On HuDoKu's page on Uniqueness, under Unique Rectangle, it says
A common mistake when looking for URs is to violate the "2 blocks" rule (see the Sudopedia article for an example).
Unfortunately, the Sudopedia link they give is broken.
I understand the "2 blocks rule" from the section above that note as meaning that the rectangle must be in exactly two blocks, but what mistake was the example supposed to show, and how can it cause a problem if I make that mistake?
Answer
Could this be an alternative link? Unique Rectangles
The link is saying that if you end up with the 2 blocks like this (the four pink boxes can only have either 1 or 2 in them, "MadOverlord" named this Unique Rectangle pattern the Deadly Pattern apparently!):
then either:
- the setter has not created a sudoku with a unique solution (since there are two possible ways to populate the boxes)
- or you have gone wrong somewhere.
The writer from Sudoku Wiki goes on to say that if you know that there is a unique solution then you can use potential blocks like this to your advantage when solving the puzzle.
For example, in the below:
You will be able to see that you cannot place a 7 in any of the blue boxes as this will leave behind the deadly pattern, and so the 7 must be in the bottom left of that rectangle.
There are a few variations on this example on that page - You can draw conclusions about the numbers in more than just the four rectangle boxes themselves.
Personally, I am a bit wary of using this method as making any assumption in a sudoku makes me feel a little uneasy! It was an interesting read though.
To answer you question, I don't think that finding that you have "the deadly pattern" is the mistake, but it shows you that maybe somewhere earlier in the solving, you've made a mistake. Or else, there are multiple solutions. As we can see from this question, not many people like multiple solutions:
Why must a sudoku have a unique solution
I hope that helps? Or at least that the sudokuwiki link helps!
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