Double Go / Discussion

Better defence?  

Herman: Correct me if I'm wrong, but isn't this a better defence for white? Now black's invasion has become pointless, and white gets 7 points, winning by 2. (Black must play at least one of the a marked points to prevent another point for white there)

You're right! Even more reason for B not to play this line. SiouxDenim

Example – part 2  

W cannot defend both attacks, so uses half a move to defend the cut and half a move to make an attack on the bottom B stones.

Herman: Shouldn't white's second half move be at a? That way, white can live and will not lose any stones...

Example – part 2  

W only has two eyes.

Better  

Herman: Whoops, missed that :)
So how about this then? White will either make three eyes on the left, or get both moves a and get an eye there...

Boring example  

This is about the simplest problem I can think of. B to kill some W stones.

Matti: Black cannot kill.

W chooses to save the left group  

White can choose to make 3 eyes for one group

B attacks the other group  

W needs to make 3 eyes. Capturing only produces 2 eye spaces.

W removes legal plays for B  

Now Black has a parity problem. The group on the right needs 3 stones playing within it, with a single stone played on the first move. Black has no sensible places to play the other half of the move. When I first analysed this, I assumed that Black could pass for half of a move. The rules as currently written don't make this clear.

B attacks the other group - option 2  

Black can also attack the group on the right like this.

B chooses a group to kill  

There are obviously only 2 eye spaces now on the left, so W makes the group on the right obviously alive and asks B to prove the group on the left is not seki

W clarifies the life of the group on the right  

B has a parity problem again.

Less boring example  

I think that the original problem is unambiguous if B has another liberty in the middle group.