Most difficult puzzles:

Almost all sudoku creators have their own methods for rating puzzles. Usually they are based on the subjective rating of different solving methods needed to solve a puzzle or guessing. In many cases the rating changes, if techniques are applied in the different order or applied to some variants of the original puzzle. That is why, this kind of rating methods are not very useful. The most common and freely available rating methods are gsfr, suexrat9 and Sudoku Explainer.

I use AIroot technique to rate the puzzle. By AIroot, it is possible to find out the shortest route (root) to make next elimination in all situations. Deepest root is not the only criteria to rate the puzzle, because it also increase the difficulty level, if these long elimination roots must use many times. I count the rating by following equation:

where,

P = difficulty level of puzzle = AI points

n = techniques needed to make eliminations, when it is used AIroot techniques, which need lowest number of links.

S_t= number of links needed to make next elimination in the situation t. All eliminations, which can be made by same number of links, will be realized.

k = 2.3 (coefficient, which describe how difficult the next step is compared to the earlier one).

Table 1. AI Sudoku Top10

Table 2. Other famous or the most difficult puzzles by well known sudoku creators (3 Oct 2006)