World’s Hardest Sudoku?

I haven’t tried to solve this yet, but considering the headline I feel honor-bound to give it a try. It’s a pity it wasn’t available in time to include in the Big Sudoku Book, since we have a section discussing how the difficulty of a puzzle is determined.


  1. #1 John Yancey
    Spokane, WA
    July 1, 2012

    Cracked it in about 6 hrs. Identified that the lower-right box(region) had 14 possible solutions. Tried them one-by-one. The correct one led me to try one of five possible row-7 solutions. One of those led me to one of three possible column-8 solutions. One of those led to one of two top-left box solutions. And voila!!!

  2. #2 asdf
    July 2, 2012

    When you say you tried the 14 possible solutions from the lower right box, how did you do that? Because its easy to come to the 14 solutions (I came to that part) but after every solution you still have work to do and you cant get any numbers for sure and yet have to break it down to more possible solutions which takes a very long time (trying all 14 possibilities and their ”sub-possibilities”)

  3. #3 Erik
    Los Angeles
    July 3, 2012

    there are two “1”s in a row. How can that be correct??

  4. #4 Erik
    Los Angeles
    July 3, 2012

    Diagonally, bottom left corner to top right corner

  5. #5 Jason Rosenhouse
    July 3, 2012

    Erik —

    In Sudoku, the two long diagonals are not considered to be regions in which every number from 1 to 9 must appear exactly once. Only the horizontal rows, vertical columns, or 3 by 3 blocks have that restriction. So there is no problem having two ones on the same long diagonal.

    There is another type of puzzle called Sudoku X, in which the two diagonals are considered to be restricted regions. But that is a different kind of puzzle.

  6. #6 Composer99
    July 5, 2012

    I shared this post on Facebook and was advised that this isn’t a valid sudoku puzzle on account of having less than 22 numbers given and therefore more than one valid solution.

    Is this the case?

  7. #7 David Winfrey
    July 5, 2012

    Many valid 17-clue puzzles are known; here are fifty of them, with a link to a list of 49151 more:

  8. #8 Composer99
    July 5, 2012

    Thanks, David!

  9. #9 Shelagh Robinson
    July 10, 2012

    Take this hardest puzzle, then scan it into photoshop and reversei it, so it’s a mirror image. It becomes a very different puzzle.
    I do research on mental rotation skills, and we’ve been using Mirror Readoku puzzles as stimuli. Fascinating findings.
    Search: Mirror Read for free puzzle.

  10. #10 David Winfrey
    July 11, 2012

    Do you mean that it becomes harder because the symbols are no longer instantly recognizable?

  11. #11 David Winfrey
    August 5, 2012

    Via Slashdot, here’s a good article on determining the difficulty of a Sudoku puzzle:

    Another article on the same site is about a proof by exhaustive search that there are no 16-clue puzzles:

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