BQM 287

    Keywords: EndGame, Question

Moved from L-group/Discussion:

Srki: Ogawa Tomoko says that W1 in diagram below was worth sixteen points. How? Can someone explain?

Ogawa's analysis  

Chris Hayashida: Calculating the endgame can be confusing. It's easiest to look at the basic situation with each player getting to move first.

Starting position  

Black moves first

Chris Hayashida: First, let's say Black gets to move first. This is how he will play, ending in gote.

Black gets first move  
Black gets first move, cont.  

Chris Hayashida: This is the tricky part. White won't play a right away, because it's gote. (Technically, it's reverse sente, but don't worry about that for now.) White will probably play somewhere else on the board instead.

Black gets first move, cont.  

Chris Hayashida: Eventually, Black will get around to playing the sequence starting with B1.

White moves first

White moves first  

Chris Hayashida: Now let's calculate if White gets first move.

White moves first, cont.  

Chris Hayashida: In a similar vein, Black won't play a immediately. There are still bigger moves on the board.

White moves first, cont.  

Chris Hayashida: Eventually, White will get the sequence starting with W1.

Comparing the two positions

Black moves first, final result  
  • Black 10 points + 2 prisoners
  • White 5 points
White moves first, final result  
  • Black 5 points
  • White 13 points + 1 prisoner

Chris Hayashida: As you can see, the first diagram has B+7, and the second has W+9. It's a sixteen point swing.

Other Questions

What about this?  

Srki: What about this?

What if Black plays here?  

Chris Hayashida: The problem with this line of play is that there is ko. I think White is trying to avoid ko. Note that this is *calculation* so this is (in part) why White is trying to read ahead the safer variation.

Srki: Chris, thank you very much, this was very helpful. Everything is clear now.

No ko  

This isn't even ko. If white plays captures after B3, black will play B5 and white cannot defend the double atari because the all the white stones are captured at a. If white plays at B5 instead of W4, black plays a immediately and the result will be equally devastating.

Bill's Note

Top side first  

Bill: W1 - B4 is sente. If Black allows W4, Black dies, as per the L-group.

Note that White does not have this sente if Black captures the white+circle stones.

Top side first, cont.  

Bill: Next, W1 is larger than W3. After W1, W3 - B6 is sente.

Top side first, var.1  

Bill: If W1 - W3, Black continues with B4 - W7, which is sente. The corner is small by comparison.

Because the surroundings are not shown, we do not really know how much a play is worth here. But we do know that the left side and the top side are not independent, and that the play on the left is therefore worth more than 16 points (deiri counting).

Ogawa qualified her remarks, with the sente above in mind: "The time had come to capture at W1 (white+circle in this diagram). In terms of the left edge alone, this move was worth sixteen points. It gave White additional profit on the upper edge by making White a (white+square in this diagram) sente."

Anyway, in these undefined situations, such calculations are only rough indications.

BQM 287 last edited by ChrisHayashida on October 1, 2009 - 09:46
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