by **Shazbot** » Thu Dec 29, 2005 6:35 am

That's over my head, and I would think not required in a book called "Sudoku for Dummies"...

From the previous candidate list, there's a naked quad (1459) in row 7 allowing you to remove those candidates from all other cells in that row.

This gives you locked candidates - in row 7, the only place for a 5 is in box 8, so all other 5s in box 8 (apart from row 7) can be removed.

That's as far as I could get too. Simple Sudoku couldn't give any more hints. Dubbed into Pappocom Sudoku, I'm told the clues do not form a valid puzzle - if I try 1 in r6c4 I'm told it's invalid, and if I try 9 it's accepted. Dubbed into Sadman Sudoku I'm told there IS only 1 solution, but it gives the next step as a guess.

I believe from what you have above, you can apply forcing chains, but I'm only learning the technique, so I may be wrong in saying that's the way to go. If you try a 1 in r6c4, it'll eventually lead you to a state where r6c4 is 1, and r4c5 is 5, leaving r4c6 with no candidates. So r6c4 cannot be 1, and must be 9. I'll get an argument here, but to me that's guessing - there's no logical next step, so you have to pick a cell and try a number, and continue down that path until you either solve the puzzle or run into an impossible situation.

So, getting back to my first statement, I also think "forcing chains" is a technique not appropriate for a book called "Sudoku for Dummies", so I'm wondering if the publisher believes a guess is an acceptable technique. Out of interest, what's the difficulty rating given to that puzzle? (and, would you mind double-checking to make sure you've copied all the numbers correctly, and possibly posting the original puzzle as it appears in the book, without your own progress?)