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Killer Sudoku is a variant of Sudoku challenges that adds new twists and turns to the classic number puzzle. Being a variant instead of a puzzle in its own right means it largely shares the same rules and dynamics as the original. It’s the unique feature it introduces, the cages, that makes it so addictive and popular even among the fiercest Sudoku oldschool players.
Killer Sudoku is an adaptation of traditional Sudoku. To the concept of rows, columns, and groups, Killer adds that of cages. These are groups of cells, limited by dotted lines, that indicate the total sum of the number contained in them. The cages cannot contain repeated numbers.
In theory, despite its lethal name, this makes puzzles easier to solve because there’s more data available to interpret and find the solution to the grid. However, the true difficulty level depends on how many arithmetic operations the players need to perform to solve the puzzle. For example, cages with one number are selfexplanatory and reveal the answer to that cell without any effort, but cages with longer chains of cells can be tricky.
This variant follows the same rules as the classic number puzzle . The goal is to complete the grid with numbers from 1 to 9, without repeating any digit per row, column, or 3x3 group.
The killer variant adds the concept of cages to these rules. These indicate the total sum of the numbers they contain. Repeated digits are not allowed within them. For example, if the sum of a twocell cage is 4, the cage contains the naked pair "1/3  1/3", since “2  2” is not permitted.
The main difference between the two is the presence of cages in the Killer Sudoku. These are an addition to the classic puzzles. They do not replace any feature of traditional Sudoku and don’t alter any of the basic rules.
Another difference is that the puzzles start with fewer solved cells. Some Killer Sudoku puzzles may even start with completely empty grids, especially in the hardest difficulty levels.
If you are new to Killer Sudoku, you might feel overwhelmed by the little or the large amount of information available on the grid. Nevertheless, despite its complicated looks, these are still Sudoku puzzles and the same strategies apply. These include strategies for beginners (naked singles and pairs, crosshatching, etc.) but also advanced strategies like the Swordfish or the XWing method.
However, the introduction of cages also prompts the existence of special tips that apply only to the Killer. Out of these, there are 2 that any player should know by heart to stand a chance at winning the hardest puzzles.
This rule applies only to traditional grids of 9 x 9. Since you cannot have repeated digits per row, column, or 3 x 3 group, the sum of the numbers in each is always 45 (1+2+3+4+5+6+7+8+9).
Cages indicate the sum of the numbers they contain, even if you haven’t solved the cells yet. They cannot contain repeated numbers either.
Combining this information, we get to the 45 strategy. This rule is useful when all cages are contained in a single row, column, or group, except for one cell that belongs to a different cage that extends to another section. Since the total sum should be 45, you can subtract the value of the cages contained and the remaining value will be the solution of the oddout cell.
Example:
Imagine you have a series of cages restricted only to the first row with the sums 8, 13, 10, and 12, but the very last cell belongs to a cage positioned vertically in column 9. Even on an empty grid, with no solved cell to guide you, you can immediately know the solution for that oddout cell on row 1:
Row value = 45
Values already known (cages restricted to 1st row) = 43 (8+13+10+12)
Oddout cell = 45  43
For the row to have a total value of 45, the solution for the oddout cell must be the number 2.
Beginner players may struggle with the arithmetics required to find the solution or at least to reduce the number of possible digits for each cell. However, some cage combinations are straightforward and can help players progress faster in the game.
Here is a quick list of the most useful cage combinations:
Sum 
Combinations 
Cages with 2 squares 

3 
1 2 
4 
1 3 
5 
1 4 / 2 3 
6 
1 5 / 2 4 
16 
7 9 
17 
8 9 
Cages with 3 squares 

6 
1 2 3 
7 
1 2 4 
8 
1 2 5 / 1 3 4 
23 
6 8 9 
24 
7 8 9 
Cages with 4 squares 

10 
1 2 3 4 
11 
1 2 3 5 
29 
5 7 8 9 
30 
6 7 8 9 
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