My Sudoku

Hard Used from hard puzzles up.

Simple Coloring

Chains of strong links

When a digit has exactly two places in a unit, one of them holds it: that is a strong link. Linked cells form chains.

Colour the cells of a chain in two alternating colours. All the cells of one colour hold the digit and none of the other colour do; which colour is which is not known yet.

So a cell that sees both colours cannot hold the digit. And if two cells of the same colour see each other, that colour is the false one: the digit goes in every cell of the other.

What to look for

Pick a digit with many units where it has two places, and follow the links from cell to cell.

Example

47586923191637237584238238381547961859257189157846374814823618593635693941891581827123823879183564258368346834835729158528918964183728 112233445566778899
Simple Coloring on 1: linked cells are the only two places of 1 in their row, column or box, so the colors alternate, and 1 is in every blue cell or in every orange one. Blue: R4C1, R5C7, R7C5; orange: R5C3, R6C7, R7C1, R9C6. R4C6 sees R4C1 (blue) and R9C6 (orange); R6C5 sees R7C5 (blue) and R6C7 (orange), so they cannot be 1.
  • The cells the technique relies on
  • Cells where it removes candidates, struck out in red
  • The two colours of the coloring

Practise it

In the app, Learn › Simple Coloring sets exercises on real positions, then a full puzzle that needs this technique and nothing harder.

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