Can an unambiguous 9 x 9 sudoku puzzle have less than 22 given numbers?

Muchas gracias - Shall try to solve it :]

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by palmagma on October 14th, 2008

Unfortunately this is an ambiguous sudoku puzzle.
It has two solutions:

SOLUTION ONE:

957 463 218
426 781 359
138 529 467

689 352 174
742 916 835
315 874 692

594 137 826
861 245 973
273 698 541

SOLUTION TWO:

957 463 218
426 781 359
831 529 467

689 352 174
742 916 835
315 874 692

594 137 826
168 245 973
273 698 541

Note the exchangeable 1 and 8 in rows three and eight.

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by palmagma on October 14th, 2008

Hm, indeed.

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by Hagar on October 15th, 2008

Have seen this happen before.
22 clues might well be the minimum
for an unambiguous 9 x 9 puzzle.

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by palmagma on October 15th, 2008

When a puzzle has a unique solution, it means that you can find the unique solution by logic only, so no guessing. If you start guessing early in the game there are of course other solutions because there are so little givens. You should never have to guess. In the above puzzle you needed to guess, so it was indeed faulty.

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by Hagar on October 15th, 2008

Have had no success with your first link.
It delivers what looks like an endless string of digits.
Maybe someone could build a puzzle out of them. Who knows?
Am looking for a 21-clue-or-less unambiguous 9x9 sudoku example
in a simple format (e.g. as a Word or Excel table).

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by palmagma on October 13th, 2008

You have to admit that your second link
is extremely obscure and difficult to follow.
The calculation might be correct.
But I am still looking for a concrete example.

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by palmagma on October 13th, 2008

Each line of that first link is one sudoku. I've taken the first line and cut it into 9 pieces below, to make your first sudoku with 17 givens.

000000010
400000000
020000000
000050407
008000300
001090000
300400200
050100000
000806000

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by Hagar on October 14th, 2008

About the second link, I agree.

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by Hagar on October 14th, 2008

Thanks - It might take a few days to get around to solving this one.

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by palmagma on October 15th, 2008

Unfortunately
000000010
400000000
020000000
000050407
008000300
001090000
300400200
050100000
000806000
appears to be a flawed sudoku (i.e. with no solution).
Correct me with your solution if I am wrong.

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by palmagma on October 15th, 2008

The puzzles are not mine, but I have no reasons to think they are not valid.

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

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by Hagar on October 15th, 2008

Yes, thanks. That convinces me.
I obviously made a mistake doing it :[

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by palmagma on October 15th, 2008

Not only is this a good 9 x 9 sudoku,
it also appears to be unambiguous.
So thanks again - it looks like 17 is the minimum clue number.

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by palmagma on October 15th, 2008

Can you give an example?

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by palmagma on October 13th, 2008