Pattern Overlay

Expert

All valid placements of a digit are enumerated; cells absent from every valid pattern have the digit eliminated.

How It Works

Enumerate all valid complete placements of a digit (one in each row, column, and box). Any cell that appears in none of the valid patterns cannot contain that digit and can have it eliminated.

Computationally intensive but theoretically the strongest single-digit technique. In practice only applied when few candidates remain.

Example

34	3489	1	2	345689	34589	58	7	68
2	5	678	467	4678	478	1	3	9
37	3789	36789	15679	1356789	135789	258	4	268
1	2	37	467	34678	3478	78	9	5
8	37	5	179	1379	1379	6	2	4
9	6	4	57	2578	2578	3	1	78
47	14789	789	1479	12479	6	27	5	3
5	1347	37	8	1247	1247	9	6	27
6	79	2	3	579	579	4	8	1

Key cells of the techniqueCells where elimination is appliedCandidate to be placedEliminated candidate (crossed out)