[math-fun] Solvable sudoku?

I was wondering: how does one determine: 1) if a particular sudoku grid is solvable at all, and 2) if a particular sudoku grid is solvable solely by "logic". I realize that there's some disagreement as to which procedures are allowed in "logic only" solutions, but still: is there some way, short of brute-force solving [either with full guess/retrace (1 above) or only using some set of deduction-rules (2 above)] for figuring out if a grid is solvable? /Bernie\ -- Bernie Cosell Fantasy Farm Fibers mailto:bernie@fantasyfarm.com Pearisburg, VA --> Too many people, too few sheep <--