Suppose - say - number 1 can only be placed in either the first or the 7th cell in the long diagonal.

So number 1 goes in eithe (1,1) or (7,7) in the diagonal from left-above to right-under.

Then I know I can exclude 1 from cell (1,7), since this cell shares a group with both (1,1) and (7,7).

For the same reason I can exclude 1 from (7,1).

- Code: Select all
`X . .|. . .|0 . .`

. . .|. . .|. . .

. . .|. . .|. . .

-----+-----+-----

. . .|. . .|. . .

. . .|. . .|. . .

. . .|. . .|. . .

-----+-----+-----

0 . .|. . .|X . .

. . .|. . .|. . .

. . .|. . .|. . .

My question now: is this related to a known solving-technique, which may even occur in plain sudoku?