4-4 point diagonal attachment, low corner enclosure
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Another reason is that when the outside White stones are strong enough, then White can look forward to attaching at and then playing at the 3-3 point. White can look forward for reducing Black's territory or increasing White's influence, or both.
(However, White does have the choice of fighting instead of trying to live in the corner-- see 4-4 point low approach low extension tenuki, diagonal attachment, 3-3 invasion for more details.)
(5d analysis by Minue)
Correct plays(locally speaking) for black are showed in following 2 diagrams.
The intent of is to defend B's corner securely. In this diagram, , moves might(may?) be put off, depending on a condition of whole board situation.
Another way to play is ,which is somewhat modest. In this case, it is often seen for w to invade at 3.3 immediately.
This is the basic diagram. There are several questions to answer here:
- Is significantly worse than a?
- How does a pincer stone at B affect the matter?
The evaluation is done using KataGo as a reference.
In a symmetrical parallel all star point opening
- is 0.2 worse than invading the corner (-0.9 vs -0.7)
- The diagonal attachment reverts back to a 0.7 lead as opposed to the preferred move, a 3-3 invasion in the opposite corner
- is the blue move
- loses 0.2 with respect to a, the blue move
In case of a Black san ren sei, White's lead is 1.1 to start with. Again, 3-3 invasions of either Black corner are preferred.
An inside approach at loses 0.6 and we start at 0.5. This is our reference for the discussion.
Interestingly, putting pressure on the white stones may prompt White to treat those stones lightly and approach at . Things can become quite heated from that point onward.
Suppose this time that Black has played a (suboptimal) move on the side . White's approach on the outside will result in this sequence. Traditionally it was considered bad for Black to kick at but nowadays, due to AI evaluation, making White stronger towards an open side is considered playable because Black gets sente.
If instead Black plays low, he'd lose about 0.7 in this position:
In this case White's stones are considered more resilient, allowing White to take sente. Both a and b are now available.
In a case of synchronicity Michael Redmond has just released this video