Extreme sudoku - stuck at end

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Extreme sudoku - stuck at end

Postby mlancas3 » Thu Aug 15, 2019 1:16 pm

I'm extremely (heh, no pun!) close to solving this guy but I'm just not seeing it. It's a valid puzzle. What am I missing?

Code: Select all
1    7    5    |   4    39   2    |   89   6    389
2    49   34   |   6    39   8    |   5    7    1
89   6    38   |   1    7    5    |   2    4    39
---------------+------------------+-------------------
5    2    7    |   9    8    3    |   6    1    4
4    89   1    |   2    5    6    |   7    3    89
89   3    6    |   7    4    1    |   89   5    2
---------------+------------------+-------------------
3    1    2    |   5    6    9    |   4    8    7
7    5    9    |   8    1    4    |   3    2    6
6    48   48   |   3    2    7    |   1    9    5
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Re: Extreme sudoku - stuck at end

Postby SpAce » Thu Aug 15, 2019 3:56 pm

mlancas3 wrote:I'm extremely (heh, no pun!) close to solving this guy but I'm just not seeing it. It's a valid puzzle. What am I missing?

Code: Select all
1    7    5    |   4    39   2    |   89   6    38+9
2    49   34   |   6    39   8    |   5    7    1
89   6    38   |   1    7    5    |   2    4    39
---------------+------------------+-------------------
5    2    7    |   9    8    3    |   6    1    4
4    89   1    |   2    5    6    |   7    3    89
89   3    6    |   7    4    1    |   89   5    2
---------------+------------------+-------------------
3    1    2    |   5    6    9    |   4    8    7
7    5    9    |   8    1    4    |   3    2    6
6    48   48   |   3    2    7    |   1    9    5

Several possibilities. For me the most obvious solution is BUG+1. The 9 in r1c9 is the only candidate preventing the BUG (a deadly pattern), so it must be true. That solves the rest. Otherwise you could see easily a Remote Pair with the (89) cells, which lets you eliminate the 9 in r3c9, also solving the puzzle. You can do the same with a Skyscraper or a 2-String Kite as well. Simple Coloring on 9s works too.
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Re: Extreme sudoku - stuck at end

Postby Wecoc » Thu Aug 15, 2019 4:04 pm

There's a remote pair.
Look at the 89 in r1c7:
- If it's an 8 then r6c7 is a 9 -> r6c1 is an 8 -> r3c1 is a 9 -> r3c9 can't be a 9
- If it's an 9 obviously r3c9 can't be a 9 either.

Edit: Sorry, didn't see SpAce's post
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Re: Extreme sudoku - stuck at end

Postby mlancas3 » Thu Aug 15, 2019 5:11 pm

Well, folks, that's what distinguishes an expert from an intermediate. It's clearly time I learn the really advanced techniques. Thank you both for that!
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Re: Extreme sudoku - stuck at end

Postby SpAce » Thu Aug 15, 2019 5:33 pm

Hi Wecoc,

Wecoc wrote:There's a remote pair.
Look at the 89 in r1c7:
- If it's an 8 then r6c7 is a 9 -> r6c1 is an 8 -> r3c1 is a 9 -> r3c9 can't be a 9
- If it's an 9 obviously r3c9 can't be a 9 either.

That's a good recommendation because Remote Pair is one of the easiest patterns to spot and it works here nicely. However, it's also good to learn to see what that pattern actually contains. It always has two parallel X-Chains, one for each digit, which makes it capable of eliminating both digits. (Similarly it can be seen as a dual XY-Chain, but it's less efficient so I don't like to see it that way). Here we're only eliminating one digit and not both, so we're actually using only one of the two chains. Hence, to be more surgical, I might rather report the move as a Skyscraper (a single X-Chain), even if I spotted it as a Remote Pair. If it could eliminate both digits, then I'd of course report it as a Remote Pair.

This is just a minor nuance. Of course there's nothing wrong with using a Remote Pair to eliminate a single digit, but why shoot a double-barrel when only one can hit a target? :)
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