I’m sure many of the Teeming Millions are familiar with Sudoku. (Rules in the link)

The puzzle in the 11 January 2008 edition of the Los Angeles Times is classified as “Diabolical”. Here it is:

```
_ _ _ _ _ _ _ _ _
_ 4 1 6 _ 7 _ 5 _
_ _ 9 2 _ 8 7 _ _
_ 8 3 _ 7 9 6 _ _
_ _ _ _ _ _ _ _ _
_ _ 2 5 4 _ 1 7 _
_ _ 7 4 _ 2 8 _ _
_ 3 _ 7 _ 5 2 6 _
_ _ _ _ _ _ _ _ _
```

(The puzzle is credited to “The Mepham Group”)

To solve the “Diabolical” puzzles of the LA Times, I use my habitual methods and then at some point I resort to trial and error - make a guess in one cell, follow the guess through until I encounter a contradiction, and then backtrack and make another guess.

Someone told me that you should never have to guess and that it’s always possible to find a logical way to solve the puzzle without guessing, using advanced methods such as the ones you find here.

I am curious to know if anyone here could solve the puzzle in my post without doing the guessing and backtracking. Needless to say, I couldn’t. In fact, I must have chosen poorly the squares in which to guess, because I guessed in one square, followed the path from my guess, had to backtrack; and then had to guess again because I only got so far after my second guess; and even after the second guess and backtracking after finding a contradiction, I only got so far and had to make a third guess!