Little Killer Sudoku Rules: How to Play This Variant

The Core Rules of Little Killer Sudoku

A Little Killer Sudoku grid starts as a standard 9x9 puzzle grid, divided into nine 3x3 boxes. All the fundamental Sudoku rules apply: each row, column, and 3x3 box must contain all digits from 1 to 9 exactly once. The defining twist is the addition of small sum clues placed just outside the grid. These clues indicate the total sum of the digits along a specific diagonal, as represented by a diagonal arrow pointing into the grid.

Unlike Killer Sudoku, which uses outlined 'cages' with sums inside the grid, Little Killer clues are always on the outside and apply only to full diagonals. The arrows can point in four main directions, creating two primary diagonal families: one from the top-left towards the bottom-right, and another from the top-right towards the bottom-left. The digit in the cell at the very tip of the arrow is included in the sum.

• Digits can repeat within a diagonal sum, as long as they obey the standard row, column, and box rules.
• The sum clue is always for the entire visible diagonal from the arrow's starting edge to the opposite edge of the grid.

How Diagonal Sum Clues Work

Clues are placed in the eight possible corner positions around the grid: the four main corners (top-left, top-right, bottom-left, bottom-right) and the four 'side' corners along the edges. An arrow shows the direction of the diagonal to be summed. For example, a clue in the top-left corner with an arrow pointing down-right applies to the main diagonal from A1 to I9.

The sum is for all digits along that diagonal line. Since digits 1-9 sum to 45, a full 9-cell diagonal's sum will always be 45. However, most clues are for shorter diagonals that start or end at the grid's edge. These partial diagonals can range from 2 to 8 cells long, and their target sums are provided. This is where the arithmetic logic comes into play, offering new ways to deduce possible digit combinations.

Key Differences from Standard Killer Sudoku

While both are arithmetic variants, they impose constraints in fundamentally different ways. Standard Killer Sudoku uses irregularly shaped, dotted-line cages *inside* the grid. Each cage has a small sum, and digits cannot repeat within a cage. This creates many small, local sum puzzles. In contrast, Little Killer uses long, straight diagonal lines. Digits *can* repeat along a diagonal if the standard Sudoku rules allow it. The logic is more about the interaction between a long, fixed line and the classic grid constraints.

Another major difference is clue placement. Killer cages are embedded throughout the puzzle, while Little Killer clues are only on the perimeter. This often makes the starting point for a Little Killer puzzle different; solvers frequently look at the intersections of major diagonals or use the known total of 45 for a full row/column/box in tandem with a diagonal clue.

Basic Solving Strategies and Tips

Start with the standard Sudoku techniques: look for Naked and Hidden Singles and basic intersections. Then, integrate the diagonal clues. A powerful first step is to note the clues for the longest diagonals. Since the digits 1-9 sum to 45, if a diagonal spans almost the full grid, its sum gives strong information about the missing cell or cells.

Use the '45 rule' common in Killer puzzles. For any set of 9 cells (like a row, column, or box), the sum is 45. If a diagonal cuts through a row or box, you can sometimes subtract known sums to find the value of a critical cell. Also, look for pairs of intersecting diagonals. The cell where they cross is part of both sums, creating a logical link. For more advanced tactics used in arithmetic variants, reviewing general Killer Sudoku Tips can be very helpful.

• The corners of the grid (cells like A1, A9, I1, I9) are often key, as they are the starting points for two different diagonals.
• If stuck, write down all possible number combinations that could satisfy a short diagonal's sum, then see which ones fit the existing Sudoku clues.