Early 2009, I read about KenKen (also known as Calcudoku) in the New York Times, and decided to write a program that creates similar puzzles, and put them online. Calcudoku puzzles are similar to Sudoku, but also require some basic math skills to solve.

The rules are:

1. the 4×4 puzzle uses the numbers 1-4, the 6×6 the numbers 1-6, and so on
2. in each row and in each column each digit can appear only once (just like in Sudoku)
3. each "cage" (the blocks with the thick border) shows a result and an operation (addition: +, subtraction: -, multiplication: ×, or division: :). The operation applied to the numbers in the cage should produce the result shown. Note that for subtraction and division the order is not fixed (!)

Look at the solutions of earlier days to make things clearer.

You can solve the puzzle on the page using the arrow and number keys, or using the mouse. Use the spacebar to clear a cell. Use backspace to undo a move. Press '=' to highlight the number in a cell. In the largest puzzles (10x10 Calcudoku puzzle (every Tuesday and Friday) and 12x12 (every Sunday and Thursday)), simply press 1 then 0, or 1 then 1, or 1 then 2 to enter 10, or 11, or 12. You can also use the a, b, and c keys to enter 10, 11, or 12. Press the Shift key and a number to store it as a guess.

You can also solve the puzzles on the iPhone/iPod Touch/iPad and the Kindle 3.

Every puzzle has a single solution.