The New Sudoku Players' Forum

[Oxboy555]

444.jpg (185.27 KiB) Viewed 1077 times

Thanks.

[JasonLion]

Look for a chain of pairs of 2 & 8.

[Oxboy555]

Look for a chain of pairs of 2 & 8.

I don't know that technique. Can you explain it or point me to a link? Thanks!

[JC Van Hay]

1. There is an error in the pencilmarks : you cannot erase the digits 28 in r2c7 because (28)r24c57 are not in the same line of boxes in ordre to apply the Uniqueness Test.
2. However, as (17)r56c58 are in the same row of boxes, you correctly erased the digits 17 from r5c8.
3. "Basics" are not completely done. Candidates could only be excluded from the set of candidates for the digits 2 and 8! Hint : there are 4 such candidates!

[JasonLion]

BrainBashers has a good tutorial.

[Oxboy555]

1. There is an error in the pencilmarks : you cannot erase the digits 28 in r2c7 because (28)r24c57 are not in the same line of boxes in ordre to apply the Uniqueness Test.
2. However, as (17)r56c58 are in the same row of boxes, you correctly erased the digits 17 from r5c8.
3. "Basics" are not completely done. Candidates could only be excluded from the set of candidates for the digits 2 and 8! Hint : there are 4 such candidates!

Thanks, JC...but why again can I erase in your point #2 but not in #1?

[JasonLion]

(28)r24c57 is not a unique rectangle because it occupies four blocks. A unique rectangle is only valid if it occupies exactly two blocks.

[Leren]

Code: Select all

*--------------------------------------------------------------*
| 28    7     4      | 3     9     15     | 15    6     28     |
| 6     1    *5-28   | 248  a28    7      | 2458  3     9      |
| 3     9     258    | 1248  6     1258   | 7     148   248    |
|--------------------+--------------------+--------------------|
| 7     3     1      | 6    b28    4      | 28    9     5      |
| 28    6     9      | 5     17    3      | 1248  48    2478   |
| 4     5    d28     | 9     17   c28     | 6     17    3      |
|--------------------+--------------------+--------------------|
| 9     24    7      | 128   3     128    | 48    5     6      |
| 5     8     6      | 7     4     9      | 3     2     1      |
| 1     24    3      | 28    5     6      | 9     478   478    |
*--------------------------------------------------------------*

This where I think you should be in your marked up diagram. Look at the cells I have marked a, b, c, d.

Suppose Cell a is not 2. Then Cell b must be 2, Cell c must not be 2 and Cell d must be 2.

You can reverse this argument and show that if Cell d is not 2 then Cell a must be 2.

So at least 1 of Cell a or d must be 2. You can use exactly the same type of argument to show that at least one of Cell a or d must be 8.

So either (1) Cell a is 2 Cell d is 8 or (2) Cell a is 8 Cell d is 2.

Either way the cell I have marked * can't be 2 or 8 because it is in the same row as Cell a and the same column as Cell d. So Cell * Row 2 Column 3 ( or r2c3 for short ) is 5 !

Also, the reason you can't remove 2 and 8 from Row 2 Column 7 using the Unique Rectangle technique is that there are 4 boxes involved in the rectangle and this technique requires that there are only 2 boxes
involved in the 4 corners of the rectangle. There are only 2 boxes involved in the rectangle of 17's so these have been correctly removed from Row 5 Column 8.

Leren