dailysudoku.com

[Paladin]

This puzzle appeared at www.onlysudoku.com.

[David Bryant]

Hi, Paladin!

The first "chain" you mentioned (r1c1 - r6c1 - r5c2) is called an XY-Wing. It's usually analyzed from the "corner", which lies at r6c1.

A. r6c1 = 7 ==> r1c1 = 5 ==> r1c2 = 4
B. r6c1 = 9 ==> r5c2 = 5 ==> r1c2 = 4

I don't quite follow the logic you used to eliminate "7" at r6c3; what I see here is another XY-Wing which allows us to eliminate the "3" at r6c3:

A. r6c1 = 7 ==> r4c3 = 3 ==> r6c3 <> 3
B. r6c1 = 9 ==> r6c5 = 3 ==> r6c3 <> 3

The XY-Wing is the simplest possible form of the "double-implication chain" technique, which has been extensively analyzed in this forum. This message contains quite a few links to disussions of the XY-Wing technique.

[Marty R.]

Paladin,

I also don't see how you eliminated that "7" at r6c3.

[dejsmith]

Guys

I realize you were only addressing Paladin's immediate question; & I am really only writing to let David, Keith, & Someone (when he returns) know I am still alive; but this puzzle should not even get one worrying about such techniques since there is a hidden pair in R1C1&3 (57) that serves to solve the puzzle. I make it a point to always read your comments on this site & the Sudocue/Nightmare site. Thanks for reviewing XY Wing, though, as I have not had the opportunity to use it lately.

Dave

[Paladin]

Dear David and Marty:

Thank you for your responses.

It seems that I may have confused this issue because I did not arrive at my conclusions, in either case, with reference to the “corner” analysis that is apparently applicable to the XY Wing rule. Since I did not understand how to correctly apply the XY Wing rule by using the “corner” concept, I used the following thought processes:

r1c1, r6c1 and r5c2 represent a “chain” where either 5,7 or 9 must appear in these cells. As it relates to the “chain”, the value 5 must appear in either f1c1 or r5c2; therefore, since r1c2 is encapsulated within, or impacted by, this certainty, r1c2 can not equal 5.

Having more fully described my thoughts, it is perhaps easier to see why I excluded 7 from r6c3, as follows:

r4c3 (3,7), r6c1 (7,9) and r6c5 (3,9) are a 3,7, 9 chain. Within that chain, the value 7 can only, and must, appear at either r4c3 or r6c1. Since r6c3 is encapsulated within, or impacted by, this certainty, r6c3 can not equal 7.

Now, having reflected upon your conclusion that an XY wing also excludes 3 from r6c3, the following has occurred to me:

Because of the relative locations of the 3,7,9 chain, (r4c3, r6c1 and r6c5), the value of r6c3 is dependent upon the values of all the cells which form the 3.7,9 chain. In this case, and based upon the positioning of the “chain” cells, 3,7 and 9 can all be removed from r6c3. R6c3 must, and in the final analysis does, equal 2.

I can see that applying this “new” (or at least new to me) chain analysis will be very helpful. I have noticed many chains which involve 3 values where the chains (although linked) are “fractured” into varying blocks, rows and columns.

Your thoughts?

Paladin

For your convenience, the puzzle is as follows:

[TKiel]

Paladin,

What you are saying is that any cell that 'sees' all three cells of an XY-wing can't contain any of the digits that make up the XY-wing.

It definitely works for this puzzle but I'm not sure that should be taken as proof that it will always work. I'd post it at
Sudoku.com to see if somebody there can yea or nay it, which somebody will definitely do.

[ravel]

[Paladin]

ravel,

Thank you very much for taking the time to share your Sudoku knowledge and expertise with us. I understand that you have indicated that a three value, three cell “chain” (configured as: xy, xz, yz) does not act as a “triple” unless all three cells share the same “unit” (i.e., the same row/column/box).

I guess my question, however, is: “Why do these non-unitary cells (r4c3, r6c1 and r6c5), apparently at least in this particular case, function as a “triple”?

Comments, anyone?

Paladin

PS- Although I do not know if this is relevant to this discussion, I do notice that this grid contains a non-productive swordfish of 7s at r1c13, r4c37 and r6c137. The grid in question appears as follows:

[ravel]