Arto Inkala created the sudoku puzzle below in 2012 and claimed that it was the most difficult one ever created. I don't usually publish other people's work but I am making an exception in this case as I want to compare this puzzle with one of my own:
I have given you the usual link to solve the puzzle yourself. I have also given you a link to run it through Andrew Stuart's grader and solver but it failed to find the solution. Finally I ran the puzzle through Hodoku, which gave up and resorted to brute force when the score got to 21342:
800000000003600000070090200050007000000045700000100030001000068008500010090000400 #1 Extreme (21342) bf
