The New Sudoku Players' Forum

[JeJ]

I took some puzzle to this stage:

Code: Select all

.-------------.-------------.---------------.
| 19  3   179 | 4    6  5   | 8     12  127 |
| 5   2   17  | 38   9  38  | 17    6   4   |
| 4   8   6   | 7    2  1   | 5     9   3   |
:-------------+-------------+---------------:
| 2   19  5   | 19   4  7   | 3     8   6   |
| 8   4   19  | 36   5  36  | 1279  12  127 |
| 7   6   3   | 19   8  2   | 19    4   5   |
:-------------+-------------+---------------:
| 6   19  2   | 5    7  49  | 14    3   8   |
| 3   5   8   | 26   1  46  | 246   7   9   |
| 19  7   4   | 268  3  689 | 126   5   12  |
'-------------'-------------'---------------'


Since there were several (1,9) couples I thought there would be good chaces of elimination using coloring while filtering on 1's, so I marked (with opposite colors depending on their location of course) r7c2, r9c1, r1c1, r4c2, r5c3, r4c4 and r6c7.

Now since r5c3 and r6c7 both have the same color, I thought all cells seen by both (r5c789), could have their 1's removed, but that is wrong.

Could someone please clarify what I am understanding wrong here.
Thanks

[ronk]

Now since r5c3 and r6c7 both have the same color, I thought all cells seen by both (r5c789), could have their 1's removed, but that is wrong.

A validly eliminated candidate must see both of the "opposing" colors. Why? We know one of the two colors must ultimately be true.

[daj95376]

Now since r5c3 and r6c7 both have the same color, I thought all cells seen by both (r5c789), could have their 1's removed, but that is wrong.

Could someone please clarify what I am understanding wrong here.
Thanks

Basic Coloring (type 1) forces eliminations in cells that see both colors; i.e., r9c7.

Coloring (type 3) was mentioned by Angus Johnson and recently discussed somewhere (that I forgot). Anyway, your puzzle has a perfect example of it. Your cells r5c3 and r6c7 are the same color, and they see the only two cells in [row 2] that contain <1>. Any cells with the color of r5c3 and r6c7 can't be true for <1>!!!

[JeJ]

Duh! I was really wrong on this.

Thanks for the clarification.