What is Killer Sudoku?
Killer Sudoku (also called Samunamupure or Sumdoku) is a variant of classic Sudoku in which the empty cells are grouped into cages with a target sum. There are no given numbers at the start: all the information comes from the cage sums and the standard Sudoku rules.
It was created in Japan by the magazine Nikoli in the 1990s, merging Sudoku with Kakuro (the cross-sums puzzle). It reached the West in 2005 via The Daily Telegraph and is today one of the most challenging number puzzles in the world.
Killer Sudoku rules
- All the Sudoku rules apply: no repeats in rows, columns or 3×3 blocks.
- The numbers in each cage must add up to exactly the stated number.
- A number cannot repeat within the same cage, even if the cage crosses several blocks.
Solving techniques
The key lies in unique combinations: a 2-cell cage summing to 3 can only be {1,2}. A 2-cell cage summing to 17 can only be {8,9}. Identifying these forced combinations eliminates candidates quickly. The "innies and outies" technique (surpluses inside/outside blocks) is essential for advanced puzzles.
Why it’s the most complete Sudoku
It combines logical reasoning (Sudoku) with combinatorial arithmetic (Kakuro) in a single puzzle, engaging more brain areas at once. It’s the favourite of mathematicians, engineers and anyone looking for the ultimate mental challenge.