sudokUI › Sudoku techniques › 3D Medusa
3D Medusa in sudoku
colouring across all digits · how it fits
Candidates of several digits, joined by conjugate pairs or by sharing a bivalue cell, form a cluster coloured alternately in two colours so linked candidates get opposite colours.
Why it works
Exactly one colour is entirely true. A whole colour is removed if it puts two digits in a cell, a digit twice in a unit, or leaves a cell with no candidates; an uncoloured candidate is removed if it is false whichever colour is true.
How to spot it
Start from a bivalue cell and spread outwards through conjugate pairs and other bivalue cells, colouring candidates alternately as you go.
Worked example
3D Medusa: candidates joined by conjugate pairs and bivalue cells (the solid links) are coloured blue and gold, and either every blue candidate is true or every gold one is. The 1 in r7c2 sees a blue 1 in r7c1 and a gold 1 in r7c3, so it is false either way.
Practise it on a real puzzle. sudokUI generates a puzzle that needs 3D Medusa and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs 3D Medusa, the puzzle's difficulty rating grows by 250. Its class is Unfair. It is needed in 1 in 16 puzzles that sudokUI generates. How the rating works.