Naked Single: The First Sudoku Technique
The simplest way to prove a digit: list what a cell could hold, and fill it when only one digit is left. Cells are named by row and column: r2c7 is row 2, column 7.
Step 1 of 4
Candidates
The small digits in row 2 are its cells' candidates: the digits a cell could still hold once you strike out every digit already in its row, its column and its box. Row 2 lacks 4, 5, 7 and 8. Column 3 already has a 5, so r2c3 could be 4, 7 or 8.
Now look at the ringed cell, r2c7. Row 2 has 1, 2, 3, 6 and 9. Column 7 adds 4, 7 and 8. Only one digit is left: 5.
Place a naked single
A cell with one candidate left is a naked single. The digit has to go there, because nothing else can.
Your move
Fill the cell in row 2 that has only one candidate.
Count the candidates
No notes this time. Work out the candidates of the ringed cell, r2c2, from its row, its column and its box.
Find the naked single
One cell in this grid has a single candidate. No other cell is forced yet. Scan for a cell whose row, column and box together hold eight different digits.
Your move
Find the naked single and fill it.
What you learned
- A cell's candidates are the digits its row, column and box don't already hold.
- A cell with one candidate is a naked single: fill it.
- Always check all three: a digit the row allows can still be in the column or the box.
In this course
Unit 1The rules
Unit 2Solving techniques
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