text:
Why do asymmetric ko fights exist?
When I play teaching games with much stronger go players, situations often come up where one side must win a ko fight in order to survive.
In these situations, my opponent will explain to me that winning this ko is worth more or is more important to the side which must win to survive.
However, this never made sense to me. At the end of the game, points are compared, so it seems that a loss by one side should be equivalent to a gain by the other. With this understanding, it seems that the side trying to kill should care just as much about the ko as the side trying to survive.
Why is it not the case that both sides should always care equally about the outcome of a ko fight?

I fear my new answer is very late, but I think you may find it clears up some important points, if you are still interested in Go.

I think the easiest way to think of this is in terms of options, and the freedom to take whatever options benefit me.
In your example, you have no options: You need to win this ko, or you will lose.
However, in my case, killing you might not be my only option. I could kill you and swing things thirty points in my favor, or if I felt like it I could grab that empty corner over there worth twenty points and strengthen my position on the right, or I could go back and reinforce my moyo on the bottom to help me with the late game fighting that hasn't started yet, or I can pick a fight with those weak stones in the centre that you forgot to connect five turns ago. I have options. You don't.
As for why the ko is asymmetric in the first place, I would be weighing not only the relative value of winning the ko, but also the actual chances I have of winning it. In most "asymmetric ko" situations, you're probably the one with the first opportunity to win the ko, and I probably won't have any threats on the board big enough for me to significantly threaten you with: Nothing I do is likely to stop you from winning it, because you need to win this ko, or you will lose. If it was just a question of "I can definitely kill you and get a hundred points, or I can definitely take two hundred points over here instead while you win the ko" that's not really an asymmetric ko.
Even if there are big enough ko threats, or somehow I had the first opportunity to win the ko, depending on how close the score is and how strong our respective positions are there might still be no real need for me to be greedy; I can forego the risk and cost of the ko (maybe those high-value ko threats will be more useful later when there's a more important fight going on) and take a lesser but safer advantage elsewhere and still maintain a comfortable lead. Why should I use up valuable aji and give you the opportunity to carve out a chunk of my territory when I don't need to?
Let's take, for the sake of example, the following situation with a 70-point life-or-death ko on the board:

I play a 70-point ko threat.
You respond.
I take the ko.
You play a 60-point ko threat.
I win the ko.
You follow through on your threat.

Now, this whole exchange, I'm ahead by ten points, and I've used up a 70-point ko threat to get there. This…isn't terrible, but not really an incredible outcome; if I had just played a ten-point ko threat in the first place and let you ignore it, I'd be in the same place score-wise, but with less wasted aji. Or I could've played any number of other ko threats that may have given me less points but otherwise strengthened my position. Options.
You, on the other hand, don't really have the opportunity to play a ten-point ko threat since, well, I'll just take the ko: That's now a 60-point advantage for me. You're the one who needs the big threats here, not me. Whether you live or die, I'm still coming out ahead while you still have to go back and secure your tenuous life situation. I'm totally okay with this.
In other words, you're painting yourself into a corner where you give your opponent every chance to dictate which battlefield he wants to fight on. This is generally a bad idea.

By symmetry, if I lose by losing the ko, doesn't that imply that you win by winning the ko? In that case, it feels like you should value it at exactly the same value as me.

That is, even if I have other options, I should still care about the ko for exactly as many points as it is worth to you. For example, if the ko represents a swing of 30 points, both players should value taking the ko vs playing elsewhere at 30 points.

@merlin2011 Go isn't a game where you need to maximize your score: A win is a win whether you win by one point or a hundred. Just because you've put all your eggs into one basket doesn't really bother me if I don't need eggs.

I'm not trying to argue that one should maximize one's score. I'm only trying to understand the conventional wisdom claiming that ko's are asymmetric. It seems that if a ko represents a swing of 30 points, then only a move bigger than 30 points for either side should be played instead of the ko. Thus, if I have a move that's worth 40 points, I should play that instead of trying to win the ko and help a group live if that group's death only causes a 30-point swing.

Consequently, only ko threats worth more than 30 points should be effective, regardless of which side is playing the ko threat. Does this make sense, or am I missing something?

@merlin2011 If you're not arguing that one should maximize one's score, why are you arguing that each move needs to maximize one's score? Having options means I get to decide whether or not there's anything more valuable to me than just adding an extra ten twenty thirty points to my score right now. Why would I waste a 40-point ko threat on something I don't even really want? Maybe it'll be more useful later when an actually important fight starts.

Oops,  I guess I was arguing that one should maximize.  I'll ask a new question about that.

@merlin2011 I've tried to expand my answer to better address the nature of the asymmetry. Please let me know if anything is still unclear.

Thanks for updating your answer! I now entirely understand why you might be willing to play a 10-point ko threat, since your explanation about being only 10 points ahead with less aji makes perfect sense. I'm still trying to wrap my head around the point of the person needing to play a 60-point ko threat, but I think I'll get there soon.

I think there's a hidden premise here. The premise is that the score would be about even if the group in question survives, but it would be heavily skewed if the group died. Therefore, the side trying to survive is willing to use bigger ko threats to win the ko and live, thereby preventing such a large skew. Since the attacking side does not need to win by a large margin, she is happy to pick up a small advantage for free without expending too many resources.

@merlin2011 That sounds about right. It's hard to be very specific considering how general the question is; every board is different, and there would be plenty of situations where players and/or observers won't even agree on how symmetrical any given battle is.

Short answer
How can a ko fight be “asymmetric”? By offering one player a strong incentive to start it.
When a ko is asymmetric
You are right that once the ko is started, the result is equally valuable to both sides. Where the asymmetry comes in is when one player, considering whether to start the ko, compares three local outcomes:

They start the ko and win. (local score W)
They avoid the ko and make less profit. (local score A)
They start the ko and lose. (local score L)

This is asymmetric if A is not midway between W and L, that is to say the extra profit from winning the ko is significantly different from that lost by losing it, i.e. W - A ≠ A - L.
Advantageous asymmetry
If the profit is significantly greater than the loss – W - A ≫ A - L –
this player will probably want to start the ko, as they do well out of it unless they either win by ignoring a threat even bigger than W - A or lose and are only compensated by following up on a ko threat even smaller than A - L.
Your opponents’ comment that “winning this ko is worth more” or “is more important to the side which must win to survive” is misleading; the difference is that starting the ko puts at risk more of the score that side would have if the ko were not played.
An example
Here is an (idealised) example from the endgame, where Black has pushed into White’s territory and White has blocked; should Black start a ko with a or connect at b?
$$Bm1 An endgame ko
$$ _XXXOOOOO_
$$ _..XO...._
$$ _c.XOa..._
$$ _.Xb12.X._
$$ ----------

This depends on how much damage the side that wins the ko can do to the other’s territory.
Perhaps Black can do a lot of damage to White by winning, but only needs to defend at (e.g. c) if they lose; if so, this is a good asymmetric ko for Black to start.
Disadvantageous asymmetry
A ko can also be asymmetric in the opposite direction: the player starting it has a lot to lose and little to gain.
One should, of course, avoid starting such kos.
This could arise in the above position if Black were somewhat weaker and White stronger.
Kos that are not asymmetric
A ko is not asymmetric if avoiding the ko is not an option; in this case the only outcomes that matter are winning or losing the ko. For example, this is the case if making a ko is the only way to save a group.
In the situation you describe – having to win a ko to survive – the defender has no options and the ko is no longer asymmetric, but the attacker may have had a different way to attack without a ko, yielding a small profit; in that case it was asymmetric at that point.
A different sort of asymmetry
Finally, though you are right to say the result is equally valuable to both sides, one player may start a ko on the grounds that though their chances are faint, it is the only way they can see to win the game! This also applies, of course, to various other risky plays, such as deep invasions.

A do or die ko is often the biggest thing on the board. The person who will die basically has to ignore everything else and win the ko.
The person who won't die can "settle" for just a little less. Say the ko is worth 30 points. The one person has to fill the 30 point ko. The other person can make a 25 point ko threat and have it ignored, and lose five points on the transaction. As long as the loss is limited (5 points, not 10-15), the second person will usually win the game.

Frankly, I fail to see why the situation should be asymetric: Either player can try to settle for something just a little less. Like: "Oh, you want to kill me with that ko (30 points), in that case I want two consecutive moves in your territory that will allow me to live there (25 points)!" A ko seems to be only asymetric if one side does not have any ko threads that are even close to being big enough. But that's pretty much the definition of ko: You play it until either side runs out of sufficiently large threats.


text:
How to evaluate this board situation under Japanese or European rules
In a game, the following board position remained after both parties passed:
$$ -----------
$$ .XO.X.X.OX.
$$ .XO.....OX.
$$ .XOOOOOOOX.
$$ .XXXXXXXXX.
$$ ...........

In the post game analysis it turned out that

Black on the move can create a seki by connecting her two stones inside the white group
White on the move can create a ko to prohibit the Seki. When black wins the ko, the result is a seki.

$$W
$$ -----------
$$ .XO4X1X6OX.
$$ .XO.325.OX.
$$ .XOOOOOOOX.
$$ .XXXXXXXXX.
$$ ...........

White takes the ko first with 7.
But the situation was left as it is on the board, how is it counted (under Japanese rules or European rules)? When playing resumes, who has the first move? Do ko threats on the board count?

I will only discuss the Japanese pro rules, since as far as I know there are no official European rules. The tournament of the European Go Congress 2015, for example, was played with AGA rules. I think most European tournaments are played with "Verbal European-Japanese Rules" which usually require to fill all dame points and kos before the end of the game. Then I think the position would be counted as a Seki by the referee.

Senseis claims that these are the official Japanese pro rules in a translation of James Davies.
Two paragraphs are relevant for the question:

9.2 After stopping, the game ends through confirmation and agreement by the two players about the life and death of stones and territory. This is called "the end of the game."
9.3 If a player requests resumption of a stopped game, his opponent must oblige and has the right to play first.

and

13.1 After the game stops according to Article 9, if the players find an effective move, which would affect the result of the game, and therefore cannot agree to end the game, both players lose.

As you already noted the best B can get is a Seki and the best W can get is killing the black stones. Therefore the only interesting situation is when B wins in case of the Seki and W wins if W can kill. Then it is important which player has more ko threats, though most likely the kos where already played as the situation appeared after both players passed.
My interpretation is the following:

B has more ko threats: B claims resumption according to Article 9.3 and W plays first. W can play the ko but cannot win it. B wins.
W has more ko threats or no ko threats: B does not want to resume the match since B will lose the ko. In the case of no ko threats W takes first and B looses the ko as well. W does not want to resume the match since it would be B's turn and B will complete the Seki which results in a loss of W. Thus both players lose according to Article 13.1

On Senseis you can also find commentaries on these rules pointing out some ambiguities around these Articles and adding interpretation.
Edit: One of the comments there is this one which I think clarifies that the position in the question would result in a loss of both players. The relevant paragraph is §13 - Interpretation of the Rules of Play. The example there is similar to the position in the question. The point is that both players will agree that there is an effective move. Therefore Article 13.1 is to be used. W cannot claim that the stones are dead as long as this effective move exists. It is not important that this move will not be played as no player will ask for resumption.

How does the resumption of 9.3 interact with the double loss of 13.1? It now seems as if resumption is always pointless as both players will automatically lose by 13.1. Surely that cannot be right?

After reading through the rulesets and their commentaries, I am struck again: When black asks for resumption of play, White moves first, captures the ko first, Black has to pass on the ko, so White can secure it no matter how many ko threats are on the board. When White asks for the resumption of play, Black secures the seki. But when noone asks for resumption of play, both players loose? Or can White claim that the white string is alive and the two black stones are dead, because the sequence 1-7 proves that?

@jknappen To your first question: I don't see why black passes on the ko?

@jknappen: To your second question I will expand my interpretation but let me say first that I am not an expert for these rules, I just found the question interesting and tried to answer them by gathering available material. In my matches I would resolve the situation as explained in the beginning: fill all dame and kos and it would be seki.

According to the Japanese pro rules is has to pass on the ko once before it is allowed to take it back–at least this is my understanding from reading them here http://www.cs.cmu.edu/~wjh/go/rules/Japanese.html . This makes sense to keep the evaluation of the position local.

I think this is a misinterpretation. This rule is only to determine life and death after the game ended. After resumption it would be a normal ko according to Article 6. W would capture first, then B would play a ko threat.

@hkBst I think it is not completely pointless. First of all the question of resumption only arises because the players made mistakes during the game, i.e. they passed too early. Resumption can happen if the players cannot agree on the life and death according to Article 9.2 but one of the players can kill/win even in the case that the other player may move first.

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text:
What's the best strategy to win a tic-tac-toe variant on a 5x5-board?
First: We assume that every player is playing at its best
Rules 5x5 board
Player 1 begins and puts every turn one cross “X”.
Player 2 puts every turn two circles “O”. They don’t have to be next to each other.
A player wins as soon as he has five of his pieces in a horizontal or vertical line.
We also agree that player 1 “X” wins if the board is completely full.

What is the best strategy for each player to win the game?

If we're assuming both players play optimally, only one of them is going to have a winning strategy.

@murgatroid99 I'm not sure that's true because the victory conditions are different for the two players.

I'm pretty sure that there is a proof somewhere that any deterministic two player game has a winning strategy for at most one player. And this game has no ties, so there will be a winning strategy for exactly one player.

It goes something like this: if player 2 has a winning strategy for every initial move player 1 can make, player 1 wins. Otherwise, player 1's strategy is to start with a move for which player 2 does not have a winning strategy, so player 1 wins.

Would this question be better suited for http://puzzling.stackexchange.com/ ?

This is a generalized version of @Guvante's answer.
O (player 2) always wins in 4. The diagonals are not necessary to achieve a win.
Examples are included in bold, but this strategy works for all choices made by X.

X places a piece at (a,b): (1,1)
O chooses a row that is not a, and places two pieces in that row, but not in column b: (2,2) and (2,5)
Now, O is attacking on the chosen row (2), and on two columns (2 and 5). X will have to defend against one of these three attacks: (2,3)
O chooses a free row and places two pieces above or below his previous pieces: (5,2) and (5,5)

This is what the board looks like at this point:
 X1|  |  |  |
   |O1|X2|  |O1
   |  |  |  |
   |  |  |  |
   |O2|  |  |O2

Now O has 3 different attacks going on. In our example these are row 5 and columns 2 and 5. X can block one of these, and O can expand the other two to three-in-a-rows. Then X can block one of those, and O completes the other. Game over.
Since this is done in 4 moves, there's no risk of X getting 5 in a row so this is of no concern.
EDIT: step by step images, colors depict dominance.

I avoided X2 being where you put it since winning is trivial in that case, having X pre-empt the O2 move makes winning much more complex (as I don't think you can win if you allow X to pre-empt past the second turn)

@Guvante Having X play at O2 is exactly equivalent to playing where this answer puts it.  In either case, after their second move O has a rectangle with three fully free 'lines', which is all this argument needs to go through..

@Guvante X's first move is wholly moot to this solution.  O just places his first move(s) on any two squares which do not share a row or column with X's first move, and then his second move on two squares which do share either rows or columns with his first move, and which leave three 'open sides' to the rectangle. X has only had two moves at this point.

@Guvante In your example, if O just makes their second move at (1,2) and (1,4) they will win by turn 4.

I have flip-flopped a few times, but think that Player 2 has an unbeatable strategy. The trick is that Player 1 needs an X in each row and each column while Player 2 needs a "box". Four O's that form a rectangle with no X's that share a row or column with said rectangle.

First two O's sharing a row with self and separated by 1 square. Never put below or above or in line with X on this move. If possible (X not in center column) also make the square not in line with the X.
If X is not above or below your O's then place two more O's above or below your current ones, at this point you can make two three in a rows on your next move, which means X cannot win.
If X is above or below your O's then they blocked the box and you need to work to make it. This involves forcing X's move every turn by making a three in a row with holes where you want him to go. The first move is to add a third O to your line forcing X to block, make this third O on a free column that does not have an X and place another O on that column not sharing a row with an X.
After X's move they have an X in three columns and rows assuming good play, if they do not it is easier. The objective is to minimize the reach to avoid X gaining the needed diversity of locations. Place an O in the same row as your fourth O and sharing a column with one of the original row's Os. Place the second O in the same column in the row that does not have an X. X is now forced to block this column without blocking a new row (as you win if they don't play in that column and both those rows are blocked by new Os).
Whichever one they block you now have two rows/columns with two O's and no X's. Place an O in each and finish whichever X does not block on your final move.

For instance the following moves show this algorithm:

X (3,3) O (2,2&2,4) X (4,4) O (1,2&1,5) X (5,2) O (2,1&2,5) X (2,3) O (1,3&3,5) X (1,1) O (4,5&5,5)

X chooses the middle and then blocks the first box. However after this X is forced to make each move it makes (or an equivalent or inferior move). In fact past the second move the only moves that would change Os moves would involve immediate wins by O.
If X goes (3,1) as their second move it ends sooner, with O (2,4&4,4) followed by two three in a rows on the next turn. (X (3,4) follows with O (2,3&4,3) then either (2,1&2,5) or (4,1&4,5) and X (2,3) follows with O (3,4&4,3) then O(1,4&5,4) or (4,1&4,5))
Player 1's strategy would be to block any three in a rows and always attempt to play in a row and column that does not contain an X, but that doesn't block the above from winning.

Your example uses (2,5) twice. Also I don't understand why X would play on (2,3) if he already has (3,3).

@freekvd: Oops, the X was (5,2) fixed it. X must play (2,3) or (2,4) to avoid O winning, since O has (2,1),(2,2), and (2,5) at that point.

O can win faster here: let their turn 3 be (1,3) & (2,3); now X has no squares in the first or second rows and whichever one he puts a piece in, O will win on turn 4 in the other.

@StevenStadnicki: Missed that, feel free to replace the moveset if you like, I only went as far as the above because I assumed it was at least somewhat complicated to win, didn't notice that the box was unblockable if you went up.

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text:
Is this double ko played reasonably by AlphaGo Zero and AlphaGo Lee?
In the 8th game of the series1 between AlphaGo Zero and AlphaGo Lee the following position with a double ko at {A,C}×{13,14} arises:

On the next move AlphaGo Lee (Black) plays the double ko exchange A14-C13 before its next effective move at T15 and subsequent three effective moves; then AlphaGo Zero joins in the fun and also makes double ko exchanges before its effective moves. This continues until move 272; after 276 Black resigns.
I do not see the benefit in these exchanges as against immediately playing at T15 etc. Are these exchanges useful or merely something that their respective training never taught them how to do? If they are useful, what purpose do they serve?
1 See the article in Nature at https://www.nature.com/articles/nature24270#t1; the full text is not free but selected game records are, in the download at https://media.nature.com/original/nature-assets/nature/journal/v550/n7676/extref/nature24270-s2.zip

There is a difference between the two semi-stable double ko positions, although a rather small one. C14 reduces the immediate number of liberties of the G13 leg. This means that black will have a free tempo when G13 runs out of liberties, if we assume that black can find a big enough ko threat to answer the C13 capture with C14 in order to force the B15 gote.
Assuming black still has a big enough ko threat when G13 runs out of liberties, the value of keeping the one liberty at C14 filled is most likely 1 point (turning a one point sente for white to a reverse-sente, as we can infer from what happens later in the game), so the two-move sequence of capturing the double ko at C13/C14 followed by an answer as A13/A14 has a (ko-related) miai value of 1 for both players. That explains why both players take the opportunity to capture this double ko whenever they can.
An other way to put it is to attribute an average of 4.5 liberties to the G13 group (alternating between 4 and 5), with white being able to force it up when they need it if black can't find a big enough ko threat.

Thanks, you have made some things a little clearer to me, though there is a lot I still do not understand, I fear, not least why Black resigns when he does, but also why Black is in a better place after 239@A14, 240@C13, 241@T15 than he would be with 239@T15 – after all, it (temporarily) gives White an extra liberty at C14. I note that White taking half the double ko threatens to remove a point at B12 and make points at A14, C14 and B15, while Black threatens to do the opposite. Of course, if Black connects at A13 or C13, B15 is not a point for White either way.

Ok, I realize I didn't pay close attention to what happens in the game. Playing 247 and canceling it with 251 does indeed seem useless, even though it does no harm. But B5 which comes right after that wastes a ko threat. So, I think black already realized the situation was hopeless and was just delaying the end of the game for all the other moves seemed closer to defeat.


text:
Having started a small avalanche, what does one do with an undefended cutting stone?
Having just looked at the small avalanche (nadare / konadare) in 38 Basic Joseki I tried it out as White in a 2-stone game against GNU Go and was surprised that it left out the move at a, immediately playing 9 although Ishida’s Dictionary says that after Black 6 the moves 7 – 8 – a – b are the only possible order.
$$Bc Up to the deviation from joseki
$$ ........-
$$ ....6...-
$$ ....12..-
$$ ..9.34..-
$$ ...5O7a.-
$$ ...b8...-
$$ ........-
$$ ---------

Is Black 9 indeed an error, and if so how should White punish Black? Please explain why your answer is ideal and suggest any general principles that apply. I suspect that one should capture with a, though that does make Black b sente.
How the game actually continued
In case it is relevant or of interest, though I am only asking about the joseki, here is the continuation.
I was playing a little hurriedly and also was not aware that 9 was an error, and continued with:
$$Wcm10 17 connects at 14, 20… in next diagram
$$ . . . . . . . .|
$$ . . . 7 O . . .|
$$ . . 1 6 X O . .|
$$ . 9 X 5 X O . .|
$$ . . 0 X O X . .|
$$ . . 4 3 O 2 . .|
$$ . . . . . . . .|
$$ ---------------+

after which both sides created moyos:
$$Wcm20 Move 29 followed by circled, then squared and finally triangled stones
$$ +-------------------------------------+
$$ |. . . . . . . . . . . . . . . . . . .|
$$ |. . . . . . . . . . . . . . . . . . .|
$$ |. . . Y Q . . . . 4 . . 6 . . . . . .|
$$ |. . X , O . . O . , . . . . . X . . .|
$$ |. . . . . . . . . . . . . . . . . . .|
$$ |. . . X . . . 5 . . . . . . . . 0 B .|
$$ |. . . . . . . . . . . . 9 . . 7 W W .|
$$ |. . . . . . . . . . . . . . . . . . .|
$$ |. . 8 . . . . . . . . . . . . . . . .|
$$ |. . . , . . . . . , . . . . . , . . .|
$$ |. . . . . . . . . . . . . . . . . . .|
$$ |. . . . B . . . . . Q . . . . . . . .|
$$ |. . . . . . . . Y . . . . . . . . . .|
$$ |. . . . . . . . . . . . 3 . O O . . .|
$$ |. # . . . . . . . . . . . O X X O . .|
$$ |. @ # X . . . . . , . . O X X X O . .|
$$ |. @ @ @ # # # . . Q . . 1 X X O X . .|
$$ |. . . . @ @ . . . . . . 2 X O O X . .|
$$ |. . . . . . . . . . . . . . . . . . .|
$$ +-------------------------------------+

9 looks like a severe mistake to me. White may simply capture the single stone and be satisfied, like this:
$$B Simple punishment
$$ ........-
$$ ....Ob..-
$$ ....XO..-
$$ ..1.XO..-
$$ ...XOX2.-
$$ ...cOa..-
$$ ........-
$$ ---------

After this, if black c, white may answer at a, or may tenuki. Tenuki is reasonable, because the black followup does not threaten white, only captures two stones and ends in gote.
(If W2 instead at a, which is sometimes a good move, then black has a cut at b. This cut is worth more than the black descend at c.)
In other words, the cut at b works a lot worse than in the joseki for black. At the same time, black is not strong (e.g. the point you played at in the game remains a weakness). Territory is white's, too. This is not an even result.
As for why B1 was played at all: It is indeed a shape point, because it makes black a work (to capture the two white stones). In some other joseki that look (vaguely) similar, it is actually the correct move.
The underlying reason why the order was wrong becomes visible if we compare with the joseki move. If black plays at W2 instead of B1, then it is significantly harder to capture him in the corner (four liberties instead of two). That is why, in the joseki, white will need to offer a trade, losing two stones but capturing the stone at B1. (There is a ladder involved etc., but those are details.)

I had also asked this in Josekipedia, and now it has been answered (by an anonymous IP-adress), see http://www.josekipedia.com/#path:qdodocpdpcqendqcrdpf. There they found White 2 good but c ideal.

c first is an interesting idea. The sequences there look reasonable, if one is not afraid of some minor complications.

this variation is disadvantage to black.

@athos Yes, that's the whole point, I think

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text:
Why choose tsuke-nobi after a double keima against hoshi?
I have been looking at §28 of 38 Basic Joseki, which deals with a double keima kakari against a stone on hoshi. I realise that this book is old and restricts itself somewhat, but I am taking it as a way of getting a grounding in joseki.
Most of the variants treated start with tsuke-nobi to pressure a squeezed stone:
$$B
$$ ---------------------
$$ - . . . . . . . . . .
$$ - . . . . . . . . . .
$$ - . . a x . W . . . .
$$ - . . b X . . . . . X
$$ - . . d w e . . . . .
$$ - . . O 1 3 z . . . .
$$ - . . v 2 c y . . . .
$$ - . . . . . . . . . .

Other variants are given, with reasons for choosing them, but I cannot formulate a strategic reason for Black to choose tsuke-nobi because White can choose so many different responses to it with different results. Of course Black could play it from habit or just hoping for a certain response, but that seems less satisfactory.
The variants considered

Tsuke-nobi: 1-3 as shown.

W4@a —

B5@x — White: side + corner; Black: large top, can swallow marked stone, sente.
B5@d — White: corner + small top, sente; Black: side high and strong.

W4@b — White: side, option on corner; Black: big top, sente.
W4@c, B5@y —

W6@z — Fight White: top weak, side medium, sente; Black: corner + cutting stones.
W6@a — Tricky White: side weak, small corner, sente; Black: strong but little territory.

W4@d — White: weakish top + side, sente; Black: decent corner, connected to centre.

Tsuke-osae: 1-2 as shown, 3@d — Peaceful White: see variants; Black: decent corner.

W probably connects at v — White: moderately strong on both edges, option to reduce corner; Black: connected to centre, sente.
W tries to enclose B (W4@3) — White: weak wall, sente; Black: shut in.

Kosumi to 5-5: B1@e, W2@a, B3@x  — Simple White: decent side + corner; Black: strong top, sente.
Kosumi to 3-3: If B has stones on both edges B1@a, W2@e, Black cuts — White: two weak groups, sente; Black: alive in corner.

My question
Examining these variants, it is clear that Black can choose tsuke-osae, 5-5 or (given pincers against both kakaris) 3-3 to achieve the results in the last three options above. But what strategic purpose is served by tsuke-nobi, regardless of where White plays 4 (a/b/c/d)?
What I hope for from the answer

It is along the lines of If you want do do X, play A, but if you want to do Y, play tsuke-nobi, i.e. states the strategic goal served, maybe even gives an algorithm for choice. Obviously that will leave me to make some judgement(s), but at least I will know which.
It assumes that I restrict myself to the above variants, and presumes my opponent does too, unless that assumption entails a severe loss for me. This because I am trying to impose some sort of limit on how much I learn for an initial grounding in this (and other) joseki. Comments on other variants may also be interesting, but are of subordinate importance to me.

I'm a go noob, but one aspect of this may be that tsuke-nobi works as a kind of a leaning attack. First lean on the one white kakari stone on the left, making the hoshi stone a bit stronger while doing that. Then put heavier pressure on the other kakari stone in the top.

Black at the 3-3 point is a special strategy that requires supporting stones in the area.
In the fuseki, without supporting stones, black's objective is to separate white, with 3 main options:

tsuke nobi against the pincer stone (your diagram)
tsuke nobi against the pincered stone
5-5 point

38 Basic Joseki is an older book that favors the first option. In more modern play you'll see the two other options as well. The tsuke nobi keeps the white stones separated, while the 5-5 point will put more emphasis on cental influence.
This page shows a very recent variation from AlphaGo.

Thanks, that helps as a start. I have not actually accepted it so far, because what I am actually hoping for, as I have edited my questions to clarify, is which question(s) to ask myself in order to make a choice between the given variants, to which I am provisionally happy to restrict myself while getting a grounding, unless that entails a severe disadvantage.

Honestly I think you're expecting too much from a forum. Beyond what I said - if you want to split: tsuke nobi, if you want central influence: 5-5 - you would have to look at the rest of the board to understand what black is attacking or defending.

Actually, corner fighting (so-called Joseki), is still evolving. With AlphaGo and AlphaZero, and the latest other AIs such as FineArt, there's no final conclusion yet.
So, to your question “what strategic purpose is served by tsuke-nobi, regardless of where White plays 4 (a/b/c/d)?” I would argue that, however White continues (a/b/c/d), Black can achieve an acceptable result, so, that's good enough to play tsuke-nobi.
On the strategy? I would rather not worry about it – so long as the journey is entertaining, one doesn't care much where is the destiny.
Or, actually, I even dare say that “strategy” is a word humans use to simplify options – a pincer is to attack, 5-5 is for more air, 3-3 is to attach both side – AIs such as AlphaZero have taught humans that there are many unknown variations and really strong players (best human ones and AIs) can be quite flexible and resourceful so the "destination" is quite unknown.
On 5-5 and 3-3, just my 2 cents, they sound a little bit slow. In the post-AlphaGo era, the best players tend to play a game in which stones conflict with each other, forcing their opponent to respond. In other words, if the pressure is not serious enough, opponent might jolly well tenuki.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is “looking for a place to resign” in go?
In a number of commentaries on professional games I have read that one of the players was “looking for a place to resign”. My impression was that this meant that they considered the game lost, and just wanted to finish the game according to certain essentially stylistic conventions. Are there such conventions, in Japan in particular? What is considered a good place to resign? Are there different conventions in clubs, amateur tournaments and professional play or in different cultures?
If my impression is right, this is not a matter of whether the game is lost or how hopeless the position should be, but of style, of a certain sort of move and response after which resignation is thought appropriate.
I imagine one might try a few desperate looking things that (unless they work) make the situation more unambiguously lost, and then resign, but that one would want to avoid moves that appear to insult one’s opponent’s intelligence. Am I on the right track?
N.B. I have wondered about this for some time, and was reminded of it by the question Do go players play to minimize score difference after realizing they cannot win?.
Some examples
I have found the following examples of players’ attitudes to resignation (my emphasis):

Invincible, the Games of Shusaku, game 33 (1851–1852): ‘Black 49 is presumably just setting the scene for resignation, as Black cannot ignore White 48.’
Invincible, the Games of Shusaku, game 63 (1854): ‘Black 19 is of course too late, but actually Black is looking for a quick exit from an embarassing situation. White 30 gives Black his cue.’
1971 Honinbo Tournament, Sakata–Ishida: ‘The game was already decided, but Sakata does not like to resign, so he played it to the bitter end. Ishida had won by 9½ points.’
1971 Honinbo Sen, Third game: ‘… was the largest point, but Ishida realised that even if he played there he would lose the game … Ishida played 86, giving Rin a chance to end the game by playing 93. Rin coyly played 89 instead, waiting to see what … Ishida would try … Ishida tried … Ishida now resigned.’
Go World Number 46, p. 30 (41st Honinbo, game 4, 1986): ‘The game ends when Black lives up to 53, but White could have prevented this; he was just looking for a way to resign’
Go World Number 46, p. 36 (41st Honinbo, game 5, 1986): ‘[With 155, Black decides to go out fighting. He cannot hope to win the ko: White has too many ko threats.] Black resigns after White 202.’
Go World Number 50, p. 74 (Sakata – Kitani Minoru, 1958): ‘White 86 is a do-or-die move, but Black cuts it off with the brilliant sequence to 105. White resigns after Black 119.’
An Younggil commenting on Ke Jie vs Lee Sedol (2016-01-05) at gogameguru.com  says ‘White couldn't keep going, because White requires so many ko threats for this battle. White connected at 172 to finish the game, which means Park just gave up. Black 173 was the most simple way to finish the game, and White resigned here.’

Even if I had the time and skill to analyse the given positions this would not tell me the general conventions.

The delightful The Treasure Chest Enigma by Noriyuki Nakayama (© 1984, privately printed) has an 11-page essay on The Art of Resigning, but while he praises some players for the perfect timing of their resignations, castigates those (amateurs) whose refusal to resign means ‘he’s saying “You’re such an imbecile that I can easily catch up 100 points”’ and says how hard it is to strike the balance, he indicates neither the right moment nor the appropriate style of play for resigning.

Sensei’s Library explains under Looking for a place to resign that the expression ‘looking for a place to resign’ means choosing a move which obviously does not work and resigning when your opponent responds correctly.
Sensei’s Library also has an article on How to resign which sketches when to resign and enumerates ways of formally indicating that you do so. It also links to more extensive discussions of the etiquette, indicating in particular that it is a bad habit not to resign when you clearly cannot win.
I apologise for not having found this earlier; my mistake was searching without quoting the entire expression ‘looking for a place to resign’ — without quotes there is an enormous amount of noise, but with quotes Duck Duck Go gave four matches: this question, Tom Au’s answer and the above articles in Sensei’s Library.

Great Q&A!  Aesthetics are definitely important in Go.  Lee Sedol talked about this aspect in interviews surrounding the AlphaGo competition, and it's a fairly common topic in general discussion of Go in particular.

Makes me want to create an "aesthetics" tag, since that factor is important in games in general, and abstract games in particular.  (Cameron Browne wrote a very interesting paper "Elegance in Game Design" and devised an algorithmic approach to gauging "shibui" in relation to abstract, combinatorial games.

@DukeZhou: Interesting links there, which I must find time to follow up, and I like your suggested [aesthetics]-tag.

@DukeZhou: I found myself able to create the tag; I wonder if you have any questions to which you could apply it.

Definitely! It's one of my primary pre-occupations in relation to games, and a factor I think makes game design one of the most difficult creative endeavors.  (i.e games are primarily mathematical and mechanical, but the also have to be appealing.)

@DukeZhou: I meant questions here in Board & Card Games; I had a quick look through recent Go questions and only saw a closed question to which I thought it probably applied: https://boardgames.stackexchange.com/questions/30198/oiling-fake-yunzi-stones. You might also check whether my texts for the tag seem to cover what you would expect.

@DukeZhou: Have now tagged appropriate [go]+[aesthetics] questions; can you contribute any from other fields?

We should probably move this discussion to a chat.
I've created a CGT page for this stack: https://chat.stackexchange.com/rooms/69177/combinatorial-game-theory

A "place to resign" in Go is one in which you are clearly behind, and in which most of the major action has been resolved, so that there is no possibility of a "swing" large enough to bring you from loser to winner.
Most players will play on if there is a reasonable chance for a "swing" in a particular position, such as a life and death fight for a group, or a big ko.
In the cases where the weaker play ignores an opposing move or a ko threat, they are playing a move that allows the stronger player to administer the coup de grace, and they resign when the stronger player does.

But why would you look for a place in which you have no chance to turn it round? The commentaries made it sound as if the losing player already knew he had hardly any chance left and was looking to go out in a certain style, as expected in (Japanese) go culture.

@PJTraill: I wrote "no reasonable chance," That's like "guilty beyond a reasonable doubt," not "guilty beyond all doubt."

I have amplified my question to clarify that I am asking about style and culture and to give examples of the sort of comments I mean.

@PJTraill: See my new latest paragraph.

That seems closer to the meaning given by Sensei, resigning in “good style”.


text:
What is this alternate cut after 3-3 against double keima round hoshi?
In 38 Basic Joseki, Kosugi & Davies remark in this position:
$$c
$$ --------------------
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . 1 . . O . . . .
$$ -. . . X . a . . . X
$$ -. . . 3 2 6 . . . .
$$ -. . O 4 5 . . . . .
$$ -. . . . 7 . . . . .
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . . X . . . . . ,

that if White blocks at 2, Black will push and cut with 3 and 5 or 3 and a.
I cannot see how one is meant to cut with 3 and a in this situation.
I have not found this in Ishida, or on Josekipedia (though someone has asked a question about a) or in Sensei’s Library; though with this severely pruned search pattern (only shaded points sought):

there were some matches (to which the “a” refers), but nothing useful that I could see.
Questions

Is this a misprint, or how is one meant to cut with 3 and a?
When is this preferable?
What continuations are there, and are they joseki?

Follow-up
I am not sure what I can have thought (though I expected a to be an answer to a different White 4, rather than achieving a different objective), but now I think I see, as Daniel T says in his answer that the marked white stones are separated:
$$c
$$ --------------------
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . X . c W . . . .
$$ -. . . X b B . . . X
$$ -. . . X W d . . . .
$$ -. . O O a . . . . .
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . . X . . . . . ,

He shows W@a–B@b, but W b and d also seem to fail, with a shaky ladder and too many cuts respectively. I still do not see when this is good; maybe the issue is which 6-3 stone B wants to separate 5-5 from, but he could just push 3 in the other direction.

The point is, that in this situation
$$c
$$ --------------------
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . X . c W . . . .
$$ -. . . X b B . . . X
$$ -. . . X W d . . . .
$$ -. . O O a . . . . .
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . . X . . . . . ,

the moves at b and c are miai: Either white allows black to connect solidly at b, or white moves at b themselves. In that case, black can still take c, yielding this situation:
$$c
$$ --------------------
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . X . X O . . . .
$$ -. . . X W B e . . X
$$ -. . . X W . . . . .
$$ -. . O O . . . . . .
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . . . . . . . . .
$$ -. . . X . . . . . ,

Now white cannot capture the black cutting stone in a ladder by playing at e, because the two white stones lack a liberty, breaking the ladder in favor of black. The cutting stone at E17 is safe as well, so there is no way that white can connect up.
Also, a black follow-up at E14 is dangerous for white, so white will need to invest another move to either protect that cut, or make it irrelevant. I'm not sure which move would be the best continuation for white, but that consideration leaves the scope of this question anyway.

It seems to me that playing 3 and 'a' cuts the E15 stone from the F17 stone.
$$Bcm1
$$ +----------
$$ |..........
$$ |..........
$$ |..1..O....
$$ |...X75...X
$$ |...32.....
$$ |..O46.....
$$ |..........
$$ |..........
$$ |..........
$$ |...X.....,

You are evidently right, though White 6 at E16 or F15 are rather more complicated to refute. I have (perhaps a little unfairly) extended my question to also ask when this cutting pattern is preferable.

Spot on that white cannot easily connect. As a side note if this actually happens in a game, I don't imagine w playing 6 in the diagram, as it will remove all options w could have with E16 at some point (now or in the future), probably tenuki is better. Black's 7 seems better at G16 or G17 maybe, to avoid white's good shape move in that area.


text:
What joseki, if any, yields this position on page 60 of 38 Basic Joseki?
When introducing pincer joseki on page 60 of 38 Basic Joseki, Kiyoshi Kosugi and James Davies use this position as an illustration:
$$Bc Illustration of pincer
$$ ---------
$$ ........-
$$ .....O..-
$$ ..X.OX..-
$$ ..O.OX..-
$$ ...OX...-
$$ ...OX...-
$$ ...XX...-
$$ ........-
$$ ........-
$$ ....,...-
$$ ........-
$$ .....1..-
$$ ........-
$$ ........-
$$ .....O..-
$$ ....,...-
$$ ....X...-
$$ ........-
$$ ........-
$$ ---------

I have tried without success to find this joseki/position with pattern search on Sensei’ s Library and Eidogo and by experimentation on Josekipedia. I also could not find it in 38 Basic Joseki, nor in Ishida’s Dictionary of Basic Joseki, though perhaps I erroneously skipped some positions that seemed to me to be excluded.
Does this position arise from a recognised joseki? If so, which, and how might I have been able to find it on-line or in a book?
Since Black has seven stones to White’s six, he must have started (probably 3-3, 3-4 or 4-5) or White must have played tenuki, but I cannot find a very plausible order.
Update: partial match
While mafu correctly remarked that it looked like a hoshi approached from both sides, low and high, with black then jumping into the corner after white attached on top of the high approaching stone, I found the following partial position in Josekipedia:
$$Wc Partial sequence
$$ ---------
$$ ........-
$$ ........-
$$ ....18..-
$$ ....54..-
$$ ...76...-
$$ ...32...-
$$ ....a...-
$$ ........-

which it attributes to Encyclopedia of Joseki - Diagram 2.9365. White 3 is a trick move, this is the refutation, but it continues at a (reverting to a standard 1-point high joseki), so this is not the answer.

Looks like a hoshi that was approached from both sides, low and high, with black then jumping into the corner after white attached on top of the  high approaching stone. I don't know the joseki sequence, but the result seems like a perfect fit for this.

@Mafu: You are right; it is 38 Basic Joseki, p 188, Dia. 4 — not sure how I missed it :(. If you do not care to provide this as an answer, I shall do it myself.

Please feel free to put up the answer! I was just guessing.

The joseki
This is, as mafu pointed out, the result of a joseki starting with high and low near kakaris against a hoshi, shown in  38 Basic Joseki, page 188, Dia. 4 (with colours swapped and rotated to match the question), and in Josekipedia:
$$Wc Double kakari joseki
$$ -----------
$$ ..........-
$$ ......c7b.-
$$ a...X.54..-
$$ O...9.O6..-
$$ d....38...-
$$ .....1X...-
$$ .....20...-
$$ ..........-

N.B. The pincer is not visible in the diagram mentioned in the question.
The position is also shown in Ishida’s Dictionary of Basic Joseki, Vol II, p. 186, Dia. 96, but with the White pincer (Black in Ishida as the colours are reversed) at a; in this case, after White 9, Black exchanges b for White c before connecting at 10, and finally White plays at d.
This joseki appeared not to be in Senseis Library, though it belonged on the page 4-4 point, low-high double approach, hane; I have since added it there. Eidogo appears to contain only double high and double low kakaris against hoshi, but not the high/low variant.
How to find it
To be worked out

I tried without success to find this joseki/position with pattern search on Sensei’ s Library and Eidogo. That was reasonable, as it appears not to have been in either source, though I have since added it to Sensei and it is now found. I also failed to find it in 38 Basic Joseki, and in Ishida’s Dictionary of Basic Joseki; that was mainly due to not guessing correctly how the joseki started.  I also failed to find it by experimentation on Josekipedia, again through not trying guessing the start. To add: suggestions for more effective searching.
Since Black has seven stones to White’s six, he must have started (probably 3-3, 3-4 or 4-5) or White must have started and then played tenuki.


text:
Has black’s moyo already given them an advantage
Playing white against the Linux programme kigo1, I (c. 6 kyu) reached the following situation, which seems typical of my games with it: (unfortunately one can only put 10 moves in a diagram, so I have split the game into three; the first two diagrams are at the end)
$$Bcm21 21-29
$$ ---------------------
$$ -???...??...????????-
$$ -???...??...????????-
$$ -???X.O?.O.X???X????-
$$ -???,.....,.????,X??-
$$ -??X..8....9????????-
$$ -??..........???????-
$$ -??..........????X??-
$$ -..7..........??????-
$$ -.............??????-
$$ -..O,.....,....?,X..-
$$ -??............1....-
$$ -??..............O..-
$$ -??O.........6....??-
$$ -??...........??????-
$$ -??...........???O??-
$$ -??O,X....,..???,???-
$$ -???OX?5..X.O???O???-
$$ -???423???...???????-
$$ -????..???...???????-
$$ ---------------------

Counting roughly and conservatively the fairly definitely claimed points of territory (shaded) I estimate that I am about 22 points behind, on my move. I give kigo about 89 points (18 top left + 64 top right + 7 bottom middle) and myself about 67 (5 top middle + 22 bottom left + 40 bottom right). This is roughly consistent with the final result3.
This analysis seems to mean one of three things, each of which leads to a follow-up question:

(A) I have already fallen behind badly in the fuseki.

If so, where did I go wrong so soon? (See earlier moves below.)

(B) I am overestimating black’s moyo (or other territory).

If so, how should one estimate such territory, and how does one reduce it2 from what it looks like?

(C) My move is worth 15 to 20 points (depending on komi).

If so, which move could be worth that much?

So my question is: which of these is the case, and what is the answer to the follow-up question?
While I am not uninterested in other comments on this game, what I really want is to understand better how to cope with a moyo strategy such as kigo appears to deploy.
Earlier moves
$$Bcm11 11-20
$$ ---------------------
$$ -...................-
$$ -...................-
$$ -...X.8..2.9...1....-
$$ -...,.....,.....,X..-
$$ -..X................-
$$ -...................-
$$ -................7..-
$$ -...................-
$$ -...................-
$$ -..0,.....,.....,5..-
$$ -...................-
$$ -................6..-
$$ -..O................-
$$ -...................-
$$ -................O..-
$$ -..O,X....,.....,...-
$$ -...OX....3.4...O...-
$$ -...................-
$$ -...................-
$$ ---------------------

…
$$Bcm1 1-10
$$ ---------------------
$$ -...................-
$$ -...................-
$$ -...3...............-
$$ -...,.....,.....,1..-
$$ -..5................-
$$ -...................-
$$ -...................-
$$ -...................-
$$ -...................-
$$ -...,.....,.....,...-
$$ -...................-
$$ -...................-
$$ -..0................-
$$ -...................-
$$ -................6..-
$$ -..4,9....,.....,...-
$$ -...87..........2...-
$$ -...................-
$$ -...................-
$$ ---------------------

Notes
1 kigo 0.5.6, a KDE programme by Sascha Peilicke available with SUSE Linux. It appears to be roughly my strength when set to its maximum level (10), or maybe a little stronger.
2 How do I chose invasion or reduction points? sounds relevant, but seems to be more about invading a rather more solid territory.
3 In spite of some blunders on my part.

I am only a weak player myself but I don't like move 22. After the exchange 22-23-24-25 black's position on the bottom is very solid. Before that move, it was possible to invade or put pressure on this group. In the fuseki it is more important to evaluate which group is strong or weak than possible territories. Weak groups makes it difficult to transform moyo into territories. The biggest problem for white here is not that black is ahead in territory, it's that all his groups are strong. Also, the double-wing formation of 1-11-15-17-19 is too nice for black. You shouldn't have let that happen.

@Djaian: Thanks for your suggestions. Looking at the answers given (which both blame B 12), I think you are right about 1–11–15–17–19, but I am not so sure about 22—25. I could equally imagine that one might consider the result overconcentrated for White: an ideal extension from 25 would probably not be at 13. I am also not sure how significantly 22—25 strengthens my corner.

@PJTraill The main problem with 22-25 is how much is strengthens black, especially while you're trying to gear up and attack that moyo; now any attempts to attack will be wedged between two strong groups. It may look like a fair trade-off on a local level, but you need to take the whole board situation into consideration.

Just a few things to note here:

The vast majority of the moves, black and white, are played on the third line; this emphasises territory over influence. As far as third-line territory is concerned, you're both fairly equal.
Black starts to break into the fourth line early with E4; this is a common joseki move which serves to strengthen black's group while also putting pressure on white's corner.
White doesn't crack the fourth line until N7; this is a rather simple extension from white's already strong corner and doesn't really put any extra pressure on black.

As you noted, black is more developed: He not only started building influence earlier, but he did it while also securing territory and pressuring white. Any move that can do three things at once is intrinsically more valuable than taking three moves to do it one at a time, and over the course of a game that can seriously add up.
Black's influence gain with E4 isn't really a big deal this early since most joseki moves are balanced to take that into account; you're trading off his improved influence for your own more secure territory, and many of your own joseki moves are similarly putting pressure on black to respond.
However, you can see the same idea play out after move 21 when joseki moves are no longer involved: Black plays at P9 which puts more pressure on white's R8 while simultaneously defending his territory and expanding his influence, and his later extension at L15 looks poised to cap (or possibly cut off) white's J17 stone in much the same manner. Meanwhile, white's N7 extension and later extension at F15 are simple extensions which really don't threaten black's strong groups at all; they're basically only doing one thing each where black is still achieving multiple benefits from each stone. Same idea, except here the influence that black is building while pushing on white is also developing into the moyo you were worried about.
I wouldn't call black's lead insurmountable: It's definitely there at this point, but nothing that can't be taken down by just playing more aggressively. Ideally, you want to play a balance of low (third-line territory) and high (fourth-line influence) moves in the opening, since the influence from the high stones really comes into play during the middle game. Where exactly that balance is is entirely up to you and your play style, but playing a territory-heavy opening like you did does make it more difficult to fight against an enemy who has already established a moyo.
In other words, if you're not choosing to build influence early for the mid-game fighting needed to break down a moyo, your best strategy is simply to not let him build the moyo in the first place.
Me, personally, I probably would've changed up the game at move twelve, ignoring the top at J17 and playing at K4 instead. Don't get me wrong, J17 is still a solid move and a big point, but K4 would've (a) established a bit more influence, (b) been a decent extension from a solid corner, and (c) put pressure on black's not-yet-established E4 group. When you can reliably notice these sorts of multi-purpose moves your game will improve significantly; like I said before, they can really add up. Even something as simple as playing F15 at J15, which extends/defends the top group about as well while simultaneously threatening an attack around N17 (and also giving you influence closer to his budding moyo), could make a difference.

Thanks, plenty to chew on there which sounds useful, so maybe I will come back once I have absorbed it.

I had trouble choosing which answer to accept, but decided on yours because I took away two main general points from this (play enough high points; seek multi-purpose moves) and one from Tom Au’s answer (do not follow Black so much). Thanks to you two, perhaps, I pushed the programme (kigo) to a handicap of three stones before I lost (I said four, but have since seen that kigo confusingly said that “Patrick (Black) won” by 10).

Your big mistake was your slack move at 12. It's a bit scary to have Black make two shimaris. The reason he shouldn't have done this is that his two stones in the bottom left corner were left stranded. You should have played 12 one or two points to the left of 13, and chased these stones. This would have given you an initiative that would have more than compensated for the two shimaris. Assuming that you play as well as Black from this point, you should win the game.
When Black got to play 13, he fixed his main weakness AND limited value of your shimari in the lower right, killing two birds with one stone. Now it is your stone at 12 that is in the middle of Black's influence. The 13-14 exchange was all in Black's favor. Ditto for the 15-16 exchange, after he solidified with 17. Your 18 was basically forced, but the 18-19 exchange was again in Black's favor. After you played 20 and he played 21, your (ultimate) response at 26 was too little too late. You have fallen one step behind, all because of move 12.
You are about 20 points behind on the board, although komi will reduce Black's lead to less than 15. By falling a move behind at 12, you, in essence gave Black the equivalent of a "two stone" handicap instead of just playing White. You will probably lose this game, even with komi.
I've noticed a pattern that will likely cause you to lose as White. You "follow" Black around, playing 12 in response to 11, 14 in response to 13, 16 in response to 15, 26 in response to 21, etc. This gives Black the advantage of every exchange, and creates the Black moyo pattern you referred to. (You also have a bunch of moyos, but they are much smaller.)
Komi can offset only one such exchange, so you need to find ways to break the pattern (or punish Black for improperly doing so). Moves 8 and 10 were a good idea, because you had sente in that part of the board. Black needed to play J3 (give or take), and then you could have grabbed the kakari at C15 for 12.
The shimari at 11 was big; it's worth about 1.5 stones because of the synergy with the rest of the position. The pincer (or protective move) at J3 was even bigger, because it affected the fate of two stones. That is, your pincer would either have captured them, or more likely, yielded an equivalent benefit, that would have put you ahead (2>1.5).

Thanks, I see you agree with goldPseudo’s answer that 12 was the mistake, but disagree a bit on where to pressure B E4+E5. Having my attention drawn to the bottom right, I looked for references to follow-ups to the joseki. I saw that 8, although not in Ishida, is considered ideal by Josekipedia (where they call it a Shusaku move), but that they call 10 slack and a mistake, saying it should be one point lower (C6). They also say the B extension 13 should be one line to the left, on the third or fourth line.

P.S. They give no follow-up when W moves next after 10 at C6. P.P.S. What is your view of Djaians comment on 22—25 and my response?

@PJTraill; K3 doesn't pincer Black hard enough; he gets "enough" room with H3, and if you reinforce your K stone  with 14 (as you should), Black can then take the shimari with 15 instead of 11. Black should have played at  J3 (the ideal point) for 11. White 22 was "aji keshi" (potential destroying). because it removed the chance for an invasion at h3. But the main mistake was 12; the invasion isn't nearly as severe as the pincer.

@PJTraill: Note the new paragraphs on Black's moyo, and your playing style generally.

It was hard to choose, but I fear I accepted GoldPseudo’s answer in the end. Thanks to you two, perhaps, I pushed the programme (kigo) to a handicap of three stones before I lost (I said four, but have since seen that kigo confusingly said that “Patrick (Black) won” by 10).


text:
Does qGo + GNU Go save or load handicap games incorrectly and how can one work around it
When I save a 2-stone handicap game with qGo 2.0.0 using GNU Go 3.9.1 as engine and then open it with the same software, the handicap stones are missing. This is quite annoying, as one has to imagine the missing stones and the scoring function will count them as territory or dame or treat the position as unsettled.
I can add them to the SGF file by hand, in which case they appear as unnumbered stones when I click the button for the first move, followed by white’s first move numbered 1.
It is evident that either the save or the load function is incorrect, and I suspect that it is the former, as handicap stones need not be on hoshi.

Does the file format prescribe how handicap stones should be saved?

If so, does qGo+GNU Go do it right?

Is this function performed by qGo or GNU Go?
Is the manual addition of the stones the best workaround?

Here is an example of an SGF-file saved and edited, with the handicap stones added by hand (AB[pd][dp] at the end of the second line):
(;GM[1]FF[4]AP[qGo:2.0.0]ST[1]
SZ[19]HA[2]KM[0]PW[Human]PB[Computer]AB[pd][dp]

;W[pp];B[cd];W[ed];B[hd];W[fc];B[np];W[qm];B[cg];W[cc];B[bc]
;W[dc];B[dk];W[cn];B[en];W[cq];B[cp];W[bp];B[bo];W[bq];B[bn]
;W[dq];B[gq];W[eq];B[oq];W[pg];B[nd];W[bb];B[pq];W[qq];B[qr]
;W[rq];B[qf];W[qg];B[pe];W[qc];B[rd];W[rb];B[pj];W[pb];B[nb]
;W[qd];B[re];W[rc];B[sc];W[ri];B[gb];W[gc];B[hc];W[fb];B[hb]
;W[ef];B[bd];W[nc];B[mc];W[oc];B[sb];W[mb];B[lb];W[md];B[ma]
;W[od];B[ne];W[oe];B[kc];W[of];B[me];W[ld];B[le];W[qe];B[jq]
;W[mg];B[eg];W[gf];B[fg];W[ff];B[rr];W[sr];B[qp];W[rp];B[qo]
;W[ro];B[po];W[qn];B[hj];W[gg];B[ep];W[mj];B[ml];W[ok];B[kk]
;W[on];B[mn];W[oo];B[op];W[pn];B[fi];W[gr];B[fr];W[fq];B[hr]
;W[gp];B[hq];W[fp];B[hp];W[er];B[cm];W[ho];B[go];W[fo];B[gn]
;W[fn];B[fm];W[gm];B[hn];W[hm];B[io];W[in];B[ho];W[fk];B[im]
;W[em];B[fl];W[el];B[gl];W[hl];B[gk];W[dn];B[dm];W[eo];B[fj]
;W[ek];B[dj];W[dg];B[dh];W[df];B[cf];W[ij];B[ji];W[ii];B[ik]
;W[jj];B[kj];W[jh];B[ki];W[hh];B[gs];W[co];B[nj];W[nk];B[pp]
;W[mk];B[kh];W[jg];B[mi];W[ni];B[nh];W[oi];B[mh];W[oh];B[lg]
;W[ng];B[nl];W[ol];B[ab];W[il];B[jk];W[cb];B[je];W[if];B[nm]
;W[om];B[ie];W[ll];B[lk];W[nn];B[mm];W[mo];B[mp];W[ln];B[jn]
;W[lp];B[lq];W[kp];B[kq];W[lf];B[kf];W[mf];B[no];W[lm];B[ko]
;W[kn];B[jp];W[jm];B[lo];W[in];B[hk];W[jo];B[ge];W[fe];B[ob]
;W[kg];B[jf];W[lh];B[li];W[ig];B[hi];W[gh];B[ih];W[lg];B[mn]
;W[mm];B[gd];W[gi];B[gj];W[de];B[fh];W[ce];B[be];W[rs];B[pr]
;W[ao];B[bm];W[an];B[am];W[ap];B[dl];W[oa];B[ej];W[en];B[jl]
;W[im];B[ii];W[nf];B[ke];W[ga];B[ha];W[fa];B[jn];W[km];B[qs]
;W[es];B[fs];W[hf];B[sq];W[sp];B[ss];W[rn];B[sq];W[lc];B[mb]
;W[sr];B[jo];W[sq];B[mo];W[ac];B[ad];W[dd];B[rs];W[ba];B[ac]
;W[aa];B[af];W[pa];B[he];W[na];B[fd];W[kl];B[lj];W[tt];B[tt]
TB[ae][ag][ah][ai][aj][ak][al][bf][bg][bh][bi][bj][bk][bl][ch]
  [ci][cj][ck][cl][di][eh][ei][gr][hs][ia][ib][ic][id][ij][ip]
  [iq][ir][is][ja][jb][jc][jd][jj][jr][js][ka][kb][kd][kp][kr]
  [ks][la][lc][ld][lp][lr][ls][md][mq][mr][ms][nq][nr][ns][or]
  [os][ps]
TW[aq][ar][as][br][bs][ca][cp][cr][cs][da][db][do][dp][dr][ds]
  [ea][eb][ec][ee][ep][hg][ml][nj][nl][nm][og][oj][pc][pd][pe]
  [pf][ph][pi][pj][pk][pl][pm][qa][qb][qf][qh][qi][qj][qk][ql]
  [ra][rd][re][rf][rg][rh][rj][rk][rl][rm][sa][sb][sc][sd][se]
  [sf][sg][sh][si][sj][sk][sl][sm][sn][so]
)

An annoying bug, and good analysis in the answer. But this does not pertain to Go as a game, only to a specific implementation, and thus is not a core topic on this site. How about posting this on their issue tracker instead? It seems to be located at https://sourceforge.net/p/qgo/bugs

@mafu: I wondered whether it was on-topic, but was unsure; it seemed to me that it could be useful for anyone wanting to play Go. The site tour did not clear it up for me. I see this as a practical question about coping with unreliable equipment, like “How can I deal with the tendency of my much used Flingle set to reveal the Bloppers?”. The reason given (computer-based games) does not seem to hold up, because it goes on to say “except for computer-based versions of board or card games”. I suppose I ought indeed to report it, though I also thought it could not be too hard for me to fix.

It is definitely useful research you did. It felt too technically advanced to me (and others, it seems) even for a "computer-based version of a board game", more like what I would expect from SO. But the question can be reopened if most people like the question - I have no strong opinion on this issue.

@PJTraill Though the question is related to Go, the issue itself is just about  a specific bug in a specific program. If you look at the other computers tagged questions you can see that they are usually pretty general. That said, this is a good question and a good answer. If you would like to propose a change to what is on and off topic you are welcome to make a meta post.

@Malco: At the moment I do feel that questions about the practicalities of using computers where they are really just another bit of equipment one wants to get the best out of in order to play the game one enjoys should be on topic, so I hope to find time to formulate this in a metaquestion.

@PJTraill I look forward to it, you seem to have done a pretty good job with this self-answer Q, so a self answered meta-Q seems right up your alley. It will be interesting to see what the community thinks

Summary
The specification shows that the file is wrongly saved and can be corrected by adding the AB property. The game seems to be saved by qGo.
Details
How to do it correctly
We can find the specification from Sensei’s Library SGF, which refers us to [SGF on Red Bean]http://www.red-bean.com/sgf/; there the specific properties for Go link to https://www.red-bean.com/sgf/go.html. There we see:
Property:   HA
Propvalue:  number
Propertytype:   game-info
Function:   Defines the number of handicap stones (>=2).
        If there is a handicap, the position should be set up with
        AB within the same node.
        HA itself doesn't add any stones to the board, nor does
        it imply any particular way of placing the handicap stones.
Related:    KM, RE, RU

So we conclude that the standard does specify how to set up handicap stones and that qGo (or GNU Go) does it wrong. Moreover, adding the AB property by hand is correct, as it yields the file as it should have been saved.
For n conventionally placed handicap stones, copy the line above n up to and including the “]” above the n from:
AB[dp][pd][dd][pp][jj]
         2   3   4   5
AB[dp][pd][dd][pp][dj][pj][jj]
                         6   7
AB[dp][pd][dd][pp][dj][pj][jd][jp][jj]
                                 8   9

Which code is in error
We can start GNU Go with a command-line interface with gnugo -level 20 (for example, and see in the help text that save file saves the game. Testing this, I see that the saved game (with two moves) includes the AB property:
(;GM[1]FF[4]
SZ[19]
GN[GNU Go 3.9.1 Random Seed 1519478141 level 20]
DT[2018-02-24]
KM[0.0]HA[2]RU[Japanese]AP[GNU Go:3.9.1]AB[pd][dp]
;W[pp];B[dd])

I conclude that qGo implements its own save function and does it wrong; perhaps it implements this in order to include the player names (PW & PB), territory (TW & TB) and something called style (ST[1]) which is described at https://www.red-bean.com/sgf/properties.html#ST:
Property:   ST
Propvalue:  number (range: 0-3)
Propertytype:   root
Function:   Defines how variations should be shown (this is needed to
        synchronize the comments with the variations). If ST is omitted
        viewers should offer the possibility to change the mode online.
        Basically most programs show variations in two ways:
        as markup on the board (if the variation contains a move)
        and/or as a list (in a separate window).
        The style number consists two options.
        1) show variations of successor node (children) (value: 0)
           show variations of current node   (siblings) (value: 1)
           affects markup & list
        2) do board markup         (value: 0)
           no (auto-) board markup (value: 2)
           affects markup only.
           Using no board markup could be used in problem collections
           or if variations are marked by subsequent properties.
           Viewers should take care, that the automatic variation
           board markup DOESN'T overwrite any markup of other
           properties.
        The  final number is calculated by adding the values of each
        option. Example: 3 = no board markup/variations of current node
                 1 = board markup/variations of current node
        Default value: 0
Related:    C, FF, GM, SZ, AP, CA


text:
Are shell and slate go stones fragile?
I'm looking to buy a set of fine (but not luxurious) go equipment (board, stones, and bowls).
I have a cheap set of glass stones with a blue plastic sheet "board". Unfortunately my home's floor is covered in ceramic tiles, so when the glass stones drop on the floor, the stones easily shatter or break in halves (I have broken a few already).
Would shell and slate stones be just as fragile? Or are they generally resistant to breakage from dropping? Alternatively, would yunzi be solid enough?
While I could go completely practical and just buy a magnetic set, I would like to avoid that. I find that magnetic sets just don't have the right feel to it and detracts from the appeal and esthetics of the game.

As long as someone else is paying for it, I love that shell and slate stones have little to no reflection on them. If I were to get some for free, I'd play with them in a carpeted area.

I have no sources except personal experience, but from that, it seems there is little practical difference between glass, yunzi and slate/shell. I wish I could justify buying slate and shell stones with their increased durability.

From the Wikipedia article on Yunzi stones

Like glass Go stones, yunzi are very durable. However, like glass stones, yunzi are still at risk for chipping if they are dropped or otherwise come into sharp contact with hard surfaces such as metal, tile, or stone.

The problem you describe is more a property of your (very hard) tile floor. I would suggest acquiring a small area rug and placing it under your board or table when playing to protect your stones.
I have never enjoyed the pleasure of playing with shell and slate stones, but I suspect that the very properties that make them so beautiful also make them susceptible to chipping from such a hard surface. Being safe seems more prudent than being sorry.


text:
In go, is it possible to guess the opponent's strength from their play?
Context : while playing against a complete stranger that hide its rank/level.
I feel like it would be possible to guess the opponent's strength by analyzing its responses to tesujis, probes, etc. during the game.
Is there some patterns (or moves or errors) that indicates that a player has or has not reach a certain plateau ?
If it's the case, is there any on-line resource on the subject ?

I'm not sure if there is a real answer to this question. Skill in Go is not a linear progression: You may be great at tesuji, but your fuseki may be terrible - or the other way around. And in particular, you often don't know that you're bad at something (thus cannot judge others correctly on that topic). --- That being said, I am currently working on a project which involves gathering evidence on this very issue, so maybe there is some kind of answer, after all.

@mafu "There is no way to do this, and here's why" is actually an answer to the question, though!

I’ve heard the odd anecdote about players not letting on about how strong they are or being asked to comment on a professional game which they were told was by weak amateurs. They back up the idea that it is not as easy as you might think. The main thing is that if one cannot understand why a play is good it will not help you to estimate the strength. And go is full of surprising switches of emphasis and subtle timing. The one giveaway is when one can be absolutely certain that a move is forced and a  player does not see it, but it is hard to estimate the size of many threats. (@mafu, too)

In addition to the way proposed by Nathan, I would like to add a few further ideas.
Winning percentage
One simple way to judge the relative strength of two players is to look at their mutual game history (if it is large enough, of course!) and consider the percentage of won and lost games. For instance the EGF rating system has this property:

This setting gives about 30% probability for beating a 1 grade stronger opponent.

So if one players wins only 30% of the games, we may assume he is one stone weaker. Other percentages can be translated via their formula.
Of course, a large number of games between the two players is required. Also, relative strength is not transitive: It may well be that player A usually beats B, B usually beats C, and yet C beats A more often than not.
Score
Historically, the average score difference in (sufficiently many) even games is a good indicator.
One stone difference in strength equals two komi, and komi is usually around 6-7 points. So, for instance, if one players wins by around 26 points on average, we may assume he is 2 stones stronger.
Obviously, this approach suffers from resignations - the players would need to continue lost games until scoring. simply applying a threshold seems to be impossible in general, and may only be useful in specific instances.
Judging moves
I was interested in seeing how accurately we can judge the level of Go players from their moves and created a website for this purpose: https://kyudan.net
It displays the first 100 moves of an anonymized game. Once you submit your estimation of the respective level of the players, their real rank is displayed. All players are dan level up (so the options rank from 1 dan to 9 dan only). More information about the process are described here.
When I last looked, there were more than 2000 results submitted by more than 100 people. There was a significant (from appearance, I did not really number crunch this) difference in estimation accuracy between different judges. The most accurate people were often within 1 - 2 stones of the real ranks, but on average people were off by 2 - 3 stones. You can see my personal results here.
Considering that simply always selecting "5 dan" will lead to being within 4 stones of the correct answer, it seems apparent that judging player level from a single game is not easy for most of us.

This is a fantastic answer ! You've done a great job !

Probably the best way to gauge a player's strength (aside from playing against them of course) is to present them with Tsumego of varying difficulties. From Sensei's Library:

Tsumego, a Japanese go term adopted into English, are problems mainly about life and death, but also about ko, capturing races, cutting, connecting, etc. As a rule they are local problems, but a few involve the whole board.
There are many published collections of tsumego, or life-and-death problems, many in Asian languages but increasingly many in Western languages. Many collections are available on the internet. SL also has a growing collection of go problems.
Most professional players and top amateur players agree that solving tsumego is the best way to improve.

Sensei's Library has a plethora of tsumego from a variety of sources.
Personally, I've also used GoProblems.com a lot to improve my own skills. I like that they try to associate individual tsumego with what rank should be able to solve them. They also have an incredible number to choose from. As of this posting they claim to have 11,196 problems.

Sorry I wasn't clear. I meant how to know their strength during the game. Thanks for the answer anyway !

And how would you weight its strength during a game ?

My mistake. It's tough to say in-game how good a player. I agree with @mafu that different players have different strengths; there's no set standard of "knowing these tesuji and these joseki makes you a 12 kyu." Analyzing their play holistically (use of tesuji, use of sente, responding to ko, etc.) may give you a sense, but it's especially difficult to judge against opponents better than you. That said, one VERY general rule of thumb is that every 20 point difference in score represents a difference in rank. i.e. - if your opponent wins by 60 points, they're roughly 3 ranks better than you.

@NathanThompson The score difference is a very good point (from many games, not just one, of course). I've heard of it with 7, 10 or 14 points instead of 20, it seems everyone uses a different metric there. Considering that 1 handicap stone equals 2 komi in theory, something around 14 sounded most reasonable to me, but I've never really thought about this.

@mafu could you write an answer, I'll validate it


text:
Does playing a ko add a dimension to the game?
Playing a game of Baduk I was thinking that playing a Ko adds a dimension to the Tree of possibilities. Meaning it at least double the number of next moves the player may think of.
Am I kind of right ?

Even if you don't play it, all the paths resulting from a ko are already present in the tree of possibilities.

"Ko" is effectively one of those situations where you can hope to get two moves in a row (e.g. if your opponent spends his next move filling, or otherwise removing the ko).
On the other hand, if your opponent chooses to allow you this, it means that the value of the ko is large. (Otherwise he would answer your ko threat and ignore the ko).
Balancing the value of a two move "ko threat" against a ko itself, adds dimension to the game. So does the art of creating these kos, that allow you to make these threats.

I would say the added dimension comes from the "long-range" nature of kos. It is possible, through a ko, for two, otherwise loosely linked regions to be directly compared. Each ko evaluates the entire board in the context of the local position.
In that sense, it creates considerable added complexity to a game.


text:
Alive strings inside a string?
$$ ------------------
$$ | O . O X . . . .
$$ | . O O X . . . .
$$ | O O O X . . . .
$$ | X X X X . . . .
$$ | . . . X . . . .
$$ | . . . X . . . .
$$ | X X X X . . . .
$$ | . . . . . . . .

How is this position scored, because white and black cannot be captured. Who controls that territory?

note that black still can be captured :-)

Summary
It does not matter if a group is “surrounded” (which is hard to define); all that matters for the score is enclosed vacant points.
The rules
Vacant points only connected to stones of one player count as territory for that player; if “area scoring” is in use, points occupied by a player also count for them, but this seldom alters the result(!). Stones which players agree are captured are removed before counting like this and count as territory points for the the capturer but are ignored in area scoring.
Some traditions recognise several special cases, which you can find in or via the article on Scoring in Sensei’s Library. See there also for how the rules are applied in practice.
Your case
$$ ------------------
$$ | O w O X . . . .
$$ | w O O X . . . .
$$ | O O O X . . . .
$$ | X X X X . . . .
$$ | b b b X . . . .
$$ | b b b X . . . .
$$ | X X X X . . . .
$$ | . . . . . . . .

White has 2 points of territory marked w and 7 stones played, making 9 area points. Black has 6 points of territory marked b and 13 stones making 19 area points.
The remaining unoccupied intersections theoretically count for Black if there are no other white stones on the rest of the board but in practice White would still be able to make more points somewhere else. If there are white stones, then neither player has surrounded the remaining intersections, so these do not fall under the scoring rules and count as neutral.
Surrounding live groups makes no difference
Note that the rules are not interested in whether one group surrounds another: the white group in your example may be surrounded by black stones, but since it cannot be captured, it earns White points.
It is just as well it does not matter, as it is not always clear who has surrounded whom: consider two opposing lines of stones from top to bottom.
$$ Who surrounds whom?
$$ -------
$$ |..OX.|
$$ |O.OX.|
$$ |.O,X.|
$$ |.OX.X|
$$ |.OX..|
$$ -------

It is better not to think of “surrounding”, i.e. going all round the “outside”, to capture a group. What really matters is whether all the strings (connected chains) in the group have liberties (adjacent vacant points). This approach lets you play using logical rules on an arbitrary finite graph, where “outside” may be meaningless.
A note on efficiency
Incidentally, since Black has played more stones than White in your example, it is sensible to compare how efficiently they have made their territory: for White this is ²₇ ≈ 0·29 and for Black ⁶₁₃ ≈ 0·46, so Black is doing roughly 1¹₂ times as well as White, even though their territory is 3 times the size.

why the odd formatting in the efficiency calculation? It seems to me you've just calculated 2/7 = 0.29 and 6/13 = 0.46, so why mess around with superscript/subscript glyphs and the centered dot instead of the more usual division line and regular decimal point?

@ilkkachu: I am sorry if you found my formatting hard to read. I grew up with centered dots for decimal points, and definitely prefer the look of them and of fractions formatted like that, which is the same as in “1¹₂”, which is common on road signs in Britain — I find them less cluttered and more readable than “2/7” and “6/31”, so it does not feel like messing around to me. If we had mathematical markup in this community I might have used that.

Depends on the ruleset.
White is alive on its own, enclosing two points.
Black however, might suffer from a ko threat and might seki.
$$ -----------------
$$ | O . O X O . . .
$$ | . O O X O . . .
$$ | O O O X O . . .
$$ | X X X X O . . .
$$ | 6 2 4 X O . . .
$$ | . 5 . X O . . .
$$ | X X X X O . . 3
$$ | O O O O O . . .

Here, 2 is the ko-threat, 3 is taking the ko and 4 is w following its ko-threat.
So, if the game ends without the the ko threat, assuming Japanese rules, w has 2 points while b has 6.
With Chinese rules, however, if the ko threat is not played, w has 9 points while b has 19 points.

You have a good point about the ko threat — though it can only happen if White gets something like all those extra stones on the board, which are not there in the question! As to the score, one needs to bear in mind that White has made 2 or 9 points with 7 stones, while Black has made 6 or 19 with 13 stones, so in either case Black has played more efficiently — and efficiency is vital in Go!

Counting the points requires isolating the counted areas, otherwise w gets 2 points and b gets 342, i.e., rest of the board. You can't comment on efficiency from such a small part of the board. For all I know, b traded lower right corner for upper left corner and w invaded to reduce the corner, or w passed 7 times.


text:
How to run strong Go (baduk) playing program locally?
There are some strong bot-players on KGS (from 4d to 2K).
I saw pachi, ayamc and even gnugo having dan ranks on KGS. I wonder how to  run such a strong bots offline.
I tried gnugo and aya, both programs did not used even 5% of my CPU, made fast moves (about 1/2 sec per move) and played very weak (about 6K, I think). I did not found any options to make them use more CPU.
I tried also to run pachi (over GTP, using gogui). Pachi used 100% (6 cores) of my CPU, a lot of RAM but played on about 4K level. It plays a little bit stronger with gigantic fuseki dictionary, but it does not help much.
So, my question is: Is there way to run strong (2k+) bot locally?

The strongest bots often require cluster computing, and are not available to most users. However, dan-level bots can surely be bought, e.g. http://www.remi-coulom.fr/CrazyStone/

@mafu a new release of CrazyStone is soon coming. It's current level is 7dan KGS. And will be available there :http://www.unbalance.co.jp/igo/eng/contents.html

I don't think you're going to get any better free versions. If you purchase a copy of Crazy Stone as mafu suggested it may be stronger. If you like the way those bots play and want an offline version it's probably worth trying it out, as that's the engine that several bots use. However, I say may be stronger because I am not sure that the commercial version is the same as the version run for KGS bots.
I have no experience with Crazy Stone as GnuGo can still regularly beat me.

There is still should be possibility to run strong gnugo (which has open source). I did not find any option to make it to think more. In my PC it always uses less than 1/2 sec per move (and it is not normal). So, by default it does not actually use any CPU, and even cluster computing could not help him. There should be some option, that makes him stronger. The same for ayamc.

A new, much stronger version of Crazy Stone was released on May 16. The highest level is advertised to be 7D.


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How many times can you use a port in settlers of catan?
The instructions don't say (from what I see) and I need to know for the game.

As many times as you want. Almanac, page 9:

Example: Nick, the orange player, built a settlement at the ore
  special harbor. Nick may exchange 2 ore cards for any 1 other
  Resource Card. He can also trade 4 ore cards for any 2 other cards.

Just to add on, make sure you're only using the port when it is your turn and during your trading step. In case it needed clarification.

... Especially not in the end-of-turn build phase of a 5-6 player game.

@ChronoD - The "trading phase" concept was only through 3rd edition.  Starting with 4th edition, trading and building may be intermixed, they are no longer separate phases.

@theamigo I did not know that! Thanks for the update.


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Shuffle destination cards in Ticket to Ride
I'm considering a house rule where discarded Destination cards are always shuffled into the deck instead of returned to the bottom. The pros and cons seem balanced to me:
Pros :-
We may need the cards that someone else discards. By shuffling them in, we have a higher chance of getting them.
Cons :-
Cards that we discard come back to us.
Thoughts and comments?

Welcome to boardgames stackexchange. This is a Q&A site. Your question is more a discussion starter. Maybe you could edit into a real question.

I would stick with the original rules. The actual effect your house rule has on game balance is probably negligible. Most of the time you won't end up drawing anyone's discard anyway, and even when you do it's only slightly better on average than a random card (unless you were the one who discarded it, in which case it's a lot worse). Thus even if balance is fractionally improved (and I'm not convinced it is), I'd focus instead on a few other effects:

Shuffling a lot is annoying and makes the game take longer. (thanks @Othya for pointing this out in the comments)
For me at least, drawing my own discard would feel like the game giving me the middle finger - sure there are other useless cards I could have drawn, but I'd feel particularly screwed over by getting the same worthless card twice.

As a side note, I don't quite use the original rules myself - I make a separate discard pile and then shuffle it when the deck runs out.

Additionally, shuffling every single time someone discards will become a nuisance, and will increase the time it takes to play the game..


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Can I deal damage to Hornet Nest multiple times before it dies?
If I have a Hornet Nest on the battlefield, can I cast 2x Lightning Axe on it to get 10 tokens? Or does the creature die between the first Lightning Axe and the space that the opponent has to respond? I think I can play as many instants as I want on the same creature before the stack resolves and the creature dies, right?

Edited title and grammar and formatting for clarity.

"Can I play multiple spells before my opponent gets a chance to respond?" is a unique question as far as I can tell. "When does my creature die if multiple spells would kill it?" is a duplicate of Toughness and the Stack, so you might look there for information.

If it is your turn, you may cast two instances of Lightning Axe (or any other instant) before your opponent gets priority. However, they do not resolve until your opponent has a chance to respond.
The order is:

You get priority
You cast Lightning Axe, choosing targets and paying the costs; Lightning Axe is on the stack
You intentionally retain priority.
You cast Lightning Axe, choosing targets and paying the costs; second Lightning Axe is on stack.
You get priority, you pass
Opponent gets priority, they may cast a spell.
Opponent passes priority.
Second Lightning Axe checks if the target is still legal. It is, it resolves. Creature has lethal damage.
a. 5 Damage has been dealt to Hornet Nest, so next time priority would be given, ability that creates 5 Hornet tokens would be put on stack.
State-based actions are checked, creature dies.
Ability that creates 5 Hornet tokens goes on stack. (It resolves)
You get priority, pass
Opponent gets priority, pass.
First Lightning Axe checks if target is still legal.  It is not, so it fizzles. No damage is dealt to Hornet Nest

Final result: 5 Hornet tokens

Note that (3)  doesn't happen by default in tournaments. You must explicitly retain priority.

Why does it matter whose turn it is?

@diego The original question seemed to me that he wanted to do it before the opponent could respond.

It doesn't matter whose turn it is though, when you cast a spell you always get priority afterwards. It is after a spell resolves that the AP always gets priority.

While you can cast 2x Lightning Axe, both targeting the Hornet Nest, both of them will not deal damage to the Hornet Nest. After the first one resolves, the Hornet Nest will die, and then when the second one resolves, it will be countered (fizzle) because its target no longer exists.
State-based effects are checked by the game in between each thing on the stack resolving. So the stack would build up and tear down like this (assuming it is currently your turn):

Cast Lightning Axe(1), targeting Hornet Nest.
In response, cast Lightning Axe(2), targeting Hornet Nest.
Both players pass priority.
Lighting Axe(2) resolves, dealing 5 damage to Hornet Nest and triggering Hornet Nest's triggered ability.
Hornet Nest dies as a state-based action.
Hornet Nest's triggered ability is placed on the stack.
Both players pass priority.
Hornet Nests's tirggered ability resolves, giving you 5 token creatures.
Both players pass priority.
Lightning Axe(1) tries to resolve, but is countered because it has no legal target.


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Landfall, land creature enter effect
If I have a creature like Embodiment of Fury and I play a land and use its landfall effect on the land that just entered will it trigger entry effects, for example Elemental Bond?
Is it seen as entering as a land then turning into a creature or is it seen as entering as a creature?

Possible duplicate of If a permanent transforms into a creature, has it entered the battlefield?

I'm retracting my close vote - this question is about the grayer-seeming area of a triggered ability that "immediately" turns the land into a creature. The answer is ultimately the same (no, it wasn't a creature when it entered), but it's worth discussing how triggered abilities are not instantaneous.

The land enters as a land, and only later turns into a creature. This is because Embodiment of Fury's effect is a triggered ability, so the ability has to go on the stack after the land enters, and only later when the ability resolves (perhaps much later, if the players feel like responding to it) does the land become a creature. If instead you controlled e.g. a static ability which said something like "All lands are also 3/3 creatures", the land would enter as a creature and trigger effects like Elemental Bond.

thanks, BTW not a magic related question but how did you make the links on the cards i mentioned

That was actually the previous editor @murgatroid99. You do it like this: "[mtg:Embodiment of Fury]", and then magical behind the scenes stuff turns it into a Gatherer link. (And for future reference, if you ever want to see the markdown that created a post, just click the edit button.)


text:
In Phase 10, can a Skip card be played in the first round of a hand?
Can a Skip card be played before all the players have had at least one chance to play? Or can it be played at anytime?

There is no such restriction. From the rules:

The player ends the turn by discarding one card of choice onto the discard pile.

And

SKIP CARDS: When discarded, a "Skip " card causes another player to lose their next turn. The player who discards the "Skip " card chooses the player who loses their turn. When a player draws a "Skip " card, the player may discard it immediately or save it for a later turn. A "Skip " card may never be used in making Phase 8, or any other Phase. A "Skip " card may never be picked up from the discard pile.

In fact, the game can play a Skip card before anyone has had a chance to play: "If the dealer starts the discard pile with a "Skip" card, the first player's turn is automatically skipped."


text:
Does a bid of 'Double' make you the declarer?
South bids, say 2 spades, then west bids 'double', then all others pass.
Does this mean that south is the declarer with the contract being 2 spades doubled?
Or does west win the contract with 2 spades doubled?
My intuition in reading so far suggests that south would still win the auction, but I have not explicitly read that this is the case.
Also, this is not intended to touch on any bidding conventions either, just simply how this affects who wins the auction. I do believe I understand the consequences in the game play or rather the scoring of a doubled bid, but I am simply uncertain who would have won the auction under these circumstances.

To assist in future interpretation, note the distinction between bids, which include only the calls naming a denomination and number of odd-tricks from 1 to 7, and calls which also include Pass, Double, and Redouble. Only Bids have the potential to make one player Declarer, and only if that Player is the first by her side to mention the denomination of the bid.

Here are the Laws of Rubber Bridge - 2014 Edition: http://cdn.acbl.org/wp-content/uploads/2015/02/Laws-of-Rubber-Bridge.pdf

Not a bad question for a beginner. I hope that the answers and comments helped you to learn more about the game.

Yes they certainly did, I like to be explicitly clear on one level of understanding before moving to the next... In that vein, I feel more comfortable learning about the actual budding language as it pertains to the strategy as mentioned her in the posts and comments. Also, I imagine had I actually played a game with someone who knew how to play this would be a question I would not have needed to ask here. It's not such a popular game with my generation however

@user74091: I can strongly recommend Richard Pavlick's site: http://www.rpbridge.net/

Side note: I consider myself a beginner, but as far as I understand, doubling 2S is usually a bad idea. 2SX scores a lot more than 2S when made (because of the game bonus).

@TwiNight: There are times when 2S* is an excellent contract for the Defenders - when they can collect a "phone number penalty" of at least 800.

Your understanding is correct: South would be the declarer, playing a contract of two spades doubled. Doubles (and redoubles) never change who is declarer, just the scores for making or failing to make the contract.

Thank you, Im learning to teach a group of friends how to play and none of us have ever played before, and want to make sure i understand all the basic game play thx