Psychological Tesuji / Black keeps sente

Sub-page of PsychologicalTesuji
False Tesuji  

B1 seems to be a tesuji, but in fact it is not.

Gobble, gobble  

If White responds to B1 with the seemingly natural move of W2, Black would keep sente because B3 puts the White group into atari and forces W4.

Possible refutation  

Instead, White can play W2 as shown.

Possible refutation, continued (B7 at black+circle)  

After W6, Black could capture back, but that would lose sente.

Variation (B7 = tenuki)  

To retain sente, Black could play B7 elsewhere instead, which allows White to follow up with the sente sequence of W8-B9. In terms of points, the resulting position is equivalent to the alternative of allowing White to play at white+circle in sente earlier (Black captured one White stone, while White has captured two Black stones and has an extra stone at white+square). But this position is worse for Black because the ko threat at a is lost (see aji keshi). If Black wanted to take sente, it would be better to simply play B1 elsewhere rather than playing out the entire sequence from B1 to B9, which loses one ko threat for nothing.

(Note: Technically, B5 in the third diagram is wasting a ko threat as well. If Black is not going to follow up locally with B7, it would be better to play elsewhere as early as B5. But even then, White can still play W6 at W8, which again forces B7 at B9 and leads to the exact same final board position. Besides, this result is still inferior to the alternative of playing elsewhere with B1 from the very beginning, as B5 is a smaller ko threat than a in the scenario where white+square is absent.)


With all of the above in mind, the trick play at a is clearly worse than the correct play of B1.

Hyperpape: I'm confused, since the parent page says that this play is not incorrect and keeps sente, but this page seems to directly contradict that.

MrTenuki: I think what happened is that someone found a refutation to the trick move. For the sake of completeness, I reintroduced the information from the previous version of the page.

Thanatos13: ... ... this seems funny. after 5, that is atari on the big group, thus 6 is completely necessary. if you wanted sente, then tenuki right away.

MrTenuki: The question is whether Black can take sente with B7, not whether White can take sente with W6...

Thanatos13: Oh, okay then. Sorry about that. A couple of growth points though. If you're not going to play B7 at B5, B5 is dubious and you have really just wasted a ko threat. W2 in both diagrams are also not as big as just taking the laddered stone.

Psychological Tesuji / Black keeps sente last edited by MrTenuki on May 9, 2012 - 17:22
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