Sudoku candidate notation: write only the digits that can still fit
Small notes inside an empty cell record the digits that can still fit. Each possible digit is called a candidate. The notes are useful only when they match the current grid.
One visible proof
After every placement, remove that digit from candidate notes in the same row, column, and 3×3 box.
- 1. Check the rowRemove digits already used in the row.
- 2. Check the column and boxKeep only digits absent from all three places.
- 3. Update after a placementIf a nearby cell receives 6, change {2,6,8} to {2,8}.
A 6 placed in the same column means the outlined cell can no longer contain 6. Candidates 2 and 8 remain possible.
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