How to see why single candidates are obviously correct?

Puzzling Asked by bryc on November 14, 2020

I am trying to understand something that should be simple. I have a "easy" puzzle here, and immediately found where 8 should be in the middle box.

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But in various Sudoku Apps, they show "candidates" to the user. And sometimes there is only one candidate available, essentially giving away an answer for free.

It says that 6 and 7 MUST be in those boxes, but I can’t see why, it is not obvious at all to me.

Ideally I would rather disable ‘candidates’ and find such candidates myself. What strategy can be used to show, just by glancing, that 6 and 7 are the only possibilities in those boxes?

One Answer

A naked single occurs when

Basically, there will be numbers that rule out 8 of 9 possibilities. So for the 7 bottom right:

As all the others are ruled out in various ways, it must be a 7.

The strategy would be to look at a cell and go through 1-9 and see what is ruled out. If all but one number is ruled out, it must be that number!

Hope this helps!

Correct answer by Beastly Gerbil on November 14, 2020

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