stuck with hard sudoku

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stuck with hard sudoku

Postby urhegyi » Tue Sep 15, 2020 10:48 am

119173024_10157253080641360_715841574267469136_o.jpg
119173024_10157253080641360_715841574267469136_o.jpg (69.3 KiB) Viewed 84 times
I saw this hard rated sudoku on a forum. I can solve it with an x-wing, 2 grouped discontinuous nice loops with als chain included, an xy-wing, an AIC and a scyscraper. But as you can see on the picture it's a hard one. Am I missing a (new) technique, or is it under-rated and must it be solved the extreme way.
Code: Select all
.1.........8.4.7.....8....6..3..58.116......9............28.6...9.6...3..371...5.
Last edited by urhegyi on Tue Sep 15, 2020 3:43 pm, edited 3 times in total.
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Re: stuck with hard sudoku

Postby Leren » Tue Sep 15, 2020 11:12 am

You need to remove the first . in the line format - otherwise the puzzle is invalid.

Both Hodoku and Andrew Stuart's solvers used at least one forcing chain, so it looks genuinely tough to me.

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Re: stuck with hard sudoku

Postby Hajime » Tue Sep 15, 2020 2:09 pm

Leren wrote:You need to remove the first . in the line format - otherwise the puzzle is invalid.

But the second cell has a given "1". So an empty first cell must be a "." . That also complies with the 116 sequence with a 1 as last cell of row 4 and 16 as the first 2 cells of row 5.
So the first "." cannot be skipped....
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Re: stuck with hard sudoku

Postby urhegyi » Tue Sep 15, 2020 2:52 pm

Hajime wrote:
Leren wrote:You need to remove the first . in the line format - otherwise the puzzle is invalid.

But the second cell has a given "1". So an empty first cell must be a "." . That also complies with the 116 sequence with a 1 as last cell of row 4 and 16 as the first 2 cells of row 5.
So the first "." cannot be skipped....

There was a . from the last sentence that came by accident into the line format, but meanwhile it's corrected.
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Re: stuck with hard sudoku

Postby mith » Tue Sep 15, 2020 5:14 pm

SE has this as an 8.3, FWIW.
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Re: stuck with hard sudoku

Postby denis_berthier » Wed Sep 16, 2020 10:16 am

Relatively easy

Code: Select all
(solve ".1.........8.4.7.....8....6..3..58.116......9............28.6...9.6...3..371...5.")
***********************************************************************************************
***  SudoRules 20.1.s based on CSP-Rules 2.1.s, config = W+SFin
***  Using CLIPS 6.32-r770
***********************************************************************************************
singles
146 candidates, 674 csp-links and 674 links. Density = 6.37%
x-wing-in-rows: n9{r2 r4}{c1 c4} ==> r6c4 ≠ 9, r6c1 ≠ 9, r3c1 ≠ 9, r1c4 ≠ 9, r1c1 ≠ 9
z-chain[3]: r1n4{c1 c7} - r3c8{n4 n2} - r2n2{c9 .} ==> r1c1 ≠ 2
z-chain[3]: r1n4{c1 c7} - c7n9{r1 r3} - r3n5{c7 .} ==> r1c1 ≠ 5
biv-chain[4]: r7c1{n5 n4} - r1n4{c1 c7} - r1n9{c7 c6} - r2n9{c4 c1} ==> r2c1 ≠ 5
t-whip[4]: r3c8{n4 n2} - c9n2{r2 r6} - c6n2{r6 r1} - r1n9{c6 .} ==> r1c7 ≠ 4
singles
stte
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