A Sudoku is given in which at least one clue is missing, for example, the one that started the previous thread:

- Code: Select all
`. . . | 3 4 . | . . 1`

. . . | . . . | 5 . .

. 9 . | 7 . 6 | . . 3

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

. 2 1 | . . . | 6 . .

. . 8 | . 6 . | 9 . .

. . 9 | . . . | 1 3 .

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

6 . . | 2 . ? | . 5 .

. . 4 | . . . | . . .

9 . . | . 5 3 | . . .

The solver must figure out which number will lead to a unique solution. The composer must make sure that no other digit will lead to a unique solution. The cell where the missing clue goes could be marked, as above, or merely implied by the symmetry. Puzzles of this type would defeat lazy solvers who might take a guess. They would seem to be simple to create, (The example was created accidentally.), changing what would have been an easy puzzle into a hard one, a hard one into a devious one, etc.

Similar ideas that may or may not be possible:

The composer sets one clue intentionally incorrect -- but the solver is not told which one. The composer must somehow make sure that there is only ONE possible change in only ONE clue that will create a valid Sudoku -- and do it in such a way that the solver has some reasonable chance of figuring it out.

For a more out-of-the-box challenge, a puzzle might be created in which the missing digit could be ANYWHERE in the grid, ignoring symmetry. The solver would have to be fully informed and the composer would have to create a puzzle such that only ONE digit in ONE cell would lead to a unique solution -- all other possibilities would either have no solution or multiple solutions. Sounds perfect for a World Puzzle Championship, not so good for Monday morning on the train.