## Problem with very hard puzzle (dailysudoku 28-May-05)

Post the puzzle or solving technique that's causing you trouble and someone will help

### Problem with very hard puzzle (dailysudoku 28-May-05)

[ subject-line edited by Moderator ]

I solved the puzzle this far:

030200781
008000304
074030960
000180607
017609038
800375149
041060890
700000410
980413070

Robert Craven

Posts: 6
Joined: 11 June 2005

Check carefully where a 6 can go in box 1.

Then look for possible groups in row 2.
SteveF

Posts: 86
Joined: 26 March 2005

If you need a more detailed clue, then here it is:

There is an X-Wing in the 6s in boxes 1 and 2, and there are disjoint subsets in row 2. For my explanation of disjoint subsets, click here and scroll down.

Hope this helps,

George
george-no1

Posts: 150
Joined: 20 May 2005

Please, do everyone a favor: if you copy the puzzle from here then add a reference to it... It's the same grid as the one posted in http://forum.enjoysudoku.com/viewtopic.php?t=373 (I found the 2/5 pairs in box 3, 6 and 9 way too familiar so I checked some other grids I had.)

George, in the starting grid there is no X-wing in the number 6.

Once you do what SteveF was hitting on (the number 6 in column 1) then there is a (useless) X-wing, but not right from the start.
Animator

Posts: 469
Joined: 08 April 2005

Animator wrote:George, in the starting grid there is no X-wing in the number 6.

Well I'm not sure why you think there isn't, because I used one in rows 1 and 2 to eliminate the possibility of a 6 in r1c3 and r2c2, which enabled me to use the disjoint subsets and then solve the puzzle (having not spotted that the 1 could only go in r1c1 or r2c1). However, I may be completely wrong, as I have been in the past.

If I am wrong, please explain why that isn't an X-Wing - I'm not too familiar with the ins and outs of all the techniques - I simply make it up as I go along (before I knew what an X-Wing was, I called it 'the rectangular rule' because I discovered it and saw that it used 4 cells that make a rectangle).

George
george-no1

Posts: 150
Joined: 20 May 2005

Here are the candidates for the number 6 (the one with the questionmark) right from the grid that was posted. (without making any eliminations):

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

The main thing about an X-wing is that you find two rows/columns with only two possible cells for the number 6. Do you see that in these candidates? I don't.

Row 1 and row 2 both have 3 possible cells for the number 6.

Yes you can elminate the number 6 from r1c3 and r2c2 but it's not because there's an X-wing.
Animator

Posts: 469
Joined: 08 April 2005

Animator wrote:The main thing about an X-wing is that you find two rows/columns with only two possible cells for the number 6. Do you see that in these candidates? I don't.

What about the 6s in r1c1, r2c1, r1c6 and r2c6? Don't they form an X-Wing meaning that all other possibilities for 6s in rows 1 and 2 can be removed? Doesn't this mean that an X-Wing removes the 6s form r1c3 and r2c2? Or am I going mad?

George
george-no1

Posts: 150
Joined: 20 May 2005

You are correct... apperently I didn't look at the columns for some reason...
Animator

Posts: 469
Joined: 08 April 2005

OK, thanks for verifying that. We all make mistakes, as I know very well!

George
george-no1

Posts: 150
Joined: 20 May 2005

Thanks alot for your help. With your hints the puzzle became quite easy.

Sorry for not referencing the puzzle (It is from dailysudoku 28-May-05)
Robert Craven

Posts: 6
Joined: 11 June 2005