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[Louise56]

Usually someone else posts and I check my efforts against theirs. I'm not sure what the technique is called but in r8c6 there is a 14 pair. Whether it is a 1 or a 4, the 3 in r4c9 will be eliminated. Then you have a 239 y-wing with the pivot in r5c8.

[Louise56]

It is less confusing if you look at the 47 pair in r4c6. Whether it is a 4 or a 7 the 3 in r4c9 is eliminated.

[Marty R.]

Louise, could you post the puzzle? That may generate some response.

(I don't recall if you have posted grids before, but if you're unfamiliar with the how-to's, this will explain:

http://www.dailysudoku.co.uk/sudoku/forums/viewtopic.php?t=1905

[Louise56]

Thanks Marty. I have never posted code before. Thanks for the directions. This puzzle was in my newspaper on Saturday. Every Sat there is a 6 star puzzle that requires advanced techniques. Someone always posts how they solved it, but not this week. The code looks a bit sloppy but here it is:

+---------+--------------+-----------+
| 1 5 4 | 8 7 3 | 2 9 6 |
| 6 7 9 | 4 1 2 | 3 8 5 |
| 2 3 8 | 9 56 56 | 1 7 4 |
+---------+--------------+-----------+
| 379 8 5 | 37 24 47 | 6 1 239 |
| 39 4 1 | 36 256 568 | 7 23 2389 |
| 37 2 6 | 137 9 178 | 4 5 38 |
+---------+--------------+-----------+
| 8 9 3 | 167 46 1467 | 5 24 12 |
| 5 6 7 | 2 8 14 | 9 34 13 |
| 4 1 2 | 5 3 9 | 8 6 7 |
+---------+--------------+-----------+

[Mogulmeister]

Hope this helps

[Louise56]

Thanks so much for posting it. What did I do wrong?

[Marty R.]

Louise, what you have described is two different bivalue cells. Looking at each cell, either of the two values will cause the same result in another cell.
That looks to me like a Forcing Chain.

[peterj]

Louise, the way you descibe it is as a forcing chain - as Marty says. But a more useful and politcally acceptable way to look at it is an xy-chain (one of my favourites!).

You can write it like this...

(3=7)r4c4-(7=4)r4c6-(4=1)r8c6-(1=3)r8c9; r4c9<>3

which means

if r4c4 is not 3 then it's 7, and so r4c6 is not 7 and so it's 4, and so r8c6 is not 4 and so it's 1 etc. till r8c9 is 3

This type of chain is a double implication chain which means it's reversible so you can play it backwards - one end of the chain must be a 3 (or both could be 3) and so anything seen by both ends can't be a 3.

Hence your elimination - good spot!

[Louise56]

Thank-you Peter and Marty. I have the code figured out now so I won't hesitate to post next time.

[keith]

[tlanglet]