The New Sudoku Players' Forum

[kurbads]

Sorry, this is beyond my comprehension.

The hint on the app says, there neither R8 or 9 C5 can be 4 because it would create a deadly pattern.

I wonder why!?


Why both? Where is the logic?


4 can appear in 3-4 places in rows and columns. What particularly eliminates them from the cells R8C5 and R9C5?

Where is the deadly pattern. If R8C5 is not 4, what makes it a deadly pattern if R9C5 is and vice versa?

Thank you in advance!


EDIT:

Paraphrasing Uniqueness Test 4 to match this example, it says
One digit being 1 and the other coming from one of the extra candidates, there is simply no room for either of these two cells to contain a digit 7.

Still do not get it. If one is know not to have 7 and has 1, why does the other have to contain extra candidate? There is no deadly pattern if it is and the other is known not have one.

[eleven]

There are 3 ways to look at it (note that the 1's are in an x-wing pattern)

Code: Select all

+----------------+----------------+----------------+
| 2    5    36   | 36   7    8    | 4    9    1    |
| 9    4    36   | 1    36   2    | 8    7    5    |
| 8    1    7    | 9    5    4    | 6    3    2    |
+----------------+----------------+----------------+
| 3    78   9    | 46   2    67   | 5    1    48   |
| 5    2    4    | 8    9    1    | 3    6    7    |
| 6    78   1    | 345  34   57   | 2    48   9    |
+----------------+----------------+----------------+
| 7    9    8    | 2    46   56   | 1    45   3    |
| 14   6    5    | 7    148  3    | 9    2    48   |
| 14   3    2    | 45   148  9    | 7    458  6    |
+----------------+----------------+----------------+

You have to avoid the unique rectangle 14 in r89c15.
(a) If one of r89c5 is 8, the other will be 1 - so both cannot be 4
(b) 4 must be in one of r67c5, otherwise there would be a hidden pair 14 in r89c5 (nowhere else in c5)
(c) if r8c5 is 4, r8c1=1, r9c1=4, r9c5=1 (deadly) and if r9c5 is 4, r9c1=1, r8c1=4, r8c5=1

[Added:]The simple answer is, that there is a triple 456 in box8.

[Leren]

Note that there are only two 1's in Column5 - in Rows 89. So if r8c5 was 4, in particular, it is not 1, so r9c5 must be 1, which exposes a deadly pattern on 14 in r89c15.

Similarly, if r9c5 is 4, r8c5 is 1, and you get the deadly pattern the "other" way around.

You can read about Unique Rectangles Type 4, for example, here.

Leren

PS: I see I have cross posted with eleven

[Pupp]

Honestly, I think your app's hint is terrible.

Under those circumstances, it should be talking about the triplet in column 5.

No app should give a hint about deadly patterns unless the entire puzzle is an actual deadly pattern. (Either it was solved wrong, or the puzzle is not valid or has multiple solutions).

I think the issue was the devs thought a hint should be something that suggests looking someplace else on the board. But hints should be showing the exact technique needed, because a human wouldn't need a hint if they knew what they were looking for.

The one exception is if the app is trying explain (not hint) about certain techniques like unique rectangles.