Sudoku Players' Forum (clone)

Okay I'm having trouble understanding why the xy wing that I used did not work. If the puzzle is labeled a to I from left to right and 1-9 from top to bottom then I tried using a y thing between the cells H2, I2, G7. Which gave me a 9 in cell H3. I just don't understand how it's not right and pissing me the hell off. Can some explain this to me because as far as I'm concerned all the criteria is met they are pairs xy,yz,xz and the 9 in cell H3 results from the elimination of the because that box is shared by H7 and I2 which contains 7's

Code: Select all

*-----------*
|...|..1|.45|
|.3.|...|...|
|8..|3..|..6|
|---+---+---|
|7..|.9.|1..|
|.5.|..2|..9|
|...|7..|...|
|---+---+---|
|5.6|.2.|...|
|.2.|.1.|...|
|..9|.8.|.54|
*-----------*

This is where I think you are, when you make your error.

Code: Select all

*---------------------------------------------*
| 29 69   27   | 8  67    1    | 3    4   5   |
| 46 3    457  | 2  4567  469  | 8    19A 17B |
| 8  14   1457 | 3  457  b49   |a79   2   6   |
|--------------+---------------+--------------|
| 7  46   234  | 45 9     4568 | 1    368 238 |
| 36 5    8    | 1  36    2    | 4    7   9   |
| 29 1469 1234 | 7  346   468  | 5    368 238 |
|--------------+---------------+--------------|
| 5  8    6    | 49 2    c47   | 9-7C 13  13  |
| 34 2    34   | 59 1     57   | 6    89  78  |
| 1  7    9    | 6  8     3    | 2    5   4   |
*---------------------------------------------*

There is an XY Wing in the cells marked abc that removes 7 from r7c7, which I think you would call G7.

In words, this works as follows : If cell a is not 7 it's 9, so cell b is not 9, so it's 4, so cell c is not 4, so it's 7. You can reverse the argument, start at cell c, assume it's not 7 and conclude that cell a is 7.

The result of all of this is that at least one of cells a and c must be 7. So r7c7, which can see cells a and c, can't be 7.

I've also marked the cells you call H2, I2, G7 as A, B and C. There is no XY wing in those three cells because cells A and B don't see cell C, so the argumentation I used above breaks down.

Some links to good teaching sites that explain how XY Wings work are here and here.

Leren