. ( could be at a if that had some useful follow up on the black group there, but means black could live bigger at b instead of due to the extra outside liberty):

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