The New Sudoku Players' Forum

[Houdini]

Why is box 9 a weak link in this chain?

[eleven]

Welcome, Houdini.

It is a strong link, which here works as a weak link.

The logic is:
If 8 "is not" in r5c8, it "must be" in r8c8 (strong link),
if it "is" in r8c8, it "can't be" in r7c7 (weak link, also correct with a strong link),
if it "is not" in r7c7, ...
(you can do that from the other side as well)

At the end you know, that if 8 is not in r5c8, it must be in r6c6, i.e. either 8 is in r5c8 or in r6c6 or in both of them.

[Houdini]

Thanks eleven. I didn't realize a strong link could take the place of a weak like that in an x-chain. I see that following the logic around makes sense. I will have to change the way I search for these chains.