The New Sudoku Players' Forum

[Lardarse]

I reached this stage.

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368    38    9   |168   4     7   |5    2    16 
7      5     4   |3     2     16  |8    16   9   
68     1     2   |9     68    5   |4    3    7   
------------------------------------------------
348    6     38  |148   5     2   |9    7    138
23489  7     1   |48    38    89  |6    5    238
2389   238   5   |7     1368  689 |13   4    238
------------------------------------------------
1      9     68  |5     7     3   |2    68   4   
238    238   7   |68    168   4   |13   9    5     
5      4     368 |2     9     168 |7    168  36

Unable to proceed I used T&E on the 68 in r3c5.

I'm just wondering whether I could have used the uniqueness principle in placing the 8 in r1c4

Yes you can. Uniqueness is usually demonstrated with row/column interactions, but row/box and column/box also works.

Look at r1c4,r1c9,r2c6,r2c8:

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168 | 16
16  | 16

r1c4 must be an 8, and then the whole puzzle unlocks.

[r.e.s.]

I'm just wondering whether I could have used the uniqueness principle in placing the 8 in r1c4

Look at r1c4,r1c9,r2c6,r2c8:

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168 | 16
16  | 16

r1c4 must be an 8, and then the whole puzzle unlocks.

No, the uniqueness argument doesn't apply to that four-cell pattern in the grid -- the only four-cell uniqueness patterns are rectangular, which this one isn't; i.e., the pattern relevant to the present case does not imply multiple solutions ...

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16  .   .   |   .   .   16
.   .   16  |   .   16  .
.   .   .   |   .   .   .

If the four cells don't form a rectangle, uniqueness could be forced by cells elsewhere in the row or column (in this case a column) of one or more of the four cells.

[Lardarse]

This is why I suck, you don't, and uniqueness is a minefield...