## Very Hard...

Advanced methods and approaches for solving Sudoku puzzles

### Very Hard...

A second Very Hard from the Sudoku programme has me looking at a pair of 2,8 in column 2 but cannot see where this takes me...I cannot see any Xwings so any help appreciated....

6*5 3*2 *18
*** 16* *7*
*9* 5** 6**

5*2 736 *8*
*** 485 ***
*3* 921 75*

**4 **3 *9*
*5* **4 ***
3** 85* 142
Arnie

Posts: 49
Joined: 19 May 2005

Did you spot the pair in column 7? If yes, then you should be able to see an X-wing in the number 4.
Animator

Posts: 469
Joined: 08 April 2005

now spotted it...thanks!! allows 4 to be inserted at r3c5...
Arnie

Posts: 49
Joined: 19 May 2005

Okay, newbie question. What pair in column 7? Do you mean R1 v. R4?

Mind, I was able to solve the puzzle, but only by a combination of brute force and intuition. Indeed, insofar as I can tell, not a single open value is determined by the given values, whether tested by row, column or square. Am I missing something?
PBear42

Posts: 6
Joined: 08 June 2005

column 7 has a pair of 4 and 9 in r1 and r4. Looking in column 2 , 4 can only be in r1 and 4 so there is an Xwing of 4's . From studying box 2, 4 can only now go in r3c5 as the xwing precludes 4 from being placed in r1c5
Arnie

Posts: 49
Joined: 19 May 2005

I got there, albeit by a different route (which in certain respects resembled dumb luck). Mostly, wanted to make sure I understood the lingo, for which your reply was perfect. Thanks.
PBear42

Posts: 6
Joined: 08 June 2005

Okay, I'm new, but don't see it. (Couldn't figure it out last night either, but figured I was just tired.) What in column two (or anywhere else) precludes a four at 2,2 (r2c2), which breaks the x-wing. For example, seems to me that, from the given values, you could have, 1,2 = 7; 1,7 = 4; 1,5 and 4,7 = 9; and 3,5 = 7. I realize those aren't, in fact, the correct values. But I don't see how you're excluding them at this stage.

On the other hand, as stated, I'm a newbie. Maybe I'm missing something.
PBear42

Posts: 6
Joined: 08 June 2005

look at the middle stack (ie boxes 2,5,8). Look where 4 can go in box 2...only 2 possible cells.

The Xwing of 4's means there cannot be a 4 at r1c5. Does that help?
Arnie

Posts: 49
Joined: 19 May 2005

Please give the coordinates of your x-wing. As mentioned, the one I think you're trying to use is broken (I think) at 2,2.
PBear42

Posts: 6
Joined: 08 June 2005

It isn't... look at Arnie's first post... he talks about the pair of 2 and 9 in column 2...

Because of that pair it is impossible to have the number 4 in r2c2.
Animator

Posts: 469
Joined: 08 April 2005

Well, yes, I did see that in the OP. That's why I've been asking what precludes the four at 2,2. I've triple-checked and don't see no pair [sic] in column two, nor yet in box 1. Coordinates please.
PBear42

Posts: 6
Joined: 08 June 2005

Look at column 2. Where can the number 2 go? Where can the number 8 go? do you see something special about these places?
Animator

Posts: 469
Joined: 08 April 2005

Thanks. Now I get it.
PBear42

Posts: 6
Joined: 08 June 2005