World's Hardest Sudoku

jones_supa writes: A Finnish PhD in mathematics, Arto Inkala, has allegedly created the world's toughest sudoku puzzle. 'There's no straightforward way to define the difficulty level of a sudoku. I myself doubt if this is the hardest in the world, but definitely harder than my previous ones.', Inkala sets off humbly. The news agencies around Europe, including The Telegraph, are nonetheless excited. The particular difficulty in this version lies in the number of deductions you have to make in order to fill in a single number on the grid. 'It is a common misception that the less initial numbers, the harder the puzzle. The most challenging ones have 21-25', the creator adds.