The New Sudoku Players' Forum

[SarC]

I'm trying to understand the basic uniqueness test (1a I think its called).
I thought I could eliminate candadites 1 and 7 from R5C2, and I can't see why it's not possible, in reference to the u-test. Any explanation would be appreciated

[Nick67]

Hi SarC,

The key is that each corner of a unique rectangle must share a box
with another corner. In your rectangle, each corner is in its own box,
so you can't use the uniqueness logic.

Last week, some guy wrote this long-winded explanation about why this is so.

[Lummox JR]

Indeed uniqueness does not apply, because the standard test requires 2 columns, 2 rows, 2 boxes; this has 4 boxes.

However if you look at the cells you highlighted in column 2, you'll see a naked triple. You also have a naked pair in column 7, and a hidden pair in row 9.

[SarC]

Thanks for the replies - I think I get it now.

However if you look at the cells you highlighted in column 2, you'll see a naked triple. You also have a naked pair in column 7, and a hidden pair in row 9.

That was very nice of you Lummox, but I have no problem solving it - it was just the principles of the uniqueness test I wanted to learn

[quillpig]

However if you look at the cells you highlighted in column 2, you'll see a naked triple. You also have a naked pair in column 7, and a hidden pair in row 9.

OK, this is where I could use some help. Instead of me explaining how I don't see the naket triple and naked pair, could you explain why they are there, and what tests have to be met for them to be there?

Thanks,

QP

[stuartn]

Interestingly, a good example of UT turned up in todays Times 'difficult'

at this stage:

Code: Select all

796354812
431289567
 [258]  [58]  [28] 7164 [39]  [39]
 [18] 2 [78] 943 [17] 56
 [15]  [15] 4 [68]  [67]  [78]  [239]  [239]  [39]
963521 [379] 48
6 [178]  [289] 4 [79] 5 [1239]  [2379]  [139]
 [12]  [17] 5 [18] 3 [278] 6 [279] 4
34 [29]  [16]  [679]  [27]  [129] 85


I believe that R5C8 has to be a 2?

[SarC]

@quillpig:
For the naked triple, the cells are R2C2, R5C2 and R8C2, with the values [75], [175] and [17].
The naked pair is in R5C7 and R8C7, with the values 1 and 7.
The hidden pair is in R9C2 and R9C3 with the values 2 and 8.

For an guide to naked and hidden subset, try http://www.simes.clara.co.uk/programs/sudokutechniques.htm

[CathyW]

I believe that R5C8 has to be a 2?

It sure does! Didn't spot it at the time after redrawing the puzzle because my DH messed it up!

I'm still not sure I fully understand why the uniqueness test can't apply when the situation is as shown above, i.e. in 4 separate boxes (despite Nick's explanation in the other thread), but just knowing it can't is helpful

[emm]

Me too, Cathy. Not that I don't believe Nick, but MadOverlord's Unique Guide definitely had some examples of UT in 4 boxes - I just need to get my head around them.

[CathyW]

Managed to use the uniqueness test again today in the Times Fiendish puzzle. Check rows 4 and 5, columns 2 and 9 which had candidates of 6 and 9, and 7 in one of the cells! I knew it was OK to apply because only 2 boxes were involved.

Will have to reread MadOverlord's descriptions of other possible situations.