Always trying to refute the expert solution I thought I found the refutation.

Click Here To Show Diagram Code

[go]$$c
$$ +-----------------------
$$ | . . . . 6 . 1 5 . . .
$$ | . O 2 O X X . . 3 4 .
$$ | 8 X 7 X X O X X O . .
$$ | . O X X O O . . O , .
$$ | . O O O . . . a . . .
$$ | . . . . . . . . . . .[/go]

But, of course, it fails. W's containment is too weak. B easily walks out at a. The problem depends on W being weak around there. Correct?

Yes, I quite like the problem for the fact that it contains such subtle secondary concerns. This one, as well as the double ko death in the corner in entropi's attempt.

Nice sequence cyclops, I would not even consider white giving up those two stones. But if escaping at h15 (or g15) does not lead anywhere (maybe because white is already too strong outside), then black can still play at a18, leading to a 2 step (or 3 step??) ko for life. This time no double-ko, I guess. Or am I again missing something?

_________________
If you say no, Elwood and I will come here for breakfast, lunch, and dinner every day of the week.

Click Here To Show Diagram Code

[go]$$c
$$ +-----------------------
$$ | . . . . 6 . 1 5 . . .
$$ | 7 O 2 O X X . . 3 4 .
$$ | . X 8 X X O X X O . .
$$ | . O X X O O . . O , O
$$ | . O O O . . . . O . .
$$ | . . . . . O . . . . .[/go]

Thx for the compliment. I wouldn't mind to get some more. But I have no idea where your ko's would arise, b17 seems dead as a stone to me.

Nowhere, it is dead indeed. I hallucinated a16 would produce a ko, which of course does not. Avoiding such no-brain type obvious mistakes could make me somewhat stronger perhaps...

_________________
If you say no, Elwood and I will come here for breakfast, lunch, and dinner every day of the week.

This is an ancient problem. In ancient problems escape was a factor. Modern compositions tend to be self contained. Different conventions, I suppose, the ancient convention being that the unshown part of the board is empty, the modern convention being that the unshown part of the board is unknown.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

It can be found in "Shikatsu Myoki".

Click Here To Show Diagram Code

[go]$$
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . O . O X X O O . . . . . . . . . . . |
$$ | . X . X X O X X O . . . . . . . . . . |
$$ | . O X X O O . . T , . . . . . , . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |[/go]

Please take notice that there is no White stone on the marked point.

_________________
The really most difficult Go problem ever: https://igohatsuyoron120.de/index.htm
Igo Hatsuyōron #120 (really solved by KataGo)

Click Here To Show Diagram Code

[go]$$W White to kill
$$ | . . . . . . . . . . .
$$ | . . . . . . . . . . .
$$ | . W . . . . . . . . .
$$ | . . O O O O O O . , .
$$ | O O X X . X X O . . .
$$ | X X . X . . X O . W .
$$ | X X . . . . . X . . .
$$ ----------------------[/go]

Ah, I see - so that's why modern problems have stones like the marked ones, so you know to focus on the corner?

heh, I like this problem. I should do more Tsumego.

@jts: That's not a modern problem!

@Marcus:

@Herman: Interesting. Now I need to figure out what I missed.

I was relieved when someone suggested the shown 2 as my first instinct was that black still didn't have two eyes.

Assuming the rest of the board is unknown the white stones still haven't got two eyes either... I had been expecting a cleaner solution but I think that it is a more solution than you often see in modern problems.

I haven't played go for a very long time (if you don't count a computer program that was described on one website as the strongest free program but so many weaknesses didn't have a live stone.) and I am not sure that I would have expected the line to run to 'a'.

It's just my way of saying "I'm too lazy to figure out why this doesn't work", but I liked the variations I spotted, and couldn't immediately see a refutation. Silly me, I should have known there would be one ... I still haven't taken the time to look at it yet ... and, I just found it, I think, while typing this:

here a sgf of the ancient solution from Herman's link:

...I'll tell you, that's not the solution I expected.

_________________
Someday I want to be strong enough to earn KGS[-].

I know! Actually, it's a great problem for studying how many different ways black can live, if White deviates almost anywhere.

Indeed, such an innocent looking position being the (one of the) most difficult problem(s) ever. I wonder what comes next!!!

_________________
If you say no, Elwood and I will come here for breakfast, lunch, and dinner every day of the week.

Well, if we're posting innocent looking positions....

Click Here To Show Diagram Code

[go]$$W White to play
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . O O . O O . . .
$$ | . . . . . . O . .
$$ | . X X . . . . O .
$$ | . . . . . X . O .
$$ | . . . . . . . . .
$$ +------------------[/go]

Herman solved this intriguing tsumego in the forward drive. How about going in reverse drive.

Click Here To Show Diagram Code

[go]$$c W to move
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . a . O X X O O . . . . . . . . . . . |
$$ | . X . X X O X X O . . . . . . . . . . |
$$ | . O X X O O . . O , . . . . . , . . . |
$$ | . O O O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

It seems Wa is the only move to prevent unconditional black live. So this diagram solves the tsumego backwards. How far can we thus rewind the position?
Is reverse tsumego a respectable aberation? Or should I seek help? Had it a name already?
And then what about allowing tenuki ? ( But not two consecutive tenuki, and only if alternate reversing is impossible )