kvasir wrote:
Now I see white starting a ko like this
- Click Here To Show Diagram Code
[go]$$W
$$ ------------------
$$ | . O 8 X 7 O . . .
$$ | 1 5 3 X O O . . .
$$ | 4 O 6 X O O . . .
$$ | 9 2 X X O . . . .
$$ | X X O O O . . . .
$$ | . X O . . . . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$[/go]
Yes kvasir this is a ko but I do not think it is the best sequence for white point of view because in this sequence Black needs only one ko threat to win the ko (surely this is not a kyu level exercice!).
What about this other sequence:
- Click Here To Show Diagram Code
[go]$$W
$$ ------------------
$$ | . O . X 5 O . . .
$$ | 1 . 3 X O O . . .
$$ | 4 O 6 X O O . . .
$$ | 7 2 X X O . . . .
$$ | X X O O O . . . .
$$ | . X O . . . . . .
$$ | O O O . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$[/go]
kvasir wrote:
It is W+14 because it is the value of a move in the ko is 12 pts. and white will be one move from finishing the ko for W+26 (and for that matter black finishing the ko is B+10).
So we have W+14 in gote or B+9 in gote from the following position, assuming white plays "a". That is the move is 11.5 pts. which is actually less than the 12 pts move when assuming black plays "c". Therefore, you can play "c" because it is "good enough" or play "a" going -0.5 pts. for the exchange which could favor white because the 12 pts. per move could be heavy. Also white might tenuki for a less favorable ko and that is another exchange.
If I am not wrong you applied the rule of thumb described in
https://senseis.xmp.net/?KoThreatRuleOfThumbIn principle I agree with your count but you know also that this rule of thumb cannot be applied to big ko (compared to the ambient temperature).
In the same article you find the following warning:
"The books, e.g. Rob van Zeijst's All About Ko, tell you to compare the ko with the threat, 2K/3 vs. H. That is not very good advice, unfortunately.
Note that the values of your own threats, as well as the value of other plays on the board, are relevant. In particular, you can afford to reply to a small threat of the opponent if your corresponding threat is large enough.
This formula works when you make two key simplifying assumptions:
The ko is only fought when the ko is the same size as the ambient temperature (miai values are equal: K/3 = T). This will be true for many kos, because if the ko is smaller than the ambient temperature, players will simply ignore it until the temperature drops to the same value as the ko, and only at that moment fight the ko. However, sometimes a ko arises from local fighting, and the size of the ko is larger than the ambient temperature (K/3 > T).
They ignore the size of the threats that you still have in reserve. (TODO: Add some discussion about why ignoring the threats you have in reserve is bad)"
When I analysed the proposed position and I discovered the ko I immediately considered it is probably a ko too big for the rule of thumb.
Let's take a typical example :
- Click Here To Show Diagram Code
[go]$$W
$$ -------------------------
$$ | . O . X . O X X X X X |
$$ | a X . X O O X X X X X |
$$ | . O . X O O O O O O g |
$$ | . c X X O X X X X X X |
$$ | X X O O O X . O O O O |
$$ | . X O . O X X . O O . |
$$ | O O O O . O X O O O X |
$$ | . O . k O O X . O O . |
$$ | O O . X O X X O O O O |
$$ | . O . X O X . X X X X |
$$ | O O . X O X . . . . . |
$$ -------------------------
$$[/go]
It seems white "c" is a good choice for two reasons :
1) taking sente in the upper left corner allows to take the big yose at "g"
2) black has a big ko threat at "k".
Even in this case the best move is white "a" because white is able to win ko
kvasir wrote:
You could add stones to the black group to make the ko clearly favored in terms of the above analysis.
I can only agree kvasir but the ko is already a very large ko, isn't it?