What is Killer Sudoku?
Killer sudoku is classic sudoku with dashed cages drawn over the grid. The small number in the corner of each cage is the sum of the digits inside it. A killer usually has no given digits at all: the sums are the clues.
The rules
The usual sudoku rules apply: every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. On top of that: the digits in each cage add up to the number in its corner, and no digit repeats inside a cage, even when the cage crosses into another box.
A 7 in three cells can only be 1+2+4. A 22 in three cells is 5+8+9 or 6+7+9. A 9 in two cells is 1+8, 2+7, 3+6 or 4+5.
The rule of 45
The digits 1 to 9 add up to 45, so every row, every column and every box adds up to 45. This is the main tool of killer sudoku. If a box is covered by cages except for one cell, that cell holds 45 minus the sums of those cages. If a cage sticks out of a box by one cell, the digit in that cell is the cage sum minus what the cage contributes inside the box. The same works for a row, a column, or two boxes together (90).
How to get started
- Start with the sums that have only one digit set: 3 in two cells is 1+2, 4 is 1+3, 16 is 7+9, 17 is 8+9. In three cells, 6 is 1+2+3, 7 is 1+2+4, 23 is 6+8+9 and 24 is 7+8+9.
- Write the possible digit sets of a cage as pencil marks, then remove the sets that clash with a digit already in the row, column or box.
- Apply the rule of 45 to every box and row that is almost covered by whole cages. A single cell left over is a digit for free.
- A digit in a cage cannot appear again in that cage. In a two-cell cage with sum 10, a 5 is therefore impossible.