Surely the most prominent variation are CalcuDoku puzzles, which first appeared in 2008. They are originally known under the name KenKen** , **but since this word has been trademarked by Nextoy, LLC, the rest of the world had to come up with a different one.

Leaving the naming issues aside, lets go back to the puzzles themselves. They share some common rules with Sudoku and in fact, very much resemble Killer Sudoku. Your goal is still to fill in the grid without repeating any numbers in rows and columns, however there aren't any 3×3 boxes as in classic Sudoku. In fact, the puzzles need not necessarily be 9×9 in size – they range from 4×4 to 9×9.

Similarly to Killer Sudoku, the grid is not populated with any "givens"; instead, groups of cells are enclosed in "cages" with a number and one of the four basic arithmetic operators (addition, multiplication, division and subtraction). You must solve each cage so that the numbers in it result in the total given for that cage using the appropriate arithmetic operation. It sounds complicated, but it's much easier when you look at this sample puzzle.

So, for example, since we are working on a 6×6 grid (using only the numbers 1 to 6) the cages consisting of two cells, with "4-" as the clue, could only be either "6-2" or "5-1". Get it? Okay, this doesn't solve any cells, but it limits the candidate numbers for some cells.

I'll give you one more clue for this puzzle: the sum of numbers from 1 to 6 is 21. In column 4, you have two cages with clues "8+" and "12+". Well, those two add up to 20. Now, since we know that in this column, column 4, as in any other column, all numbers must add up to 21, it means that the remaining cell in this column MUST BE 21-20=1. Therefore, R1C4 (row 1, column 4) = 1, which immediately also solves it's "buddy" cell (the cell in the same cage) – R1C5 = 5. (Why?) This technique is called "innies/outies" and it's use is not as common in CalcuDoku puzzles as it is in Killer Sudoku.

Oh, by the way, if the operator next to a number is missing, it's not a mistake. You have to figure out what it must be! In this particular puzzle, it's quite easy to do that, unlike in some more difficult ones.

As you can see, unlike Sudoku, there is some math involved. Don't be put off by this. These puzzles are very addicting but arguably even more satisfying when finished and in fact, they are being used for educational purposes in schools.

]]>To be honest, I haven't fallen so much into the trap of solving Sudoku puzzles. However, I have become addicted to another aspect of Sudoku puzzles: I enjoy creating them. Actually, I focus on its variants that have mushroomed after the initial success of the original "number-place" puzzles.

It is difficult to keep track of all the different variants that have been invented in the past two years. Two of the most successful ones are surely Killer Sudoku and Samurai Sudoku. They extend the original puzzle in two different ways. While Samurai demand even more patience because of its size, Killer Sudoku asks that you use some basic arithmetics in order to solve a puzzle.

But that is not all; there are many variants that impose additional restrictions on the placement of numbers in a puzzle: diagonal, consecutive, odd/even, or greater/less than Sudoku, to name a few. Then there are jigsaw Sudoku puzzles. Some authors even combine a couple of these extra rules into one puzzle.

And then, once you've become familiar with the family of Sudoku puzzles, you learn that there are other logic brain teasers that ask from you to play with the numbers. Kakuro is a hybrid between Killer Sudoku and good old crosswords; Hanjie puzzles (which also go under different names: griddlers, nonograms, pic-a-pix, or paint-by-numbers) reveal interesting images after you solve them; to extend your Japanese vocabulary, you learn about Hashi, Masyu, or Hitori puzzles.

It is my intention to write a series of articles here at Blogcritics and introduce some of these puzzles to you. I hope that you, dear reader, will also become addicted to these magnificent mind-benders!

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