The New Sudoku Players' Forum

[pajr777]

Hi everyone, looks like this place might be a good place to ask for help!

I've been using the program to learn and have finally attempted a few 'Very Hard' puzzles with mixed success. I think there must be a line of logic that I haven't had to use before as I'm now stumped!

Here is the original puzzle:

6?? ??? 12?
??8 ?5? ?7?
??? 3?8 ??5

??? 6?? 89?
??? ??? ???
?15 ??9 ???

1?? 2?3 ???
?7? ?6? 4??
?34 ??? ??1

And here is where I've got to:

653 [79][79]4 128
[24][249]8 156 [39]7[349]
7[49]1 328 [69][46]5

[234][24][27] 615 89[2347]
[3489]6[79] [478][378]2 51[347]
[2348]15 [478][378]9 [237][34]6

186 243 [79]5[79]
[259]7[29] [589]61 4[38][23]
[259]34 [589][89]7 [26][68]1

Any help on the next step would be greatly appreciated!

[scrose]

Make an elimination based on where the candidate 9's are in column 2.
Look for an x-wing in the candidate 2's and make some eliminations.
You can now fill all of the cells, one after the other, in box 1.

Please reply if you need a more specific hint.

[pajr777]

Thanks for the quick reply, can you elaborate please? I can see that the 9 in column two must be at ether r2c2 or r3c2, but as there are no other 9s in that 3x3 box I can't see what to eliminate..

[scrose]

It sounds like you've already eliminated the candidate 9 from r2c1. (Edit: Yes, my mistake. I didn't look closely enough at your candidates.) You can go ahead and look for the x-wing.

[pajr777]

Thanks, the x-wing was the key.

[EDIT: Ignore the rest, I figured it out. Thanks!]

I assume that you can eliminate all other candidates in the two rows and columns concerned once you've found an x-wing, and not just those within the square created by the candidate pairs?

Is there an easy to understand explanation somewhere of how the x-wing works? Although I've just used it to finish that puzzle, I'd like to understand how it works.

Thanks for your help, I think this might be a bit of a breakthrough for me

[tom375]

pajr777 writes above:

"Is there an easy to understand explanation somewhere of how the x-wing works?"

I'd like to know too. angusj.com seems to have the best instructions for how to solve but I'm still far from understanding. Reading these entries makes me feel pretty dumb. Are X-wing, Turbot fish, etc., only Sudoku terms or are they from Set Theory Maths?

[simes]

XWing and Turbot are Sudoku terms. I doubt they're anything to do with set theory.

For an alternative explanation of XWings,here's my attempt

[pajr777]

pajr777 writes above:

"Is there an easy to understand explanation somewhere of how the x-wing works?"

I'd like to know too. angusj.com seems to have the best instructions for how to solve but I'm still far from understanding. Reading these entries makes me feel pretty dumb. Are X-wing, Turbot fish, etc., only Sudoku terms or are they from Set Theory Maths?

Xwing works because there are two rows or columns with only two possible cells for a given candidate and the cells form four corners of a square. This means that there are only two possible configurations which will solve that candidate - the candidate must be placed in the top left/bottom right or the top right/bottom left of the square. Therefore any other cells in the same two rows or the same two columns of the xwing cannot contain the candidate and can therefore be eliminated.

[scrose]

tom375, I recently wrote an explanation of how to identify an x-wing based on a puzzle that was being discussed. You might find it helpful.

[Cec]

pajr777 and tom375 wrote "Is there an easy to understand explanation somewhere of how the x-wing works".

I think the explanation Scrose is referring to is explained in "Can't solve hard puzzles" (Post July 16) which helped me a lot. Ignore the "rubbish" I submitted in trying to come up with a simpler explanation.

Regards Bonsai Cec

[scrose]

Here is the explanation that cecbevwr is referring to.