Life In 19x19

Hi all.

I ask you some sequence to better understand the outcomes.

In the game below I follow a pro's advice against the bad 8 move pincer. After move 13 Katago gives in fact advantage (around 65%, which is a lot starting around 45%) but I can't properly understand why. Of course I notice black's thickness, clearly, but I think that it's easy to counter: the pincer stone after something like P17 and a move like C6 afterwards can erase all, and top lefts corner territory altough not a lot it's 15-18 point in sente and well that's not bad either.

Can you give me some sort of explanation where the advantage is?

Thanks!

I think there are two flaws in your assessment.

(1) You are overestimating the size of White's upper-left corner. Black has forcing moves against it. To keep things simple, imagine a Black sagari 9descent) on both sides followed by a hanetsugi. That leaves White with more like 10 points. Following the simplest rule for evaluating Black's thickness, that is 3 points for each stone in the wall (but you don't normally count 1st-line stones in a wall), that would be 7x3 = 21 points for Black. But wrap-around thickness is usually counted as significantly more.

If you look at the rest of the board, each side has two stones (a kind of equality), so the position is B21++ vs W10. Black is almost a whole move better off.

(2) You are misunderstanding sente. Sente in fuseki means the initiative. Black has the initiative here (i.e. he is the one who really decides what happens next in the game as a whole). White's stone at the top is far too close to Black's thickness. That means he needs another here, so he has to use his next move to defend it. He is the one in "gote". Furthermore, while he does nominally then have three stones to Black's two in the board area outside the upper left, two stones at the top will essentially be doing the work of one. Simplifying again, we can say that White's extra move at the top is worth more like just half a stone. In addition, even a two-space extension there will remain weak because of the perfect thickness and so Black can expect to overconcentrate White further in this area.

In toto, Black is clearly ahead by half a move or more after just 13 moves.

Click Here To Show Diagram Code

[go]$$Wc Position at move 13
$$ ---------------------------------------
$$ | T T T T . . . . . . . . . . . . . . . |
$$ | T T T T . . . . . . . . . . . . . . . |
$$ | T T O T O X . . . O . . . . . . . . . |
$$ | T T T O . X . . . , . . . . . , X . . |
$$ | . . O . X . . . . . . . . . . . . . . |
$$ | . . X X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

White has no more than 14 points, but that's just points: White is completely surrounded and doesn't interact with the rest of the board, while Black's group has no weaknesses and faces two directions; the influence towards the left side of the board is not canceled by any white stone. In addition, the sequence is not really sente for White because White needs to defend K17. If White plays at P17 next, then Black may respond with Q14 and then White still needs to add a move to prevent an invasion between the two stones, thus letting Black reinforce is upper right corner in sente. Or Black can put directly a stone inbetween.

Another way to have it is to do some tewari-like feeling about the efficiency of the shapes. Like, it's just seems to me that white's move at 3-3 hunkering in the corner, overlaps a bit too much white white's other stones.

Like, if it were like this, which seems like a plausible kind of position if white was overly insistent on holding the corner.

Click Here To Show Diagram Code

[go]$$Wc White's turn
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . . X . . O . . . . . . . . . . |
$$ | . . . O . . . . . , . . . . . , . . . |
$$ | . . . . X . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |[/go]

Would white really want to play the 1-2 3-4 exchanges? Black gets stronger outside and fixes all cutting points, white gets two stones that overlap heavily with their existing stones, making a few more points.

Click Here To Show Diagram Code

[go]$$Wc White hunkers in the corner
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O . 1 X . . O . . . . . . . . . . |
$$ | . . . O . 2 . . . , . . . . . , . . . |
$$ | . . 3 . X . . . . . . . . . . . . . . |
$$ | . . X 4 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |[/go]

To add my 2¢ to the excellent replies so far, back in the day I used to estimate the value of a wraparound wall as 40 pts. A rough estimate, to be sure. I'm OK with giving White 14 pts. in the corner. Either side can make a hane on the 1st line. Let's also give White 14 pts. for the stone on the top side, even though the bots tell us that that is an overestimate. So in the top left quadrant we estimate 40 pts. for Black and 28 pts. for White, a difference of 12 pts. in Black's favor. Each side has made 5 moves there. At this stage of the game the net result of 5 moves by each player should be zero. Since KataGo only gives Black a winrate difference of 20%, Black may only have made a gain of 5 pts. or less. But still, the play in the top left is a disaster for White.

Thanks! That helped

Lightvector's first diagram of a 4-4 that ignored a double approach, got surrounded at 5-5, and then lived at 3-3 raises an interesting question. Yes, that particular way of living in the corner with 3-3 after being surrounded is bad for white, and the double kicks make it worse, but the more general strategic principle of a 4-4 allowing itself to be surrounded and living in the corner is actually not so bad (as we used to think) and is one of the key lessons of AI fuseki. But if you do allow your corner to get surrounded like that, you should do 3 4 things:
1) get something sufficiently valuable from the tenukis, for example in my Leela Zero Opening Gospel video I look at the Kobayashi denied fuseki, and munching a 3-4 after an ignored press is well worth allowing your 4-4 to get surrounded, see https://youtu.be/vupa_IM1wWY?t=521
2) live efficiently later, which means play 1 kick and 1 knights move to 2nd line, or 2 knight moves, not 2 kicks (makes black too strong and still weakness inside), and not 3-3
3) live at a good time later, not too early, not too late. If they play 5-5 and you immediately live in the corner in gote, it's probably a good exchange for them because they surround you in the corner (good topologically, making their 2 approach groups connected) and get sente for another big opening point, and that's better for them than if they took the big opening point and you got to play first in the double approached corner which not only gets your group out to the centre but separates their groups and gives them potentially weak groups. But it might not actually be so great for them depending what happens, maybe they would later wished to have taken the territory and base in the corner themselves. If living creates aims of cuts on the outside wall which doesn't have ample eyespace then it raises in priority. In general it might come a dozen moves later in the opening, it's still a pretty big opening move.
2 / 3 a) it's also ok to not live there and let the opponent spend another move to kill it off whilst you do something else valuable, and even then you still have useful aji.
4) not make your previous stones sad (this is a general principle of efficiency), but in OP example as John points out the white top side pincer stone becomes weak when black surrounds the corner and gets strong (stay away from thickness/strength), so has become a liability, and would be better placed elsewhere.

Here's an example game of mine putting these ideas into practice. Note:
1) tick, I got nice wall press and then the sente turn
2) tick
3) faily-tick, though checking with a bot now it says it would have been good to have lived there even earlier
4) tick, no weak stones close to black's wall