The New Sudoku Players' Forum

[paulcoz]

This is one of the first ‘Very Hard’ puzzles I have tried.

Pappocom (partially completed):

Code: Select all

4.5|931|2.6
92.|645|.31
631|827|459
---+---+---
59.|172|.4.
2..|394|1.5
.14|568|92.
---+---+---
152|786|394
.4.|259|.1.
8.9|413|5.2


Could someone please explain how I can finish this logically? I would like to know which techniques are used and the next eliminations (and thus placements).

I can't do anything with these criss-crossing pairs and triples.

Thanks,
Paul.

[Sped]

There is an X Wing that eliminates the 6 in r5c3:

Code: Select all

 *--------------------------------------------------*
 | 4    78   5    | 9    3    1    | 2    78   6    |
 | 9    2    78   | 6    4    5    | 78   3    1    |
 | 6    3    1    | 8    2    7    | 4    5    9    |
 |----------------+----------------+----------------|
 | 5    9   *368  | 1    7    2    |*68   4    38   |
 | 2    678 (6)78 | 3    9    4    | 1    678  5    |
 | 37   1    4    | 5    6    8    | 9    2    37   |
 |----------------+----------------+----------------|
 | 1    5    2    | 7    8    6    | 3    9    4    |
 | 37   4   *367  | 2    5    9    |*678  1    78   |
 | 8    67   9    | 4    1    3    | 5    67   2    |
 *--------------------------------------------------*


Row 5 has just two sixes, in columns 3 and 7. Likewise row 8 has two 6s in the same two columns. Any way you put 6s in row 5 and row 8, it's guaranteed that one will be in column 3 and the other in column 7. Therefore all other 6s in columns 3 and 7 can be eliminated. r5c3 loses its 6.

Then there's a naked pair in column 3 and it's simple after that.

edit: removed reference to unique rectangle in r1c28/r5c28. the cells must be in 2 boxes, not 4. Doh!

[paulcoz]

Hi Sped, thankyou for the prompt reply.

I have read about unique rectangles, but I still find it hard to identify them. I need to do some more reading in that regard.

I can now see the x-wing (great!) and I don't doubt that the unique rectangle is on the mark too.

Thanks again,
Paul.

[Sped]

Hi Sped, thankyou for the prompt reply.

I have read about unique rectangles, but I still find it hard to identify them. I need to do some more reading in that regard.

I can now see the x-wing (great!) and I don't doubt that the unique rectangle is on the mark too.

Thanks again,
Paul.

Do doubt I goofed on the unique rectangle. It's not there.

The 4 cells must be in 2 boxes, not 4.

I like unique rectangles so much I see them where they don't exist! Gotta watch that.

The X wing works though!

[kissafool]

+-----+-----+-----+
|4 7 5|9 3 1|2 8 6|
|9 2 8|6 4 5|7 3 1|
|6 3 1|8 2 7|4 5 9|
+-----+-----+-----+
|5 9 6|1 7 2|8 4 3|
|2 8 7|3 9 4|1 6 5|
|3 1 4|5 6 8|9 2 7|
+-----+-----+-----+
|1 5 2|7 8 6|3 9 4|
|7 4 3|2 5 9|6 1 8|
|8 6 9|4 1 3|5 7 2|
+-----+-----+-----+