19.08.2011, 23:35
(19.08.2011, 18:04)RobertBe schrieb: And a generalization of this rule is the following:
If there is a chain of 2s with a 1 on each end (e.g. 1-2-2-2-1) then the outer diagonals can not point at the 1s.
Proof: Let n be the number of 2s in the chain. The total number of cells covered by the chain is then 2n+6. The sum of the givens in the chain is 2n+2, so the 4 outer diagonals must have a total of 0 (pointing towards the chain).
I'm pretty sure, that's what he meant when he wrote 12...21 in the first point of his list. Although this is a special case of his general rule I guess is deserves a special point as it is used very often.