The New Sudoku Players' Forum

[Aiesh]

Again a question about the very hard ones... I can solve a hard sudoku, but i get stuck in each very hard sudoku
I guess I need a technique or something...
Here's an example of a sudoku in which I am stuck, what do I miss?

[Aiesh]

Sorry, I didn't totally understand the X-wing, now I do, I guess that was my problem

[Karyobin]

I've been there too. Nice trick innit? (When you spot 'em amongst all the mess, that is.)

[Aiesh]

True
I only know sudoku since yesterday, so still a long way to go I guess

[blackadder]

Check the x-wing pattern with candidate 5 in columns 5 and 6. Let me know if you need more help.

Best,

Gil

[blackadder]

Check the x-wing pattern with candidate 5 in columns 5 and 6.

Sorry, this is not an x-wing. You should find a true x-wing with candidate 2 on rows 2 and 4.

[Wolfgang]

You should find a true x-wing with candidate 2 on rows 2 and 4.

And the one in 8 in rows 1 and 5 will help you.

[Wolfgang]

And the one in 8 in rows 1 and 5 will help you.

After that look at columns 1 and 9

[Bunnybuck]

You should find a true x-wing with candidate 2 on rows 2 and 4.

And the one in 8 in rows 1 and 5 will help you.

There is another helpful x-wing in columns 7 and 9 with candidate 5.

[Animator]

First of all, re-check your pencilmarks (there is already a 5 in column 9).

Second, there is a useless X-Wing in the number 2 as pointed out, and there is a helpful X-wing in the number 8.

So take a really good look at the number 8.

[chabo]

There is an X-Wing pattern for the value 8 in the cells (1,3) (1,7) (5,3) and (5,7). This eliminates the value 8 for cell (2,3).
==> The value 1 is the only candidate for cell (2,3)

Read more about X-Wings at:
http://daily-sudoku.tk

[Jan Otto]

The left box in the middle, row 4,5 and 6, column 1,2 and 3, have 5 cells with a single value and 4 cells with a combination of two numbers. The box can be expressed in two different ways, alternative A setting cell (4.1) to the value of 2, or alternative B setting cell (4.1) to 5. The other cells will be determined accordingly for each of the two alternatives. Continue the Sudoku puzzle with alternative A (all cells having only one unique number) and see what it leads to for the other boxes. If you reach an impossible combination for one of the cells in the other boxes, then the alternative A is impossible, and you know that alternative B is the right one. Continue in this way and select alternatives only from boxes with cells having a combination of two numbers.
Regards Jan Otto

[tom375]

If anyone is still reading this - Jan Otto's suggestion above just looks like trial and error to me. I have been following this problem to try to improve my technique. I have looked at the excellent angusj.com web site. I kind of understand what X-wings are but I don't understand the theory behind them. I tried Chabo's advice about the X-wing for 8 but finding 1 for r2c3 didn't get me anywhere.