“But what is their lure? A mathematician I spoke with dismissed the puzzles as mere ‘bookkeeping’ — keeping track of where things go. And there surely is some of that, since one technique for solving them involves tentatively writing miniature numbers in each little square to figure out the various possibilities. The grid for a difficult puzzle can begin to look like the first draft of a major corporation’s balance sheet. This is hardly higher mathematics. In fact, numbers are hardly necessary: the same puzzle can be posed using nine colors or nine national flags.

Yet mathematicians have been taking more of an interest in sudoku — not necessarily in solving the puzzles, but in understanding more about their character. In a recent essay in The American Scientist, Brian Hayes described the difficulty of determining the difficulty of these puzzles: it bears little connection with how many numbers are given at the start.” (New York Times )