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?
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.
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.