Click Here To Show Diagram Code

[go]$$Bm93
$$ +---------------------------+
$$ | . . O X X X X X O . O . O |
$$ | . 2 O O O O X X O O . O O |
$$ | . O X X O X X X O O O O O |
$$ | . O X O X X , . X X X X O |
$$ | . . 1 O O X . X X X . X X |
$$ | . O X . O X . . X . X O . |
$$ | O O O O O X O O O X X X O |
$$ | X O X X O O X X O X O X X |
$$ | X X X . X O X X X X O O O |
$$ | O . X X X O O X X X O . X |
$$ | O O O X X O O O X X O X . |
$$ | O . O X O O . O X O O O X |
$$ | O X O X O O . O X X O . O |
$$ +---------------------------+[/go]

W++

Oops sorry Dany. A notation error. This problem was already mentionned and solved.
BTW I found the final position already in the post https://lifein19x19.com/viewtopic.php?p=270976#p270976 but with an uncorrect sequence.

in principle, the solution too
but imho 94 white move is easier for black - black can immediately finish an unclear ko

Click Here To Show Diagram Code

[go]$$Wm94
$$ +---------------------------+
$$ | . 2 O X X X X X O . O . O |
$$ | 8 X O O O O X X O O . O O |
$$ | 6 O X X O X X X O O O O O |
$$ | 7 O X O X X , . X X X X O |
$$ | . 5 4 O O X . X X X . X X |
$$ | . O X . O X . . X . X O . |
$$ | O O O O O X O O O X X X O |
$$ | X O X X O O X X O X O X X |
$$ | X X X . X O X X X X O O O |
$$ | O . X X X O O X X X O 3 X |
$$ | O O O X X O O O X X O X 1 |
$$ | O 9 O X O O . O X O O O . |
$$ | O X O X O O . O X X O . O |
$$ +---------------------------+[/go]

B+0.5

so I prefer white's other move, which continues the ko

Click Here To Show Diagram Code

[go]$$Wm94
$$ +---------------------------+
$$ | . . O X X X X X O . O . O |
$$ | . X O O O O X X O O . O O |
$$ | . O . . O X X X O O O O O |
$$ | . O . O X X , . X X X X O |
$$ | . . 1 O O X . X X X . X X |
$$ | . O X . O X . . X . X O . |
$$ | O O O O O X O O O X X X O |
$$ | X O X X O O X X O X O X X |
$$ | X X X . X O X X X X O O O |
$$ | O . X X X O O X X X O . X |
$$ | O O O X X O O O X X O X . |
$$ | O . O X O O . O X O O O X |
$$ | O X O X O O . O X X O . O |
$$ +---------------------------+[/go]

P.S. with ko trick the solution can be extended by 4 moves

Oops as long as we talk about "preference" we have to define what is the objective.
For me I consider each player have to respect the best possible result. This best result being a black win by 0.5 point any move that will be a loss regarding this result has to be eliminated.

Click Here To Show Diagram Code

[go]$$Wm94
$$ +---------------------------+
$$ | . . O X X X X X O . O . O |
$$ | . X O O O O X X O O . O O |
$$ | . O . . O X X X O O O O O |
$$ | . O . O X X , . X X X X O |
$$ | . . 1 O O X . X X X . X X |
$$ | . O X . O X . . X . X O . |
$$ | O O O O O X O O O X X X O |
$$ | X O X X O O X X O X O X X |
$$ | X X X . X O X X X X O O O |
$$ | O a X X X O O X X X O . X |
$$ | O O O X X O O O X X O X . |
$$ | O . O X O O . O X O O O X |
$$ | O X O X O O . O X X O 2 O |
$$ +---------------------------+[/go]

Here after black white cannot play at "a" because it is great loss isn't it?


For the time being, and with this respect my best sequence has "only" 101 moves:



Can you tell me if we can reach a better figure?

Is this what you have meant?

Yes, this little trick))

Very good point Jmeinh.
Now the longest best sequence reaches 105 moves:



My feeling is that we did not find how to use the last white potential ko threat in the center (g8, g9, h10). I guess another improvment could be found.

Click Here To Show Diagram Code

[go]$$Bm87
$$ +---------------------------+
$$ | . . O X X X X X O . O . O |
$$ | . . O O O O X X O O . O O |
$$ | . O X X O X X X O O O O O |
$$ | . . . O X X , . X X X X O |
$$ | . . . O O X . X X X . X X |
$$ | . O X . O X . . . O X . 1 |
$$ | O O O O O X O O O X X X . |
$$ | X O X X O O X X O X O X X |
$$ | X X X . X O X X X X O O O |
$$ | . . X X X O O X X X O . X |
$$ | O O O X X O O O X X O X . |
$$ | . X O X O O . O X O O O X |
$$ | O X O X O O . O X X O . O |
$$ +---------------------------+[/go]

87 mistake
W++

Yes, White suddenly has 2 more ko threats after the preparatory move G9 instead of just one. However, if Black captures the other stone, G9 gives only one more ko threat (as usual), because of the stone J8.

Click Here To Show Diagram Code

[go]$$Wm92
$$ +---------------------------+
$$ | . . O X X X X X O . O . O |
$$ | . . O O O O X X O O . O O |
$$ | . O X X O X X X O O O O O |
$$ | . O X O X X . a X X X X O |
$$ | . . . O O X 1 X X X . X X |
$$ | . O X . O X b c . O X . X |
$$ | O O O O O X O O O X X X . |
$$ | X O X X O O X X O X O X X |
$$ | X X X . X O X X X X O O O |
$$ | . . X X X O O X X X O . X |
$$ | O O O X X O O O X X O X . |
$$ | . X O X O O . O X O O O X |
$$ | O X O X O O . O X X O . O |
$$ +---------------------------+[/go]

White starts with 'a'; if Black defends with 'b', White has the second ko threat at 'c'.

Yes of course. You mentionned already the correct move in your post https://lifein19x19.com/viewtopic.php?p=270963#p270963. A typing error because on my goban I always played the right move.

Black wins by 0.5 points after 105 moves.

Will we ever get to see Dany's "Not yet" again?
Well, I used to hate it - a little bit, at least
But if it doesn't come again - I will certainly miss it. A little bit, at least.

I think there are multi objectives to compare variations

black objectives (as winning side):
- max black score
- min variation length
- min complication

white objectives (as losing side):
- min black score
- max variation length
- max complication

that is, there are 3 objectives - score, length, complication

and comparison GT94 with D94 imho looks like:
score, GT94 = D94
length, GT94 > D94
complication, GT94 < D94

the solution finally found

What move do you think was the most difficult?

Click Here To Show Diagram Code

[go]$$ Komi = 0.5; no prisoners
$$ +---------------------------+
$$ | . . O X X X X . O . X X . |
$$ | . . O O O . . . O . . . . |
$$ | . . . . O X X X O O O O a |
$$ | . . . O . X , . . X X X . |
$$ | . . . O O X . X X X . X X |
$$ | . O X . O X . . . . . O . |
$$ | O O O O O X , . O X X X . |
$$ | X O X X O . X . . X O X X |
$$ | X X X . X O X . . X O O O |
$$ | . . X , b O , X X X O . . |
$$ | O . . X X O O O X X O . . |
$$ | X . O 1 O . X O X O O . . |
$$ | . X O X . O . . X X O . . |
$$ +---------------------------+[/go]

The first white move was for me the most difficult white move. Seeing the big point at "a" my intuition as white is to find a sente sequence in the bottom right corner and to play at "b". It is not natural to me to find a seki gote allowing black to play at "a".

Click Here To Show Diagram Code

[go]$$Bm39 Komi = 0.5; prisoners
$$ +---------------------------+
$$ | . . O X X X X X O X X X . |
$$ | . . O O O O X O O b X X O |
$$ | . . . . O X X X O O O O O |
$$ | . . . O O X , . X X X X O |
$$ | . . . O O X . X X X . X X |
$$ | . O X . O X . . . . . O . |
$$ | O O O O O X c . O X X X . |
$$ | X O X X O O X . . X O X X |
$$ | X X X . X O X . . X O O O |
$$ | . . X , X O X X X X O . X |
$$ | O O O X X O O O X X O X . |
$$ | X X O X O . X O X O O O X |
$$ | . X O X O O a O X X O . O |
$$ +---------------------------+[/go]

For black the most difficult move for me was at the very beginning of the second phase. Seeing the two ko threats at "a" and "b" I naturally chose the move at "a" because this ko threat cost nothing while the ko threat at "b" costs 12 points. The point is that I was too lazy to calculte the exact value of the ko and the exact value of the ko threats.
When I discovered that the value of the ko was 30 points and the value of ko theats was 30 points for "b" and "only" 29 points for "a" all become clear. The ko threat at "a" can be used only after a white move move at "c" which loses 1 point.

OC it would be completly wrong to conclude I consider the other moves easy !!!

A beautiful problem which help me a lot to understand basic subtilities for handling a two-step ko.

I agree. For me "b" the the most difficult move in the problem. But your explanation "The ko threat at "a" can be used only after a white move move at "c" which loses 1 point" is wrong. In the current position "a" is a valid ko threat. Beacause black has not yet added a stone in the left bottom corner. The real reason why "b" is better than "a" is described in Cassandra's diagrams.

You are right Dany. In fact it is difficult to answer the question "What move do you think was the most difficult" because for me the difficulty was not a specific move. The difficulty for me was to compare a ko threat at "a" with a ko threat in the upper right corner. Depending of the exact position different arguments are used and that is a great difficulty.
I showed the diagram after the 40th move where the problem began and it lasted then for several moves. Really difficult indeed.

Oops, it is not yet the solution. Strictly speaking white can reach 106 moves !

Yes, Black captures at the top left with move 99. Although at the cost of "complexity" (but that is a matter of taste).

I do not see to which post you answer.
It seems it is not my last one because the purpose of black is to have the shortest sequence isn't it?

Well, actually it was your last one, but I had confused the objectives of W and S, so mine was nonsense