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Here is a puzzle that I think deserves a lot of attention: TriDoku! It’s a great puzzle-type but for some reason it’s very rarely seen in the Sudoku world. I want to change that! Even though it may not appear obvious, there are 81 cells in this puzzle. And there are 9 nonets. Just like in your ordinary Sudokus. But there aren’t 9 rows and 9 columns. Why? Because the puzzle is triangular! The rules: 1. There are 9 nonets in forms of triangles. These are drawn with thick lines. All must contain all numbers 1-9. 2. Each edge of the big triangle contains 9 numbers – again, no repeats there either. These cells are shaded in DARK GRAY. 3. There is an INNER triangle, shaded in LIGHT GRAY. Each side of the inner triangle contains 9 numbers. No repeats on those sides, please. 4. And finally: two cells that are touching each other must not contain the same number! Make sure you use this rule! Each cell is touching up to 12 other cells! Be careful! All solving techniques come from classic Sudoku. Use naked and hidden singles and subsets in nonets and edges. Use interactions between the nonets and the corresponding edges. But finally, you will have to use: the hexagon rule! The Hexagon rule comes directly from rule number 4. Since two touching cells cannot contain the same numbers, then a group of cells that are ALL touching each other cannot contain any repeats. Simple? Yes. Well then, each hexagon (6 cells pointing to each other) must not contain duplicates, so you CAN use the subsets solving technique, but be careful, there are 6 cells and 9 possible numbers! Some of the techniques I will explain in detail in the coming days. Now, lets see the puzzle! Oh, and one quick note: 3 cells are marked half dark gray half light gray. They are not split in any way, they contain one number, but they belong to both an outside edge and an inner edge, so I marked them this way. TriDoku for Friday, February 12, 2010. Difficulty: THINKER
(click to download or right-click to save the image!)

Tridoku book Large print

To see the solution to this puzzle click here
Comments? Questions? Please!
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  1. Scott Urman
    Posted February 12, 2010 at 9:48 am | Permalink

    Neat! Took me about 10 minutes for this one, so I’m not sure that it merits thinker status (at least based on my time for other thinkers). But it’s definitely tricky and a very cool variation!

  2. gvfaker
    Posted February 18, 2010 at 10:22 pm | Permalink

    I liked the first one because of the novelty, but I solved it pretty quickly.
    If you want to continue publishing this kind of puzzles, I think they have to be more difficult.
    Anyway, I prefer jigsaw or outside.

  3. Posted October 5, 2011 at 2:23 am | Permalink

    nice,I think my nose will bleed by answering this one.

  4. Patricia Patel
    Posted January 26, 2012 at 6:56 am | Permalink

    Learning should be a joy and full of excitement. It is life’s greatest adventure; it is an illustrated excursion into the minds of the noble and the learned. – Taylor Caldwell

  5. Annemarie Staley
    Posted June 20, 2013 at 8:17 pm | Permalink

    Hi! I’m addicted to Tridoku but can’t find many books on them… Are you planning on coming out with another? I’d definitely buy it!

    • Posted June 21, 2013 at 2:16 am | Permalink

      Annemarie, there are some tridokus in the Sudoku Variations book. I might publish a new volume of just Tri Doku, but not sure when.

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