I haven’t tried to solve this yet, but considering the headline I feel honorbound 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.
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Cracked it in about 6 hrs. Identified that the lowerright box(region) had 14 possible solutions. Tried them onebyone. The correct one led me to try one of five possible row7 solutions. One of those led me to one of three possible column8 solutions. One of those led to one of two topleft box solutions. And voila!!!

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 ”subpossibilities”)

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

Diagonally, bottom left corner to top right corner

#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.

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 David Winfrey July 5, 2012
Many valid 17clue puzzles are known; here are fifty of them, with a link to a list of 49151 more:

Thanks, David!

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 David Winfrey July 11, 2012
Do you mean that it becomes harder because the symbols are no longer instantly recognizable?

#11 David Winfrey August 5, 2012
Via Slashdot, here’s a good article on determining the difficulty of a Sudoku puzzle:
http://www.iprogrammer.info/news/181algorithms/4598thechaoswithinsudokuarichterscale.html
Another article on the same site is about a proof by exhaustive search that there are no 16clue puzzles:
http://www.iprogrammer.info/news/181algorithms/3572no16cluesudoku.html
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