Is black allowed to take the ko?

my intuition is to take the ko, it seems urgent. If there wasnt any urgent situation, I would say top right is very big, so c15 would be my move.

So let me compare those: If I take the ko, white seems to have 3 options:
1. Try to fight the ko (most likely r14 as ko threat)
2. Say the ko is small and take a big move that is not a ko threat (I nominate c14 or at least that area)
3. Submit at g3

3 seems very bad and in that case it was definitely good to take the ko, so I`m going to discard. In case of 1, both answering at s14 and trying for mutual destruction at g3 seem plausible. I think I would be tempted to try g3, but for this argument I think it is enough to say, if I just answer the ko threat it was already a good idea to take the ko and make him spend that threat that also loses him a liberty.

So the variation I really have to look at is 2.

On the other hand, if black doesnt take the ko, but plays upper left, white is supposed to punish him by doing something in the lower left, so I should figure out what that is.

My variations:

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[go]$$Bcm25 Variation, black takes the ko
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$$ | . . . O . . . . . . . . . . O . . . . |
$$ | . . . , . . . . . , . . . . . , X . . |
$$ | . . . . . . . . . . . . . . . O X . . |
$$ | . . 2 . . . . . . . . . . . . O . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
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$$ | . . O . O . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . X X O X . . . . . . . . X . . . |
$$ | . . X , O 1 O X . , . . . . . , . . . |
$$ | . . . . O O 3 O . . . . . . . X . . . |
$$ | . . . . . . 4 . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

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[go]$$Bcm25 Variation, white makes the ko big, black resists
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$$ | . . . O . . . . . . . . . . O . . . . |
$$ | . . . , . . . . . , . . . . . , X . . |
$$ | . . 1 . . . . . . . . . . . . O X . . |
$$ | . . . . . . . . . . . . . . . O 4 6 . |
$$ | . . . . . . . . . . . . . . . . X . . |
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$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . O . . . . . . . . . . . . . . |
$$ | . . . . 2 X . . . . . . . . . . . . . |
$$ | . . . X X O X . . . . . . . . X . . . |
$$ | . . X , O 3 O X . , . . . . . , . . . |
$$ | . . . . O O 5 O . . . . . . . X . . . |
$$ | . . . . . . 7 . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

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[go]$$Bcm25 Variation, white defends the ko
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$$ | . . . O . . . . . . . . . . O . . . . |
$$ | . . . , . . . . . , . . . . . , X . . |
$$ | . . 1 . . . . . . . . . . . . O X . . |
$$ | . . . . . . . . . . . . . . . O . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
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$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . O 5 . . . . . . . . . . . . . |
$$ | . . . . 3 X 4 . . . . . . . . . . . . |
$$ | . . . X X O X 6 . . . . . . . X . . . |
$$ | . . X , O 2 O X . , . . . . . , . . . |
$$ | . . . . O O . O . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Actually I like all 3 variations for black...

In the first one, making wheite answer 28 seems like a success. As for the evaluation, upper left is about as good as lower right, upper right is a joseki with even number of stones, so I`ll call that even and lower left seems good for black given that its his move and he is nicely out. White is also not too bad though, he is fine on the bottom (black cant really start the ko again) and the 2 stones on the left are little funny, but work with the top left...

Variation 2 is terrible for white, because 26 is now a wasted move, so forget that.

Variation 3 Is again hard to judge. I call the top even, both corners even number of stones in reasonable places. On the bottom withe seems locally better in bottom left but does it make up for the bottom right. I dont really think so, black adds a move on the left(e8?, c10?) and he seems fine... Also I´m not sure about the whole variation altogether...

The variations seem very close and maybe even Indicate c15 was better, but my gut says take the ko and it feels really good to make white defend with 28 and probably once more if you connect, so I´ll go with that. dont know what white is supposed to do, if black doesnt take the ko though.

This position is from GoGoD 1902-07-00a between Tsuchiya Shugen (W) and Karigane Junichi, both 4 dan. Note that Tsuchiya has already played the press in the top right corner.

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[go]$$Bcm25 Moves 25 to 30
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$$ | . . . , . . . . . , . . . . . , X . . |
$$ | . . . . . . . . . . . . . . . O X . . |
$$ | . . . . . . . . . . . . . . . O . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
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$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . O . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . X X O X . . . . . . . . X . . . |
$$ | . . X , O 5 O X 1 3 . . . . . , . . . |
$$ | . . . . O O 6 O 2 . 4 . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Instead of taking the ko right away, Karigane extended on the 4th line, losing 12½% to par by Elf's reckoning. Tsuchiya crawled on the 3d line, but , , and are all errors, each losing more than 6% to par.

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[go]$$Wcm26 Variation for
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$$ | . . . O . . . . . . . . . . O . . . . |
$$ | . . . , . . . . . , . . . . . , X . . |
$$ | . . . 6 . . . . . . . . . . . O X . . |
$$ | . . . . . . . . . . . . . . . O . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
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$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . O 4 . . . . . . . . . . . . . |
$$ | . . . . 2 X 3 . . . . . . . . . . . . |
$$ | . . . X X O B 5 . . . . . . . X . . . |
$$ | . . X , O 1 O X X , . . . . . , . . . |
$$ | . . . . O O . O a . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

fills the ko and connects. After plays the turn, allowing to capture the stone. Then approaches the top left corner.

If turns at a instead, White plays atari at 28. If connects at 30, White crawls at a, threatening the hane on the 4th line.

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[go]$$Bcm25 Black takes ko with sente
$$ ---------------------------------------
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$$ | . . . O . . . . . . . . . . O . . . . |
$$ | . . . , . . . . . , . . . . . , X . . |
$$ | . . . . . . . . . . . . . . . O X . . |
$$ | . . . . . . . . . . . . . . . O a . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
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$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . O . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . X X O X . . . . . . . . X . . . |
$$ | . . X , O 1 W X b , . . . . . , . . . |
$$ | . . . . O O 2 O c . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

The saying goes that there are no ko threats in the opening. OC, there are exceptions, and White has the push (de) at a, but Black's threat to capture the stone is huge. This kind of sente ko arises frequently in the opening, though often closer to the edge, and it is almost always right to take it immediately. Note that Black's threat to capture is now the greatest, before White has played at c. Since Karigane took the ko with sente later, I suppose that he did not do so right away because he thought that the extension to b carried a bigger threat, the turn at c. And Tsuchiya did reply at c. But it appears that allowing White to fill the ko leaves too many weaknesses in Black's shape. And the bottom side is not so big, either.

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[go]$$Bcm25 Main line for
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$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . . . . 5 . O . . . . |
$$ | . . . , . . . . . , . . . . . , X . . |
$$ | . . . 3 . . . . . . . . . 7 . O X . . |
$$ | . . . . . . . . . . . . . . . O . . . |
$$ | . . . . . . . . . . . . . . . . X . . |
$$ | . . 4 . . . . . . . . . . . . . . . . |
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$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . O . . . . . . . . . . . . . . |
$$ | . . . . . X . . . . . . . . . . . . . |
$$ | . . . X X O X . . . . . . . . X . . . |
$$ | . . X , O 1 O X 6 , . . . . . , . . . |
$$ | . . . . O O 2 O . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

After taking the ko with sente, Black does not extend on the bottom side, but approaches the top left corner. After the two space low pincer, Black shifts to pincer the White stones in the top right corner. Now the bottom side is big enough for White to play atari with . continues the attack in the top right. Another example of corners before sides.

I figured that the odds were high that this was an only move problem because Elf's top choice, taking the ko, got a whopping 319.3k rollouts, with its second choice getting fewer than 1.5k rollouts.

Speculatively:
Current position:
1. Taking the ko defends a lot of cutting points and then H4 becomes light (W is already pretty much alive anyway). White might play the R14 ko threat but that loses W a little bit.
2. Connecting H5 doesn't seem to gain anything (except a larger follow up, but W just has to respond solidly.
3. Extending J4 is possible but risky even if W just fills the ko.

Opp to play
1. Double atari!

Past:
1. W's shapes are fairly good but B has sente and W is playing a little bit too close to B's lower left group. B can probably counter-attack on the left. I think I prefer B.

Check:
1. easy enough