Nine
Hidden single
Nine calls it: Only place
A digit fits in only one place left in a row, column or box. The cell itself may still have other options, but the unit has nowhere else to put that digit.
Example
952376814684954956492153521493687631249395136429546
In the center box the 6 can only go here. The 6s in column 6 and row 6 rule out the other cells, and the rest is already filled.
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.
Everything about Nine
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