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2

My late contribution:


2

The first thing I saw was: The second thing I saw:


2

Also:


3

There is only one place to put followed by the


2

What you are describing is called a deadly pattern. Essentially, if some situation would result in multiple solutions, then you can deduce that the situation is invalid (assuming the sudoku has a unique solution). This leads to advanced solving strategies where you can spot potential deadly patterns and rule out possible cell values that would lead to ...


4

If you put the $8$ into R9C1 you have multiple solutions, so you might as well go ahead and assume it is R7C1 and see if this gives a unique solution or even a solution at all. If not, write to the publisher of the Sudoku and ask for your money back!


5

Unfortunately this seems to be an invalid Sudoku. It has multiple solutions, and therefore there is no logical next move.


2

You can deduce:


2

A different approach: The logic bit: You can remove some of your original logic by noticing a contradiction on row 7: Another way: Finally, but a bit sneaky:


4



0

Once you resolve the problems with your markings, which will solve the bottom left sub-grid and reduce the markings in the right middle subgrid, you're left with a puzzle that has multiple solutions. That means it isn't a valid Sudoku at all -- at least as I've been taught, a Sudoku must have one unique solution. An automated solver I use says that there ...


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