Sudoku techniques · Extreme
3D Medusa in Sudoku
3D Medusa extends Simple Coloring into the third dimension: instead of following one number, it weaves conjugate pairs and bivalue cells into a single two-coloured network spanning several digits. Exactly one colour is true, so a cell carrying both colours sheds its other candidates, and a candidate that sees its own digit in both colours can never be true.
Why it works
Conjugate pairs and bivalue cells weave one chain of strong links across several digits, and exactly one of its two colors is true; a cell carrying both colors sheds its other candidates, and a candidate that sees its own digit in both colors can never be true.
How to spot it
Everything Simple Coloring does, with the extra move of stepping between digits inside a bivalue cell.
- Colour a strong link on one digit, then use any bivalue cell you hit to jump to its other candidate and keep going.
- Each coloured item is a (cell, digit) pair, not just a cell: that is the part people trip on.
- Look for the same cell carrying both colours on two different digits: everything else in that cell goes.
Builds on
Worked example
These are the app's own lesson boards. Step through them the same way a hint does.
- 1
Paint this chain of strong links in two colors. Wherever a candidate has only one other spot in its row, column, or box, the two spots form a strong link: exactly one of them is true. A bivalue cell links its own two candidates the same way. Alternating colors along every link means one whole color is true and the other entirely false. Cells shown with carry only marks of the first color, cells with only marks of the second, and a cell marked carries marks of both colors.
- 2
Each red cell holds the candidate 5 with no color, yet it sees 5 marked in both colors. Whichever color wins places 5 at one of those marks, so the candidate 5 can be removed from every red cell.