Sunday Problem #10

Published 9 Aug 2020 by antti (last edited 3 Sep 2020)

This is the cover problem of Mateusz Surma’s tsumego book series You won’t get dumber while thinking.

Composing go problems is tough work. It can take hours just to come up with one good problem and, once you publish it, the problems themselves are not even copyright-protected – only the problem collection and the answers. This is why I like to support original problem creators when possible.

This problem is intriguing in that it doesn’t involve very complex reading; instead, most of the time is needed to figure out just what is going on in the first place.

Black or White to play.

Solution
Black can of course start a kō fight with a while White can start a kō fight with b, but to solve this problem, we need to have a closer look.
Note that White is unable to try to avoid a kō with white
a, as then black c kills unconditionally.
If Black were to play locally, 1 is her best move. White then plays 2, and a kō fight for White’s life starts.
After 2, White is one move away from living with 8 points, while Black is two moves away from killing White with 27 points. Therefore, the estimated score is 3⅔ points for Black after an equal number of moves.
If White were to play locally, he can only start a kō with 1. Black then captures with 2.
After 2, White is two moves away from living with 8 points, while Black is one move away from killing White with 27 points. Therefore, the estimated score is 15⅓ points for Black after an equal number of moves.

In other words, it is generally correct for both players to ignore the position until one of the players finds a timing when they have enough threats to win the kō (or else get a big enough compensation elsewhere). This type of a kō fight is sometimes called ‘reserve kō’ (, horyū kō).


Comments (5)


sig wrote 2 months, 1 week ago:

I guess it’s kind of important to know the ko-threats elsewhere?


antti wrote 2 months, 1 week ago:

That will make a big difference, of course, but there is an important insight to realise even if the position is just considered statically (without the rest of the board).


Fanshawe wrote 2 months, 1 week ago:

This looks like a thousand-year ko to me. W can start a ko where B gets to take first: not a huge privilege by any stretch. But if B plays first, W can ignore that and die or, more likely, to start the same ko, except that now if B wins it, B has a wasted move (A3 would be my preference for endgame reasons in case W wins the ko, but J1 also works). So it looks like neither side might be in a huge rush to start the ko.


antti wrote 2 months, 1 week ago:

You’re close! ‘Thousand-year kō’ is not exactly the right classification, since this shape has no way to end up in a seki, but beyond that the two are very close.


antti wrote 2 months ago:

‘Solution’ added.


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