Lesson 36

3D Medusa

The rule

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.

Step by step

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.