Little Killer Sudoku: rules and tips

Little killer sudoku puts small clues outside the grid, each with an arrow. The clue is the sum of every digit along the diagonal the arrow points down.

It is killer sudoku’s lighter cousin: no cages, just diagonals. The catch is that diagonals aren’t houses, so digits on them may repeat.

The rules

• Normal sudoku rules apply: 1 to 9 once in every row, column and 3×3 box.
• A clue outside the grid gives the sum of the digits along the diagonal its arrow points to.
• Digits may repeat on a diagonal, as long as the repeated cells don’t share a row, column or box.
• Here, clued diagonals are three to eight cells long.

How to start

Start with short diagonals

A three-cell diagonal in a corner lies entirely inside one box, so its digits are all different. A total of 6 must be 1, 2 and 3; a total of 24 must be 7, 8 and 9.

Know when repeats are possible

On the four-cell diagonal r1c4, r2c3, r3c2, r4c1, the two middle cells share box 1 and must differ, but r1c4 and r4c1 sit in different rows, columns and boxes and could hold the same digit. Always check which pairs on a diagonal can see each other before listing combinations.

Bound long diagonals

For longer diagonals, work out the smallest and largest possible totals given what each cell can hold. A clue close to either extreme pins the cells down quickly.

Common mistakes

• Reading the wrong diagonal: the arrow, not the clue’s position alone, sets the direction.
• Assuming digits on a diagonal can’t repeat.
• Forgetting that the diagonal runs right to the far edge of the grid.

How the ranks scale difficulty

Every puzzle type on Hoot’s Puzzles comes in four ranks, named after the stages of a mathematical idea. Lemma is the warm-up: about a dozen more digits are given than logic strictly needs, and singles plus the variant’s own rule will carry you through. Proposition needs a real idea or two, typically pointing and claiming. Theorem is near-minimal, so almost every clue is load-bearing and you may need pairs. Conjecture gives the fewest digits and sparser rule clues: expect to think hard.

Rank sets how many diagonal clues you get: nine at Lemma, seven at Proposition, six at Theorem and five at Conjecture.

There are 99,999 puzzles per rank, each with its own number, so you can replay one or share it. Every type also has a 30-puzzle collection that climbs from Lemma to Conjecture. Every puzzle is checked to have exactly one solution that you can reach by logic alone, with no guessing.

Keep exploring

• Killer SudokuCages with sums instead of given digits. Partition arithmetic and the rule of 45.
• Sandwich SudokuOuter clues sum the digits trapped between the 1 and the 9.
• Sudoku XBoth main diagonals are extra houses. Also called diagonal sudoku.
• Solving techniquesSingles, pairs, cage arithmetic, line logic and Latin squares, all in one guide.
• All puzzle guidesEvery puzzle type on Hoot’s Puzzles, with rules and tips.