## Is logic enough

Everything about Sudoku that doesn't fit in one of the other sections

### Is logic enough

This is as far as I got with a Very Hard puzzle from the Sudoku program.
I set the Flag wrong numbers on, so I know this is right.

* * 8 7 * * 5 * *
3 4 7 * 2 * * * 9
* 9 5 * * * * * 7

5 3 9 * 1 6 * 7 8
4 8 1 * 7 * * 9 *
7 6 2 8 * * 3 * 1

8 * 4 * * * * 2 *
* * 3 * 6 * * 1 4
* * 6 * * 4 * * *

I would appreciate any comments, so I can go on to another puzzle

Thank you.
Michael

Posts: 9
Joined: 14 April 2005

It has an X-Wing in the 4's... (r1c5, r1c7, r6c5 and r6c7)

Update: actually there are two X-wings with the number 4

One is: r1c5, r1c7 and r6c5, r6c7
And another one: r3c4, r3c6 and r4c4, r4c6

(if you are still lost after that then you should take a look at column 5 (the numbers 4 and 9 to be precise) and afterwards you should check the number 5....)
Animator

Posts: 469
Joined: 08 April 2005

Dear Animator,

I've reposted the puzzle and hoping there is a typo in your reference.

If the x-wings you'r referring to are actually r1c5,r1c8,r6c5,r6c8. . . . and the second x-wing should br r?c7. . . .

If it is a typo, I spotted them and eliminated the possibility of the digit 4 in those columns and the applicable rows, but it did not help enough.

If it NOT a typo, then my concept of x-wings is faulty.

I am including my pencil marks. Maybe this will help explain my confusion. I have spent a week on this (not full time), and am at a brick wall because I believe that the puzzles created by the Sudoku program can be solved by logic alone.

Hope this helps.

Puzzle so far:

* * 8|7 * *| 5 * *
3 4 7|* 2 *| * * 9
* 9 5|* * *| * * 7
--------------------
5 3 9|* 1 6| * 7 8
4 8 1|* 7 *| * 9 *
7 6 2|8 * *| 3 * 1
--------------------
8 * 4|* * *| * 2 *
* * 3|* 6 *| * 1 4
* * 6|* * 4| * * *

Pencil marks:
126* | 12** |****| |****|49**|139*| |****|346*|23**|
**** | **** |****| |156*|****|158*| |18**|68**|****|
126* | **** |****| |1346|38**|138*| |1248|368*|****|

**** | **** |****| |24**|****|****| |24**|****|****|
**** | **** |****| |235*|****|235*| |26**|****|256*|
**** | **** |****| |****|49**|59**| |****|45**|****|

**** |157* |****| |1359|35**|1357| |679*|****|356*|
29** |257* |****| |259*|****|2578| |789*|****|****|
129* |127* |****| |129*|358*|****| |79**|358*|35**|

Sorry for the awkwardness, but it is hard to align with a proportional spaced font.

Thanks again,
Michael

Posts: 9
Joined: 14 April 2005

Looking at your pencil marks, you can eliminate 5 from box 8, except for col 5 (all the places for 5 in col 5 are in box 8). This leaves r8c2 as the only place for 5 on row 8.
Guest

Dear IJ

Thanks again. Your reply took me through the rest of the puzzle quite simply.

This is the second time you helped me. It's nice to know someone takes the time to help the less expeienced.

Michael

Posts: 9
Joined: 14 April 2005

### X-Wing

i guess "X-Wing" means there is more than one position in a box for any missing number - any advances on that? - please - and if so how does that make it any easier to place the number when there are multiple choices?
jayal

Posts: 15
Joined: 18 April 2005

Hi Javal,

X-wings is a pattern where there are four candidate cells for a particular digit that form a box of two rows and two columns, or an "X". These four cells can be anywhere in the puzzle.

The critical factor is that either both rows, or both columns must have no other candidate cells. This ensures that either the bottom-left and top-right, or the top-left and bottom-right cells in the X will be the digit in question. The neat thing is that you don't need to know which, as you have determined that for each row and column, the digit must go in one of the two cells in the X. From this, you can eliminate the digit in question as a possibility from any other cells in the two rows and columns.

Make sense?
Guest

Animator

Posts: 469
Joined: 08 April 2005