## stuck with hard sudoku

Post the puzzle or solving technique that's causing you trouble and someone will help

### stuck with hard sudoku

119173024_10157253080641360_715841574267469136_o.jpg (69.3 KiB) Viewed 239 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.
urhegyi

Posts: 406
Joined: 13 April 2020

### Re: stuck with hard sudoku

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.

Leren
Leren

Posts: 4066
Joined: 03 June 2012

### Re: stuck with hard sudoku

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....

Hajime

Posts: 355
Joined: 20 April 2018
Location: Netherlands

### Re: stuck with hard sudoku

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.
urhegyi

Posts: 406
Joined: 13 April 2020

### Re: stuck with hard sudoku

SE has this as an 8.3, FWIW.
mith

Posts: 459
Joined: 14 July 2020

### Re: stuck with hard sudoku

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***********************************************************************************************singles146 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 ≠ 9z-chain[3]: r1n4{c1 c7} - r3c8{n4 n2} - r2n2{c9 .} ==> r1c1 ≠ 2z-chain[3]: r1n4{c1 c7} - c7n9{r1 r3} - r3n5{c7 .} ==> r1c1 ≠ 5biv-chain[4]: r7c1{n5 n4} - r1n4{c1 c7} - r1n9{c7 c6} - r2n9{c4 c1} ==> r2c1 ≠ 5t-whip[4]: r3c8{n4 n2} - c9n2{r2 r6} - c6n2{r6 r1} - r1n9{c6 .} ==> r1c7 ≠ 4singlesstte`
denis_berthier
2010 Supporter

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Location: Paris