Excuse me but how is this solved?!
Unless it’s the logic of “black magically can play somewhere to force white not make the obvious two eyes” I can’t see it.
I try to like tsumego, but they frustrate me.
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Well what is black next move?
Whatever it is, W plays R1 and lives, no?
Whatever? What about a ko threat somewhere else, bigger as this life?
That’s what I mean
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So ok ko threat white answers the threat then black can take the ko back. It’s a ko.
It less big as a simple straight kill, estimated as half value of a kill, but still.
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Note that the atari on white is crucial. If white had one more liberty outside, there is no ko, white can play x
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I’ve set up an example game inspired by Groin’s position:
Maybe you want to take a look @Gia.
I hope the skill-level is appropriate.
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Me too.
I already complained about that somewhere else in the forum. 
Last night I had a live game against a player stronger than me. It could’ve been a move even game if I was able to answer correctly to an invasion and save my corner. But I didn’t.
He invaded, I didn’t kill his invasion and then played elsewhere. Then he added one more stone and said: if you’d played there, it would’ve been seki, you would’ve saved your stones.
Tsumego at first tell you to live or to kill.
But when you’re more skilled, then
you must look for the best result available.
This could be seki or ko.
I still struggle with that.
I thought my group was dead so I left that part of the board, without looking for the best result.
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Oh thanks, I’ll look into it!
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Ko < unconditional life for the side looking for life. One reason is even if you eventually won the ko fight, the killing side can play two moves in a row elsewhere. Maybe it’s not big enough to be a ko threat, but getting a move for free is almost always strictly better than nothing.
Another reason is when the opponent has enough ko threats they can kill by ko.
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I’d like to make some disambiguation here about the term “bent-4 in the corner”, because I often see confusion about it.
In the upper left, white’s eye shape is a bent-4 shape in the corner. White can live and sometimes black can make a ko.
In the lower right is a different (but related) situation. That is an example of the more (in)famous “bent-4 in the corner” situation, which has kept rules experts busy for ages.
White cannot do anything to live, but when there are no ko threats left, black can eventually take white’s outside liberties and then play at J2 to kill white (making a black bent-4 shape in the corner). So this white group is (almost) always dead.
.
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About Bent Four in the Corner
How it the bent four in the corner seki can behave under Chinese rules
I’ll try to remember that there is some ambiguity to the term.
https://senseis.xmp.net/?BentFourInTheCorner
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You are supposed to kill the 3 white cutting stones in the middle to save the lower 5 black stones.
Think about a tesuji, not about life-and-death / eyes.
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That’s the key to understand what this problem is about.
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See the post just before.
And yes basically it looks impossible to kill the white outside stones.
Yes you can! The goal here is to save the three black stones (O5) by capturing the four white stones (R5). It’s a pretty advanced problem, but we can reduce the number of variations we need to check if we notice that both black and white have three liberties, so:
- Black’s first move must take a white liberty.
- After white responds, black must be able to keep white below three liberties.
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