The Omission Technique allows you to narrow down candidates by identifying situations where a number must be restricted to specific cells within a block, row, or column. When these constraints are found, the number can be removed from other cells within the same affected area.
- Row-based Omission: If a candidate number appears only within one block in a row, it cannot appear in any other cells of that block.
- Column-based Omission: If a candidate number appears only within one block in a column, it cannot appear in any other cells of that block.
- Block-to-Row Omission: If a candidate appears only in a single row within a block, the candidate cannot appear elsewhere in that row.
- Block-to-Column Omission: If a candidate appears only in a single column within a block, the candidate cannot appear elsewhere in that column.
This contains a Row-based Omission scenario where the number 2 can only appear in columns 1 or 2 in row 2. As these are constrained to block 1, the number 2 can be removed from the other cells in block 1.
This puzzle utilizes only Naked Singles and the Omission technique.
How to spot it
Look for a digit inside a box that is confined to a single row or column, or a digit in a row or column that is confined to a single box. The candidate is locked along that line.
When to use it
It is a bridge technique between basic singles and the harder pairs and triples; check it whenever a box pins a digit to one line.
Common mistakes
- Eliminating in the wrong direction. Pointing removes from the rest of the row or column; box-line reduction removes from the rest of the box.
- Assuming it places a digit; Omission only removes candidates.
Frequently asked questions
- What is Omission in Sudoku?
- Omission, also called Locked Candidates, occurs when a digit's only possible cells in a box all lie in one row or column (or its only cells in a row or column all lie in one box). The digit can then be eliminated from the rest of that line or box.
- What is the difference between pointing and box-line reduction?
- Pointing is when a box confines a digit to one row or column, letting you remove it from the rest of that row or column. Box-line reduction is the reverse: a row or column confines a digit to one box, letting you remove it from the rest of the box.
- Why is Omission also called Locked Candidates?
- Because the candidate is locked into one line of a box (or one box of a line). It cannot escape that intersection, which is what enables the eliminations.