# How to decide what numbers to show in a sudoku grid so that it's solvable?

Let's assume I've generated, from an empty board, a complete and valid sudoku board by some means. Borrowing from this question, let's say that board is:

+-------+-------+-------+
| 5 6 4 | 9 3 7 | 2 8 1 |
| 1 7 2 | 5 4 8 | 9 6 3 |
| 9 8 3 | 1 6 2 | 4 7 5 |
+-------+-------+-------+
| 7 4 9 | 3 2 5 | 8 1 6 |
| 2 1 8 | 7 9 6 | 5 3 4 |
| 6 3 5 | 8 1 4 | 7 9 2 |
+-------+-------+-------+
| 8 2 6 | 4 7 3 | 1 5 9 |
| 4 5 1 | 6 8 9 | 3 2 7 |
| 3 9 7 | 2 5 1 | 6 4 8 |
+-------+-------+-------+


According to this paper (pdf) there must be at least 17 clues for the sudoku to be solvable. I take that to mean solvable without guessing. I'm unsure if that means there is distinctly one grid that's an answer.

If I have a grid, how can I pick the 17 squares to reveal and guarantee that the user doesn't have to guess in order to solve the grid?

• I would assume that “solvable” means to have a unique solution, but I have not read the paper. May 11, 2016 at 20:27
• @CarstenS Me either. Maybe I should... May 11, 2016 at 20:36