Killer Sudoku rules and puzzles

The case notes

Let a Killer cage rule out digits nearby

This small Killer cage sums to three. We are going to use it on a cell outside the cage, but inside the same Sudoku box. Watch a worked example, then try the idea in a puzzle.

Put it into practice

A cage can reach beyond itself

This small Killer cage sums to three. We are going to use it on a cell outside the cage, but inside the same Sudoku box.

The cage holds {1, 2}

Only one and two add to three using two different Sudoku digits. Their order is unknown, but both digits are reserved inside this box.

One here breaks the cage

Suppose this other box cell were one. Then the cage could not use one anywhere, yet its two cells must total three. That is impossible. Two fails the same way.

Remove 1 and 2 outside the cage

So this cell is neither one nor two. A cage can claim digits for a row, column or box before we know exactly where they sit inside it.

Read the transcript

This small Killer cage sums to three. We are going to use it on a cell outside the cage, but inside the same Sudoku box. Only one and two add to three using two different Sudoku digits. Their order is unknown, but both digits are reserved inside this box. Suppose this other box cell were one. Then the cage could not use one anywhere, yet its two cells must total three. That is impossible. Two fails the same way. So this cell is neither one nor two. A cage can claim digits for a row, column or box before we know exactly where they sit inside it.

Keep following the clues