The New Sudoku Players' Forum

[QBasicMac]

Well, I am tired of staring at this puzzle (taken from The Daily Sudoku Wed 2-Nov-2005).

Please tell me (preferably in detail) what is next. I know my present position has a solution. It is easily found by T&E.

Mac


Solution So Far

Code: Select all

48- 3-- ---
9-7 452 -8-
--3 1-8 74-
8-5 713 4--
7-- 946 --8
-49 825 1-7
-94 581 27-
17- 234 --5
--- 6-- -14


Pencilmarks So Far

Code: Select all

-         -         126       -         679       79        569       2569      1269     
-         16        -         -         -         -         36        -         136     
25        256       -         -         69        -         -         -         269     
-         26        -         -         -         -         -         269       269     
-         123       12        -         -         -         35        235       -       
36        -         -         -         -         -         -         36        -       
36        -         -         -         -         -         -         -         36       
-         -         68        -         -         -         689       69        -       
25        235       28        -         79        79        38        -         -       


[CathyW]

The uniqueness test would determine that r1c5 must be 6, otherwise the puzzle would have at least 2 solutions because the 7s and 9s in r1c4&5 and r9 c4&5 could be swapped around.

Then you have locked candidates (6s) in box 1 which allows you to place r6c1.

The rest should shake out with hidden and naked singles.

Hope that's what you wanted.

[Shazbot]

In column 8, candidate 3s are locked to box 6, so you can eliminate candidate 3 from r5c7, which leaves a naked single (5) in r1c8.

Then in box 3, candidate 2s are locked to column 9, so you can eliminate 2 from r4c9.

Then look for a naked triple in row 1, followed by a hidden single in column 3.

This may be more information than you needed, but the puzzle can then be completed with simple naked singles from there.

I haven't studied up on the uniqueness test CathyW mentioned, but from her explanation it sounds like you need to assume, or know, that the puzzle has one unique solution - I'll have to read up a bit on that method. The above is another method to get to the same solution.

[QBasicMac]

The uniqueness test .... Hope that's what you wanted.

Heh, Sorry, CathyW, mais je ne comprends pas. Thanks, anyway.

In column 8, candidate 3s are locked to box 6, so you can eliminate candidate 3 from r5c7, which leaves a naked single (5) in r1c8.

Then in box 3, candidate 2s are locked to column 9, so you can eliminate 2 from r4c9.

Hey, thanks!! That is exactly what I needed. I simply could not see the "candidate 3s are locked to box 6". Blind spot.

Then look for a naked triple in row 1, followed by a hidden single in column 3.

Well, I can't see the naked triple for some reason. But r1c3 and r1c9 have the only occurrences of 12, so
r1c3: {126} >> {12}
r1c9: {1269} >> {12}

Then r9c3=8 because r1c3=r5c3=12 and the rest is automatic.

Thanks again,

Mac

[Shazbot]

After you find your naked single 5 in r1c8 you can eliminate 5 from r1c7, then you're left with the following naked single in row 1 (in bold)

{4} {8} {126} {3} {679} {79} {69} {5} {1269}

Those 3 cells can ONLY contain those 3 numbers, so you can eliminate 6, 7 and 9 from all other cells in that row, leaving you with

{4} {8} {12} {3} {679} {79} {69} {5} {12}

[Lummox JR]

I haven't studied up on the uniqueness test CathyW mentioned, but from her explanation it sounds like you need to assume, or know, that the puzzle has one unique solution - I'll have to read up a bit on that method. The above is another method to get to the same solution.

You can always make that assumption, just as safely as assuming the puzzle has all its clues placed properly and isn't unsolvable. A sudoku with multiple solutions is invalid. If you're attempting to solve one, you're already wasting your time.

[CathyW]

There's quite a lot about "uniqueness" here: http://forum.enjoysudoku.com/viewtopic.php?t=2000

I have to say I always assume that a puzzle has a unique solution, unless proven otherwise through being unable to solve and/or dubbing in the clues to software that tells you there is more than one solution. I agree with Lummox about it being a waste of time trying to solve something that has more than one solution.