Nine
Hidden pair
A technique Nine explains
Two digits only fit in the same two cells of a unit. Those two cells belong to those two digits, even if they still list other options. All the others drop out.
Example
5263425364258917629532968573891426329243586732
In row 2, 3 and 6 only fit in these two cells. So those two cells are for 3 and 6, and every other digit drops out of them. That leaves row 2 with only one place for the 4: here.
In the app
Nine is the sudoku that writes this reasoning itself, on every puzzle, in the app. The explanation fills in nothing: it names the technique, colours the proof and says why. Free for seven days, then a one-time unlock. No ads, no account, no internet needed.
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