Indirect Naked Single
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An Indirect Naked Single is a single technique that determines a placement by first writing candidates on the board, then inspecting the candidates in each unfilled cell in turn.
How to spot it
- First, mark all candidates in every unfilled cell. Remove candidates that have been ruled out by placed digits or by deductions from other techniques.
- Then inspect the candidates in each unfilled cell in turn.
- If a cell has only one candidate
xremaining,xmust be placed in that cell.
How it works
The rules of Sudoku require each cell to contain exactly one digit from 1 to 9. When all other digits have been ruled out and only candidate x remains, x must be placed in the cell.
Examples
On the board below, candidates have been marked and some have already been eliminated using other techniques. Examine each unfilled cell in turn. When we reach R4C6, we find that only candidate 2 remains, so 2 must be placed in R4C6.
If we inspect only the digits already placed in the peer cells of R4C6, both 2 and 7 are missing, so a Direct Naked Single cannot determine the placement. Here, candidate 7 has already been eliminated using another technique, so inspecting the remaining candidates tells us that 2 must be placed in R4C6.