I’ve started working through a tsumego book written for 25k-20k players and the attached position was presented with the mission of saving the six black stones in the corner. The book’s answer is that the correct play is to start a ko fight at E1. I don’t see how that helps after white plays D1:
- Black E4, white E1 - white is safe, black is dead.
- Black A4, white B5 - white is safe, black is throwing away stones
- Same for Black B5, White A5, Black A4, white can easily tear black apart from there
Even when I pit katago against itself on this problem, black resigned after white responded to E1 with D1. So what am I missing?
tsumego30.sgf (198 Bytes)
E1 D1 A4 B5 E1
White plays a ko threat.
Black doesn’t respond to the threat and plays E4 instead, taking 3 stones.
If a tsumego, as is almost always the case, only shows a local position and not the entire board, it should be assumed that there may be ko threats for either player outside the displayed area.
So, it’s enough to simply “achieve ko” without worrying about how that ko will turn out.
For further reading, see the Sensei’s Library pages on ko and tsumego conventions.
Also, as jlt says, A4 is indeed a local ko threat.
On a pedantic note, if Black really needs two threats then he can first play at B5 and only then at A4. However, these would be very painful ko threats, sacrificing stones.
A5 is also a threat, but simply worse than B5, since the A5 stone might be captured later.
On a related note, btw, I found this position interesting:
It’s White’s turn to come up with a ko threat if he wants to try to kill Black in this potentially seki shape.
There are five atari available – can you rank them from best to worst?
Part II: Do you think it’s correct for White to try to kill at this point, or should he play away?
Part III: Who’s better?