also

http://thru-you.com

sudoku

I go through periods of being taken by sudoku. It is unfortunate, then, that i have a sudoku program on my computer at work. I have worked through 50 sudoku puzzles in this program in the last year or so. I have noticed that a large number of them eventually come down to me having to guess a solution, and then test it to see if it is correct. (A few even seem to have two correct solutions - symmetry is a bitch.) This really bugs me. I also do sudoku in the newspaper, and out of a book that i have, and i don't run into this problem anywhere else.

The thing that really irritates me is that it is possible to 'guess' wrong - in other words, there is a solution to these puzzles, but i can't figure out how to get at it from a logical standpoint. (Well, i suppose guessing and then proving that guess to be wrong is sort of logical, but i'd rather figure out how to eliminate the wrong answer up front.)

An example of such a puzzle is:

_9_ 762 314
637 154 982
4__ __9 675

_59 618 4_7
_46 927 15_
17_ 543 _96

_83 495 _61
9__ _76 _4_
_64 __1 __9

At this point, if i guess 5 in the upper right corner, i can fill in the rest with a correct solution. If i guess 8 in the upper right corner, i run into an eventual contradiction.

Does anyone know what i'm missing that might allow me to eliminate the incorrect solution?