tapir: in a recent blitz game my opponent tried to play similar to a known sequence when the marked black stone is a near approach. how can i exploit this best. connecting at a in sente, or starting big fight with b? in the game i played the second one and got a nice result.
Herman: b doesn't really work. White should have split black immediately
tapir: Hmm. So white made the bigger mistake... :( Thank you for the comprehensive answer.
Herman: I don't think it is the bigger mistake, actually. If black had played immediately, instead of the overplay , then white can reasonably ignore and play tenuki. Exchanging for then becomes a good followup-for black. Later, white can play endgame with ---, and you get the situation in this diagram. So I think the result is not bad for either player, but is par for the situation if white takes sente (As opposed to the "correct play" diagram below, which is gote for white).
Herman: This really hurts the stone. But black needs a move in the corner, so has no time to respond.
Herman: a further note: is the real error in this diagram, black should play a, as shown below by Bill.
Andy: To avoid the punishment, black might try to play as in 4473enclosure33invasion, which seems less bad, since black will end in sente and be able to do something about . Still, one has to think is misplayed here.
Herman: instead of , white should stretch (to ). See also the sequence posted by Bill further down.
? hard to find a great move for black here.
tapir: If white tenukis now, can she still resist Black a later? (If the ladder isn't working for white.) I think she can't, so is kind of sente, isn't it?
Herman: Yes, the black move at a is good aji for black.
As a follow-up, white can play something like this.
Note though, that black had perhaps better choose to forgo and instead jump out to a in response to . It is a success for white anyway, as black will have a weak group in white's sphere of influence...
Correct play for black would have been this . Black should play this early, because a white move at a would take a big corner while attacking black.
tapir: basically i didn't play a earlier because I wasn't sure that it takes the corner and don't want to play moves while knowing that i don't understand them.
Herman: After a, white may still lose the corner, but only under unusual circumstances, in which case white is probably getting plenty of compensation elsewhere. In this position, a is the only move played by professionals if they get first move in this corner. (For black, is the most common, but invading at 3-3 is possible, as bill shows below.