One sided unremovable ko threat
One-sided unremovable ko threat without loss
chewtonic: In the unremovable ko threat article, both sides lose points when removing the ko-threat. There is a class of positions which could be called 'one-sided unremovable ko threats' where one player loses nothing with their threat.
Black can play any of the internal points as a threat that loses nothing (leaving a position similar to the first diagram if White answers), whereas White cannot threaten anything. If a bent four in the corner happens in another corner, Black would feel cheated, under Japanese rules. Note that if you reversed the colours in the bent four in the corner diagram, you would need to reverse the colours in this diagram too, to create a similar situation for White.
cliftut; notice, though, that whoever plays first in this diagram can gain sente. If white plays one of the points first, black has to pick which is more important to him, filling in ko, or getting 10 points in the corner:
- If black fills in ko, then white can create seki in the corner(this would be pointless, of course, if winning ko gave black 10 points or more). This is also assuming white can take no further action to save points after losing ko(ko for life).
- If black prevents seki by playing in the corner, white can take back ko and the fight continues. This loses 10 points for white, though, so it could only be played if winning ko kept white from losing over ten points, and if there were no more ko threats(at least no large ones, depending on how much winning ko is worth.)
I hope I made sense. If not, say so and I'll try to make a diagram. As far as I can tell, though, my comment is valid, if only in very paticular situations(depending on who gains or loses more by winning ko, and exactly how much ko is worth to each player). I just wanted to show that this may not be completely 'one-sided'. If anyone can see a flaw in my logic, please point it out.
yong?: cliftut, you dont make sense, it is seki without white playing.
Jake: He means that black can play this as a ko threat, and white cannot play anything to negate that. Black plays, forcing white to play or black lives, while white can never play here first - assuming white responds to a play (ie threat) move by black, it does become/stay seki.
chewtonic: It is conceivable that Black's best play is to ignore a white move in the seki. For example if Black has no ko threats and the ko is worth more than 10 points. So Black wins the ko and white then plays a second stone in the seki. If stone counting is being used, this extra white stone could win the game. Of course if 2 dame exist elsewhere on the board, white could play there instead and the result would be the same.
theotherable?: In almost all cases of correct play black will ignore a white play here. I'm not even sure I can construct an example where he would play. For black to play, the ko has to be less than 10 points, or black wins anyway. This play by white is either a trick, a mistake, or a desperate attempt to get a ko threat. For example one situation where a white play here might be useful is a large 3 stage ko where black has 3 other large unremovable ko threats, and 2 white plays here make the third the 1 required ko threat. A black reponse to the first play makes it a ko threat, and makes the next play here another ko threat, so black should not repond to the first move.
theotherable?: Another interesting point about this position is that after black plays his ko threat to kill white, white then DOES have a ko threat here to kill black, though at the cost of 10 points. This means that for large 2 step ko fights, white can ignore this and use this later to build a threat of his own.
asmobia?: Here is another example:
One-sided unremovable ko threat with gain
asmobia? White has one unremovable KO threat. In addition, White can even gain some points after Black answers it.
White gains 4 points by using Area counting, or 3 points under 1989 Japanese rule.
Infinite unremovable ko threats without loss
A double ko seki has the interesting property that it contains infinitely many unremovable ko threats for both sides. This means that if a game deciding ko occurs elsewhere, then a triple ko is created. One of the oldest rules disputes also includes the double ko seki, since the infinite unremovable ko threats seem to create life even where there is none.