Sunday Problem #38

Published 21 Feb 2021 by antti (last edited 23 Feb 2021)

This is an original problem by me, inspired by a shape that occurred in an ngd league game. This is one of the few cases I’ve seen where a real-game position leads to a difficult and interesting life-and-death problem.

This problem included a small competition, where the first person to post the correct answer would get one month of Kifu newsletter as a prize. This prize went to Assu, congratulations!

Black to play.

Solution
Black fails (1)
Black’s natural-looking block of 1 doesn’t work because of white 2. If Black then tries to expand her eye shape with 3, white 4–8 directly capture the stones to the left.
White fails
Black might try the hane of 3 against white 2. White should be careful not to play 4 here, as Black can then set up a kō with 5. White can do better than this.
Black fails (2-1)
In response to black 3, white 4 is correct. Now when Black tries to expand her eye shape with 5, White throws in with 6. Black has no choice but to capture with 7, and, following . . .
Black fails (2-2)
. . . White throws in again with 8. After black 9 and white 10, there is no way for Black to live.
Black fails (3)
Black’s descent of 1 fails to white 2–4.
Black fails (4)
Similarly, black 1 fails to 2–10.
Black fails (5)
Gradually, it seems Black needs to get more creative. The diagonal of black 1 is a surprisingly good attempt, but it fails to white 2–6.
Correct first move
By process of elimination, we only have the double-keima of black 1 left. This is the correct first move.
Black fails (6-1)
White attacks with the hane of 2. Here, black 3 fails to white 4 – after black 5 and white 6, Black cannot form two eyes.
Black fails (6-2)
Black 3 here is no better. White plays another hane at 4, and after black 5 White kills with 6.
Black fails (6-3)
Black’s block of 5 may look like a more promising attempt, but here white 6 kills.
Correct moves 2–4
Here it is correct timing for Black to exchange 2–4.
Black fails (7)
Playing consecutive throw-ins first with 3 and then with 5 here is sometimes good technique, and in general has the correct idea for this problem, but unfortunately White now kills black with 6–10. However, now we are getting really close to the solution!
Solution
Instead of throwing in at 7, Black can only set up the kō with 5. White is forced to play 6, and now Black starts fighting a desperate kō with 7.
This is either a one or two-move approach kō, depending on what White wants. White can ignore Black once, respond to
a with b, and still get to fight a regular kō; or White can ignore Black twice while Black plays a and b and still get to fight a kō.

Comments (13)


comorphism wrote 1 month, 3 weeks ago:

Q19. It makes an eye at R19 and traps the white stone. If white P18 then black O18, otherwise black can take P18. Additionally, white's atari at P17 does nothing to falsify the eye.


sirocco wrote 1 month, 3 weeks ago:

O18, T18, Q18 perhaps?


TomiP wrote 1 month, 3 weeks ago:

P18, if white plays Q19 black can cut Q18, White P19 Black O19 and eye will become miai in S19 or Q19


TomiP wrote 1 month, 3 weeks ago:

Actually white T18 ruins my previous answer. So the answer is P19. If white plays T18 black will get two eyes by playing Q18


ogee wrote 1 month, 3 weeks ago:

Well Comorphism got it.

Q19-T18-S19 white may try then bad continuation 1) P18-O18-O19-P19 which doesn't work so instead white optimizes endgame by 2) O16-P17-O18


Lebertran wrote 1 month, 3 weeks ago:

My solution is p19:

P19, P18, O18, Q19 and after O19 there is miai between Q18 and T19.


Lebertran wrote 1 month, 3 weeks ago:

I mean T18.

Q19 doesn't seem to work.

Q19, P18, O18, O19, P19 and white can atari at O16.


ogee wrote 1 month, 3 weeks ago:

O19-T18-S19-Q19-Q18-P18-O18

Or
O19-P18-O18-T18-S19


Lebertran wrote 1 month, 3 weeks ago:

Ok, so my first attempt is not the best line for white.

Here my last solution:

P19, T18, T17, T16, Q18, S19, R19, P18, P17, O18, and than there is ko at T 19 for the second eye.


Tanukki wrote 1 month, 3 weeks ago:

O18 Q18 Q19 T18 S19 P19 P17 makes a ko. white takes first at p19, and there's no local threat.
It's not a very "interesting" solution, so I'm probably missing something...but I think I eliminated all the other moves...


sirocco wrote 1 month, 3 weeks ago:

Trying again. O18 is still my first move.

If white P18 black descends to O19. If white hanes at T18 black can safely atari at Q18 to live. Even if white hane's again, the two eyes can't be taken away.

If instead of white P18 if white descends at Q18, black will counter with P18. Even if white descends again, black can play T18 and is fine. If white plays T18 instead, black will play Q19 and the eyes at P19 and R19 are miai.

If white plays black blocks at Q18. White O19, black connects at P18. If white connects at P19, black plays T18 and lives. If white T18 instead, black plays P19 and lives.

Still looking for a stronger response from white. Can't find one yet. Will keep trying :D


Assu wrote 1 month, 3 weeks ago:

P19,T18,T17,T16,R19,Q18,T19,S19
3-step ko for black, white starts


antti wrote 1 month, 3 weeks ago:

Assu got the solution and the prize! I have now added the ‘solution’ section, which also includes most failing attempts.

For now, I cleaned the comments that had no solution attempts and left the solution attempts in place.