"Adv. ko"?

"Adv. ko"?

Postby garaden on Fri Feb 19, 2016 11:11 am

vougalar frequently annotates some wrong paths with "Adv. ko" (for example, the first wrong path here: http://goproblems.com/18625). What does this stand for, and what kind of mistake is it indicating? I'm not sure, but I think these paths require the player to win a ko fight that the correct path(s) avoid entirely. Though I remember one problem where a wrong path marked "Adv. ko" had a double ko and the correct path only had one ko, which is a slightly different mistake. I remember racking my brain on that one, trying to see if there was a way to avoid the ko, failing, and following that path anyway, hoping I wouldn't see "Adv. ko" on the other end! :)
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Re: "Adv. ko"?

Postby vougalar on Sat Feb 27, 2016 4:31 am

http://senseis.xmp.net/?TsumegoConventions

Conventions

The assumptions in life-and-death (tsumego) problems are rarely spelled out.

They include:

board otherwise empty;
escape from the local position is good enough for life
no effects from corners of the board not represented in the diagram;
seki is obviously a solution to a life problem.
and obviously alternating play

Sometimes it is assumed that

ko is a solution;
there are worthwhile non-local ko threats.

Life-and-death statuses

When asked for the best result in an isolated life-and-death situation, this order usually applies, assuming you are the defender. Reverse the lists, if you are the attacker.

life (any kind)
ko (any kind)
death

Life

This is the order of preference for different kinds of life:

unconditional life or escape to the center
seki

Also note, that life by double ko counts as "unconditional life".

Ko

When the best ko is needed, this order should be easy to remember:

approach ko in defender's favor
direct ko, defender takes first
direct ko, attacker takes first
approach ko in attacker's favor

Also note, that the number of internal threats can be important, since it can in effect change the player who gets to take first.

Other kinds of ko (multi-stage ko, eternal life, etc.) do not fall in this list very nicely, so you have to judge their goodness in the context of the given problem. Generally however, a thousand year ko is better for the defender than a direct ko.

Death

In an isolated problem it does not really matter what kind of ko threats are left behind, so all sequences that kill unconditionally are usually considered equal. The only notable exception is death by double ko, which gives an infinite number of ko threats, and thus has to be considered better than "just dead".

Ladders

There is usually no call to read ladders, but logically speaking we should assume all ladders are good. Note that being chased in a ladder doesn't count as escape.

To be quite precise is a little tricky. Something like this: we assume an otherwise empty board that is semi-infinite in any direction with an unmarked corner, but is a large band (to make ladders work); and we add in a second board as direct summand for ko threats to inhabit.
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Re: "Adv. ko"?

Postby zwim on Sat Mar 12, 2016 10:28 am

That still doesn't tell us what "Adv. ko" stands for...
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Re: "Adv. ko"?

Postby tamingo on Tue Mar 22, 2016 5:25 am

garaden wrote:vougalar frequently annotates some wrong paths with "Adv. ko" (for example, the first wrong path here: http://goproblems.com/18625). What does this stand for, and what kind of mistake is it indicating?

I guess it means "advantageous ko", i.e. a ko where the other opponent has to find the first ko-threat. The opposite would be "disadvantageous ko" (Disadv. ko), i.e. your opponent starts the ko and you have to find the first threat.

If "Adv. ko" is marked as wrong, you are usually supposed to find a solution without ko.
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Re: "Adv. ko"?

Postby extremeblueness on Mon Jul 03, 2017 12:51 am

I always thought it meant "avoid ko," but there's situations where that doesn't make sense.
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