Sudoku for Nikolaus 2005

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A Nikolaus Sudoku Puzzle

7
4
6 3
6
9 3
7 8
1
4
9
5
2
8
1
4
9
9
5
6
3 7 8
1

How many possibilities are there to add one symbol such that the board becomes a (uniquely solvable) Sudoku puzzle?

Number:

To prove that you have not just guessed the number, also give three of the possibilities (or all possibilities, if there are less than three):
Row: Column: Symbol:
Row: Column: Symbol:
Row: Column: Symbol:

Questions and suggestions to Frank.Luebeck@Math.RWTH-Aachen.De.

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