Re: [computer-go] Legal Configuration (fwd)

[Izzet Pembeci <pembeci@xxxxxxxxxx>]

When I replied the mail was sent to Mark so here is our private follow up
on this subject. I didn't want anybody missing his excellent reachability
suggestion for my example (will work based on the dimension of the board 
(even/odd) I think).

>>There isn't as long as pass is a legal move at any point in the game, in 
>>which case determining legal positions is trivial: the set of illegal 
>>positions is equal to the positions with chains without liberties.
>>    
>>
> [me]
>I was ignoring handicaps but completely forgot pass. What about if we ignored
>that too? It doesn't seem trivial at all.
>
>..........    B started, W's turn
>..BBB.....
>..........
>....W.....
>..........
>
>This is unreachable, one W is missing and there is no B formation indicating a
>capture (and no W formation indicating capturing B's were captured and so
>on). So even in this very simple case we can prove intuitively but formally   
>it is very hard.
>
>Yet another Go spawn intractable problem I guess.
[Mark]
I wouldn't be surprised if this position is quite reachable, although 
the proof is rather long. Think about both sides filling one side of the 
board until W captures all B's. Then continue this way until W occupies 
almost all of the board, except the three positions where you put the 
three B's. When you have that position, B will play in the middle, W 
adds a stone, B captures a board full of W stones, W plays at the point 
in your diagram and B completes the row of three B's. Next W's turn.

But it's all rather academic of course.

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