BUG+1 (Bivalue Universal Grave)

Hard

When all empty cells have exactly two candidates except one with three, the extra candidate in that cell is the solution.

How It Works

A BUG (Bivalue Universal Grave) is when all empty cells have exactly two candidates — this creates multiple solutions, which is impossible in a valid puzzle.

BUG+1: All but one empty cell have exactly two candidates; one cell has three. The "extra" candidate (the one that appears three times in its units instead of twice) must be the correct digit for that cell.

Example

8	7	5	2	9	1	6	4	3
1	2	3	46	46	8	5	7	9
4	69	69	3	5	7	1	2	8
26	4	12	7	3	5	9	8	16
3	5	16	8	2	9	4	16	7
9	8	7	14	14	6	2	3	5
5	1	8	69	7	2	3	69	4
26	3	29	19	8	4	7	5	16
7	69	4	5	16	3	8	169	2

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

8	7	5	2	9	1	6	4	3
1	2	3	46	46	8	5	7	9
4	69	69	3	5	7	1	2	8
26	4	12	7	3	5	9	8	16
3	5	16	8	2	9	4	16	7
9	8	7	14	14	6	2	3	5
5	1	8	69	7	2	3	69	4
26	3	29	19	8	4	7	5	16
7	69	4	5	16	3	8	169	2

8	7	5	2	9	1	6	4	3
1	2	3	46	46	8	5	7	9
4	69	69	3	5	7	1	2	8
26	4	12	7	3	5	9	8	16
3	5	16	8	2	9	4	16	7
9	8	7	14	14	6	2	3	5
5	1	8	69	7	2	3	69	4
26	3	29	19	8	4	7	5	16
7	69	4	5	16	3	8	169	2

8	7	5	2	9	1	6	4	3
1	2	3	46	46	8	5	7	9
4	69	69	3	5	7	1	2	8
26	4	12	7	3	5	9	8	16
3	5	16	8	2	9	4	16	7
9	8	7	14	14	6	2	3	5
5	1	8	69	7	2	3	69	4
26	3	29	19	8	4	7	5	16
7	69	4	5	16	3	8	169	2