Nine
Naked pair
A technique Nine explains
Two cells in the same unit have exactly the same two options. Whichever way round they go, together they use up those two digits, so both drop out of the other cells in that unit.
Example
35846792194621877213846397241818448127489857413478
These two cells in row 8 both have only 6 and 9 left. Together they use up those two digits, so they are out in the other cells of row 8. That leaves one cell with only one digit left: 3.
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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