dailysudoku.com

dejsmith · Joined: 23 Oct 2005 Posts: 42

I agree with Marty from his 6/27 Very Hard post. Some X Wing work & few other things makes Samgj's Very Hards qiuite enjoyable. After the basics, I solved the 6/29 Very Hard with an X Wing on 7s in R35C29, which forced R4C9=4; & the rest fell into place.

Dave

PT · Joined: 30 Jun 2006 Posts: 2

I am confused. How is it an x-wing? It is true that for c2 the 7 can only go in at r3 and r5, but for c9 it can go in at r3, r4, r5 and r6. If you look at row-wise, for r3 the 7 can go in c2, c6 and c9 while for r5 it can go in at c2, c7, and c9. There are too many cells for the usual definition of an x-wing unless there is a more general definition (not 4 corners of a rectangle).

I think the logic is as follows:

Either r1c3 7 or r1c7 7

If r1c3 7 -> r3c9 7
If r1c7 7 -> r3c2 7 -> r4c3 7

Either case r4c9 cannot be a 7.

Is there a way to solve the puzzle without using this step?

If not, does this mean the puzzles of this site have gone beyond the basic x-wing (4 corners of a rectangle)?

PT

Charles · Posted: Fri Jun 30, 2006 6:07 am Post subject:

PT: I agree with you, there is no X-Wing on 7 at the positions described. Your obversation on the (1,7) pair is excellent and correct. I asked once before if this technique has a name - I didn't see a reply. I do remember this method being described and used in previous puzzles. Charles

ravel · Joined: 21 Apr 2006 Posts: 536

This is, where you have an x-wing in 7. In rows 3 and 5 there are only 2 places for 7 in the columns 2 and 9.
Since 7 cannot be in r2c8, in box 2 it must be in r2c5 or r2c6, therefore it cannot be in r3c6 (box/box elimination)

PT · Joined: 30 Jun 2006 Posts: 2

Thanks, Ravel. Order has been restored.

I remembered reading in another thread that someone input a puzzle into the draw/play program to get a hint for the next move but the program could not do it. The explanation was that the most advanced logic the program has is the x-wing. Anything beyond x-wing the program cannot see. So when I could not see the x-wing I thought maybe the program has progressed to the point of being able to see unstructured (or amorphous) either-or scenarios like the one that I suggested. That would be a great accomplishment to program that into a computer. Alas, you dashed my hope :P

PT

Charles · Posted: Fri Jun 30, 2006 5:58 pm Post subject:

Ravel: Thanks for me too, I simply did not see the removal of 7 as a candidate in R3C6, thereby creating the X-Wing. Sometimes it seems we can look for so long that we create blind spots - this was certainly one for me. Thanks again, Charles

Marty R. · Posted: Fri Jun 30, 2006 11:09 pm Post subject:

I did it the same way as Dave, i.e., the X-Wing on "7" solved the whole puzzle. This was a disappointment compared to the June 27 puzzle, as I would like to see more of a challenge on puzzles with this rating.