The New Sudoku Players' Forum

[JFistere]

I'm getting better... but...
Well, I know how to use more techniques now but I am stuck on this one:

Code: Select all


Sterling Second Degree Black Belt #4


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


 
 *--------------------------------------------------------------------*
 | 2369   3689   238    | 39     1      5      | 2679   2789   4      |
 | 5      169    4      | 8      7      2      | 69     3      16     |
 | 1239   1389   7      | 39     6      4      | 5      289    18     |
 |----------------------+----------------------+----------------------|
 | 136    4      13     | 167    5      9      | 8      17     2      |
 | 7      16     5      | 146    2      8      | 3      14     9      |
 | 8      2      9      | 147    3      17     | 47     6      5      |
 |----------------------+----------------------+----------------------|
 | 239    3789   6      | 5      48     37     | 1      24789  378    |
 | 139    5      138    | 2      48     6      | 479    4789   378    |
 | 4      378    238    | 17     9      137    | 267    5      3678   |
 *--------------------------------------------------------------------*
 

An explanation of the next move would be very much appreciated.

Thanks,
John

[udosuk]

Next move:
1 on r6 must be locked in block 5 (r6c46), so you can eliminate it from r45c4.

Eventually, you need more advanced techniques to solve this in the later stage (such as unique rectangles or forcing chains)...

[Hud]

7 locked in row 8, box 9 also.

[udosuk]

When you ever get to this:

Code: Select all

 *-----------------------------------------------------------*
 | 69    3689  28    | 39    1     5     | 2679  789   4     |
 | 5     169   4     | 8     7     2     | 69    3     16    |
 | 129   1389  7     | 39    6     4     | 5     289   18    |
 |-------------------+-------------------+-------------------|
 |*136   4    *13    | 67    5     9     | 8     17    2     |
 | 7     16    5     | 46    2     8     | 3     14    9     |
 | 8     2     9     | 147   3     17    | 47    6     5     |
 |-------------------+-------------------+-------------------|
 | 29    789   6     | 5     48    37    | 1     248   38    |
 |*13    5    *13    | 2     48    6     | 49    489   7     |
 | 4     78    28    | 17    9     137   | 26    5     368   |
 *-----------------------------------------------------------*

Just notice the rectangle at r48c13. r4c1 must be a certain value, otherwise the grid cannot lead to a unique solution. This is the simplest application of the "Unique Rectangle" technique...

But many people have moral limitation on applying this class of techniques (me included), as it could only be used when you're certain that the puzzle under standard sudoku rules must produce a unique solution... I think someone else would be able to suggest another move such as a short xy-forcing chain or almost locked set reductions to solve this puzzle...

[re'born]

The unique rectangle is really the best way to go, but if one insists on another method:

(2,2)1 > (2,9)6 > (2,7)9 > (8,7)4 > (6,7)7 > (4,8)1 > (5,8)4 > (5,4)6 > (5,2)1 > (2,2)!1, a contradiction. Therefore, (2,2)!1 and this should solve the puzzle.

It's as close as I could find to a simple xy-chain.

[JFistere]

Having worked through a couple of books, I felt I was ready to move on. I asked Barnes & Noble for the toughest SuDoKu book they had. I am finding that I was slightly overconfident! They are much tougher than I expected!

Thanks for all your help!

John