Originally posted by afx
you can show, that in row 2 column 8 is a 1.
There are only 1 and 9 possible and you can exclude the 9 with bifucation.
The rest is easy. To see the bifucation trick, you have to eliminate a lot
of candidates, first.
I don't know, whether there exists sudokus, which only can be solved with
brute force. I have heard about a list of extreme hard, not yet solved (without
brute force, of course ) sudokus in the net.
"Unsolveables" is the term the site uses, if you're talking about the one I'm thinking about.
They are called that because they seem to resist every other known technique of solving them. Bifurcation or "brute force" isn't particularly efficient, but it always works on a legitimate sudoku given you apply it deep enough.
I did examine the above problem by examining strongly linked squares, and I have eliminated one of three answers on three different squares.
Row 2, Column 9 can't be 9, and is either 6 or 1.
Row 5, Column 3 can't be 9, and is either 8 or 1.
Row 5, Column 8 can't be 1, and is either 9 or 5.
(Strongly linked refers to two or more squares with an "either-or" relationship. With the above three above eliminations, they can't be the first value because they are in line with 2 other squares which are indirectly, but still strongly, linked. One or the other is the eliminated number, although you don't know which one for the moment.)
EDIT: Oops, Row 5, Column 3 is aligned with 2 directly strongly linked spots.. I should have seen that one easier..