Life In 19x19

This is another exercise from 321go.

Task: Black to play, kill white

Click Here To Show Diagram Code

[go]$$Bc
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | X X X . X . . .
$$ | O O O X . . . .
$$ | . . O X . . . .
$$ | O . O X . . . .
$$ | . . . X . . . .
$$ +----------------[/go]

Accepted solution:

Click Here To Show Diagram Code

[go]$$BmNaN
$$ | . . . . . . . .
$$ | . . . . . . . .
$$ | X X X . X . . .
$$ | O O O X . . . .
$$ | . . O X . . . .
$$ | O X O X . . . .
$$ | . X O X . . . .
$$ +----------------[/go]

Question: Why does this kill white?
Seki??

Click Here To Show Diagram Code

[go]$$BmNaN
$$ | . . . . . . .
$$ | . . . . . . .
$$ | X X X . X . .
$$ | O O O X . . .
$$ | O . O X . . .
$$ | O X O X . . .
$$ | . X O X . . .
$$ +--------------[/go]

Click Here To Show Diagram Code

[go]$$B
$$ | . . . . . . .
$$ | . . . . . . .
$$ | X X X . X . .
$$ | O O O X . . .
$$ | O 1 O X . . .
$$ | O X O X . . .
$$ | 2 X O X . . .
$$ +--------------[/go]

leads to black to play in following position

Click Here To Show Diagram Code

[go]$$B
$$ | . . . . . . .
$$ | . . . . . . .
$$ | X X X . X . .
$$ | O O O X . . .
$$ | O . O X . . .
$$ | O . O X . . .
$$ | O . O X . . .
$$ +--------------[/go]

The last diagram is not a seki because black can play at any of the intersections inside the shape (1-1 and 2-3). Such a move puts the whole white chain in atari, threatening to capture it next move. White's only answer is to capture black three stones but by doing so he only creates a dead shape since it is now black's turn ('Three-in-a-row' if black played the 2-3 point and 'Bent three' if black played the 1-1 point).

Ah, sorry, I see!

The exercises before were all "win a killing race to save your stones".

So I was too focused on keeping my own (black) stones alive.

Sacrificing them, I see immediately how this will kill white.

I stopped playing this out on a board too early. Obviously.

Thank you!

You might find this image, which Bonobo made on SL, a useful reference:
These black groups all have the minimum required stones for living.

https://senseis.xmp.net/?SmallestGroupWithTwoEyes

Kind of just for fun, but it's interesting to think about these small shapes that are alive.