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[Hactar Go] Why is this tsumego solution correct?
http://www.lifein19x19.com/viewtopic.php?f=11&t=17305		 Page 1 of 1
Author:  libcat [ Thu Mar 05, 2020 12:42 pm ]
Post subject:  [Hactar Go] Why is this tsumego solution correct?
-Edit - I got a bit confused with my diagrams in my first post so this is the correction version.

I have just started learning using the Interactive Way to Go website (it's so good!) and solve Tsumego on my phone, but the app I use doesn't really explain why a certain shape is the correct solution. Here is the problem from the Hactar Go Lite phone app.

White to play.

Click Here To Show Diagram Code
[go]$$Wc
$$ +-------------------+
$$ | O X . O . . . . .
$$ | O X X X X X X X O
$$ | O O O O O O O O .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ +-------------------+[/go]


I finally arrive at the solution:
Click Here To Show Diagram Code
[go]$$Wc
$$ +-------------------+
$$ | O X . O 3 . 2 1 4
$$ | O X X X X X X X O
$$ | O O O O O O O O 5
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ +-------------------+[/go]


I'm guessing that White needs to stop Black from forming two eyes and living. So why is the above correct? Because I can see that Black has two eyes, one big one of the left and a small one on the right. Maybe I have completely misunderstood the point of this tsumego or the concept of forming eyes. Assuming that Black wants to form two eyes, could it then play either a or b? It looks okay to me.

Click Here To Show Diagram Code
[go]$$Wc
$$ +-------------------+
$$ | O X a O 3 b 2 1 4
$$ | O X X X X X X X O
$$ | O O O O O O O O 5
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ | . . . . . . . . .
$$ +-------------------+[/go]
Author:  EdLee [ Thu Mar 05, 2020 4:03 pm ]
Post subject: 
Hi Libcat,

B has only one real eye, and is dead.

(a) and (b) are two liberties, not two eyes.
(c) is a False Eye.

Click Here To Show Diagram Code
[go]$$B Black Dead shape
$$ +--------------------
$$ | a O O b X c X . . .
$$ | X X X X X X O W . .
$$ | O O O O O O . . O .
$$ | . . . . . . . . . .[/go]
Click Here To Show Diagram Code
[go]$$W Black Dead shape
$$ +---------------------
$$ | . O O . X 2 X 1 . .
$$ | X X X X X X O O . .
$$ | O O O O O O . . O .
$$ | . . . . . . . . . .[/go]

As an exercise, try to prove to your satisfaction that B cannot form two real eyes with this shape:
Click Here To Show Diagram Code
[go]$$B Black Dead shape
$$ +--------------------
$$ | . O O . X X X O . .
$$ | X X X X X X O O . .
$$ | O O O O O O . . O .
$$ | . . . . . . . . . .[/go]

Did you mean to make 9x9 boards in your diagrams ?
... because your diagrams are open-ended on the right edge.
If your diagrams are supposed to be 9x9, then in your second diagram,
your :w5: at J7, in reply to :b4: J9, is correct.

In the diagrams here, I made the boards bigger than 9x9,
and in reply to :b4:, W extends toward the right with :wc: .
Author:  Kirby [ Thu Mar 05, 2020 5:06 pm ]
Post subject:  Re: [Hactar Go] Why is this tsumego solution correct?
The problem makes sense now, with the better diagrams. I removed your other thread, since it was confusing.
Author:  libcat [ Fri Mar 06, 2020 8:48 am ]
Post subject:  Re:
@EdLee Hello and thank you!
EdLee wrote:
Did you mean to make 9x9 boards in your diagrams ?
... because your diagrams are open-ended on the right edge.
If your diagrams are supposed to be 9x9, then in your second diagram,
your :w5: at J7, in reply to :b4: J9, is correct.

In the diagrams here, I made the boards bigger than 9x9,
and in reply to :b4:, W extends toward the right with :wc: .


Yes, I did not even realise this until checked my phone again. Here is the screenshot. It looks like everything was supposed to be happening on the top right of a 19x19 board, so technically yes, 9x9. I hadn't thought about this up till now, but clearly where the tsumego takes place/board size makes a huge difference! Sorry for missing that big detail.

I will have to check my understanding of all the concepts you linked to - especially false eyes since they don't look appear obvious to me.

EdLee wrote:
As an exercise, try to prove to your satisfaction that B cannot form two real eyes with this shape:
Click Here To Show Diagram Code
[go]$$B Black Dead shape
$$ +--------------------
$$ | . O O . X X X O . .
$$ | X X X X X X O O . .
$$ | O O O O O O . . O .
$$ | . . . . . . . . . .[/go]


ARGH okay yes, I see. Thanks again.
Author:  libcat [ Fri Mar 06, 2020 9:21 am ]
Post subject:  Re: [Hactar Go] Why is this tsumego solution correct?
Kirby wrote:
The problem makes sense now, with the better diagrams. I removed your other thread, since it was confusing.

Thank you! (I don't know what the etiquette is here i.e, do I 'like' your reply? No idea, but either way I'm grateful)
Author:  Joaz Banbeck [ Thu Aug 27, 2020 10:47 am ]
Post subject:  Re: Re:
libcat wrote:
... false eyes since they don't appear obvious to me.


The easiest way to understand a false eye is to ask yourself how it will eventually resolve. ( Works for lots of other shapes too, FWIW )

If you look at the example used above...

Click Here To Show Diagram Code
[go]$$B False eye
$$ +--------------------
$$ | . O O . X C B O . .
$$ | X X X X X X O O . .
$$ | O O O O O O . . O .
$$ | . . . . . . . . . .[/go]

you can realize that,sooner or later, ONE player must play BOTH of the circled points.

In other words, this:

Click Here To Show Diagram Code
[go]$$B Not in this universe
$$ +--------------------
$$ | . O O . X B W O . .
$$ | X X X X X X O O . .
$$ | O O O O O O . . O .
$$ | . . . . . . . . . .[/go]

...sharing of the two points cannot ever exist. Try it on a board to prove it to yourself.
Even if both players cooperate, you can't get there from here.

So, eventually it must end like this:

Click Here To Show Diagram Code
[go]$$B Dead with one eye
$$ +--------------------
$$ | . O O . X B B O . .
$$ | X X X X X X O O . .
$$ | O O O O O O . . O .
$$ | . . . . . . . . . .[/go]

...or this:

Click Here To Show Diagram Code
[go]$$ Dead with one eye
$$ +--------------------
$$ | . O O . X W W O . .
$$ | X X X X X X O O . .
$$ | O O O O O O . . O .
$$ | . . . . . . . . . .[/go]




BTW, I'm assuming in this post that you know basic dead/live shapes, and therefore you know that this:

Click Here To Show Diagram Code
[go]$$B Basic dead shape
$$ +--------------------
$$ | . O O . X O .
$$ | X X X X X O .
$$ | O O O O O O .
$$ | . . . . . . .[/go]

..is dead.
Author:  Joaz Banbeck [ Thu Aug 27, 2020 11:30 am ]
Post subject:  Re: [Hactar Go] Why is this tsumego solution correct?
This is not the first time that the issue has been discussed.

For a slightly different statement of the issue, have a look at this: https://lifein19x19.com/viewtopic.php?f=11&t=16767
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