Here are two options that, by making one change to the puzzle in post #6, converts it from unsolvable to one that has a unique solution:

```
Option 1: C1 empty Option 2: G1 empty
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
A | _ 2 _ | 1 5 _ | _ _ _ | A | _ 2 _ | 1 5 _ | _ _ _ |
B | _ _ 5 | 2 _ _ | _ _ 9 | B | _ _ 5 | 2 _ _ | _ _ 9 |
C | _ _ _ | _ _ 4 | _ 2 5 | C | 3 _ _ | _ _ 4 | _ 2 5 |
|-------+-------+-------| |-------+-------+-------|
D | _ 8 7 | _ _ 2 | 5 _ _ | D | _ 8 7 | _ _ 2 | 5 _ _ |
E | _ 4 _ | _ 1 7 | _ _ _ | E | _ 4 _ | _ 1 7 | _ _ _ |
F | _ _ _ | _ _ 3 | 1 _ 7 | F | _ _ _ | _ _ 3 | 1 _ 7 |
|-------+-------+-------| |-------+-------+-------|
G | 2 _ 4 | 6 _ _ | _ _ _ | G | _ _ 4 | 6 _ _ | _ _ _ |
H | 6 _ _ | _ 9 _ | _ 3 _ | H | 6 _ _ | _ 9 _ | _ 3 _ |
I | _ 5 _ | _ _ _ | 9 6 8 | I | _ 5 _ | _ _ _ | 9 6 8 |
```

Both solutions are easy to find, and are different from each other.

]]>*--------------------------*

| 4 2 6 | 1 5 9 | 3 7 0 |

| 7 0 5 | 2 3 6 | 4 1 9 |

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

|--------+-------+--------|

| 1 8 7 | 0 4 2 | 5 9 3 |

| 5 4 3 | 9 1 7 | 0 8 2 |

| 0 9 2 | 5 6 3 | 1 4 7 |

|--------+-------+--------|

| 2 3 4 | 6 0 8 | 7 5 1 |

| 6 7 1 | 4 9 5 | 2 3 0 |

| 0 5 0 | 3 2 1 | 9 6 8 |

*--------------------------*

Luckily there are plenty of resources for algorithms etc.

If you choose an Advanced puzzle you can watch it generating the puzzle. It tries solving it each time it adds a number - if it CAN solve it, then it gives you the puzzle.

In "Advanced Hard" mode, it actually removes clues until there is only ONE solution.

]]>Now it's solvable...

Also, sudoku puzzles are impossible to solve based on the predefined numbers in boxes.

The old image was

_ 2 _ |1 5 _ |_ _ _

_ _ 5 |2 _ _ |_ _ 9

3 _ _ |_ _ 4 |_ 2 5

---------------------

_ 8 7 |_ _ 2 |5 _ _

_ 4 _ |_ 1 7 |_ _ _

_ _ _ |_ _ 3 |1 _ 7

----------------------

2 _ 4 |6 _ _ |_ _ _

6 _ _ |_ 9 _ |_ 3 _

_ 5 _ |_ _ _ |9 6 8

How can one tell if a sudoku puzzle is unsolvable?

What is the least amount of numbers one may have before any puzzle becomes unsolvable?

How many unsolvable puzzles are there?

]]>

I'm going to try to solve it with a program, now.

]]>