The New Sudoku Players' Forum

[Hud]

As usual, I wasn't able to complete this one. At least I could dub it into Pappocom's program.
It was rated 6 stars.

Code: Select all

+---------+--------+----------+
| 7  6  2 | 4 8 9  | 5  1   3 |
| 9  1  3 | 2 5 6  | 8  7   4 |
| 8  5  4 | 1 7 3  | 6  9   2 |
+---------+--------+----------+
| 1  23 7 | 8 4 5  | 9  23  6 |
| 23 4  8 | 6 9 1  | 23 5   7 |
| 5  9  6 | 7 3 2  | 4  8   1 |
+---------+--------+----------+
| 24 78 9 | 3 6 78 | 1  24  5 |
| 34 38 1 | 5 2 48 | 7  6   9 |
| 6  27 5 | 9 1 47 | 23 234 8 |
+---------+--------+----------+

I thought I could eliminate the 2 at R9C8 but I'm not sure so I left it in. I really intend to study the techniques involved in solving this one. Thanks in advance.

[Ruud]

Hi Hud,

I spotted a remote pair in your grid:

Code: Select all

+---------+--------+----------+
| 7  6  2 | 4 8 9  | 5  1   3 |
| 9  1  3 | 2 5 6  | 8  7   4 |
| 8  5  4 | 1 7 3  | 6  9   2 |
+---------+--------+----------+
| 1 *23 7 | 8 4 5  | 9 *23  6 |
| 23 4  8 | 6 9 1  |*23 5   7 |
| 5  9  6 | 7 3 2  | 4  8   1 |
+---------+--------+----------+
| 24 78 9 | 3 6 78 | 1  24  5 |
| 34 38 1 | 5 2 48 | 7  6   9 |
| 6 -27 5 | 9 1 47 |*23 234 8 |
+---------+--------+----------+

There are alternatives, but remote pair is my favorite choice here:

- XY-Chain (same cells)
- Multi-coloring digit 2 (same cells)

Ruud.

PS. Scanraid has a nice explanation of remote pairs here: http://www.scanraid.com/AdvanStrategies.htm#RP

[Hud]

Ruud, thanks for the reply. I'll read it tomorrow and see if I can digest it. At first glance, it looks useful.

[re'born]

Hud,

This is also a BUG+1 grid, allowing you to immediately place r9c8=2.

[emm]

I find remote locked pairs the hardest concept of all the basic techniques to visualise. I guess it's probably the same principle but the explanation of it seems so much more complicated than either strong links, XYchains or colouring - which in this case immediately removes 5 candidate 2s.

[MCC]

There's also a fishy x-wing in the 2's.

x-wing = r4c2, r4c8, r9c2, r9c8 with r7c8 the fin.

You can eliminate the 2 in r9c7.

Then with the x-wing in 2's in rows 4 & 8 you can then eliminate the 2 in r7c8.


MCC

[tarek]

There's also a fishy x-wing in the 2's.

if you construct the finned x-wing from columns 1 & 7, you would be able to eliminate the 2 in r9c2...solving the puzzle

Code: Select all

*-----------------------------------------------*
| 7    6    2   | 4    8    9   | 5    1    3   |
| 9    1    3   | 2    5    6   | 8    7    4   |
| 8    5    4   | 1    7    3   | 6    9    2   |
|---------------+---------------+---------------|
| 1    23   7   | 8    4    5   | 9    23   6   |
|*23   4    8   | 6    9    1   |*23   5    7   |
| 5    9    6   | 7    3    2   | 4    8    1   |
|---------------+---------------+---------------|
|#24   78   9   | 3    6    78  | 1    24   5   |
| 34   38   1   | 5    2    48  | 7    6    9   |
|*6   -27   5   | 9    1    47  |*23   234  8   |
*-----------------------------------------------*

tarek

[re'born]

I find remote locked pairs the hardest concept of all the basic techniques to visualise. I guess it's probably the same principle but the explanation of it seems so much more complicated than either strong links, XYchains or colouring - which in this case immediately removes 5 candidate 2s.

emm,

I'm not positive that you were asking for this, but...

I've always thought finding remote locked pair deductions was very easy. Here is my method. First identify the bivalue cells with a particular pair of candidates {xy}. Start at any of them and say "same". Now travel to another which sees the first cell and say "different". Travel to another that sees the second and say "same". Travel to another that sees the third and say "different", and so on. Any time you get to a cell and say "different", then you can eliminate x and y from any cell which sees the initial cell and the current cell. Of course, there is no guarantee that you picked your initial cell correctly, but with practice it is easier to pick out the right starting point.

In Ruud's example, I would start at r4c2 and say "same". Then move to r4c8 and say "different", then to r5c7 and say "same" and then to r9c7 and say "different". Now any cell which sees both r4c2 and r9c7 cannot have a 2 or 3, hence the exclusion in r9c2.

I hope this helps, or if you already knew this, I hope I haven't insulted your intelligence.

[emm]

Thanks for the reply rep’nA – no worries about my intelligence, it’s pretty much uninsultable.

I understand your explanation and if that's what remote locked pairs are, then I get them. Truth is, that’s exactly how I would describe colouring and it doesn’t sound to me like the explanation of remote locked pairs in either Scanraid or Susser.

Maybe I’m missing the obvious – but I can easily visualise the elimination of 5 candidate 2s either by strong links or colouring in this example and I was surprised that Ruud suggested remote locked pairs, but perhaps in the end it's all much the same thing.

[TKiel]

I read the Scanraid link and I looked at the remote locked pairs that Ruud pointed out in the puzzle. The exclusion made in both can also be made with simple coloring. Are there instances where remote locked pairs make an exclusion that simple coloring doesn't?

Tracy

[re'born]

Are there instances where remote locked pairs make an exclusion that simple coloring doesn't?
Tracy

I don't think so. As any two cells in a chain of remote locked pairs form a locked set, necessarily the cells form a conjugate pair on both candidates. Therefore, remote locked pairs is just a special case of doing simple coloring on two different digits.

[TKiel]

Rep'nA,

Thanks for the answer.

Tracy