The New Sudoku Players' Forum

[m_b_metcalf]

There exists a puzzle with 20-odd givens based on the MC grid

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

(or its version with n=10-n), but I cannot find it anywhere. Does someone please have it on file or know of a link?

Many thanks,

Mike Metcalf

[eleven]

Not sure what you are looking for.
Maybe this post could be a help ?

[coloin]

There exists a puzzle with 20-odd givens based on the MC grid .......

well its based on the MC grid - in that it has 6 repeating minirows - but I dont think it the MC grid
im not sure how many grids have all 6 bands less than 30/416.

im sure "checker" or "gridchecker" would confirm the no 19.

C

[m_b_metcalf]

Sorry, I should have been more explicit. The grid starts

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


(n=10-n) and the puzzle starts

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. . . . . . . . .
. . . . . . . . 4


It's an absolute worst-case puzzle for a naive brute-force solver. I recall finding it on sudopedia but can't find it on its mirror.

Regards,

Mike

[tarek]

Hi Mike,

Wouldn't a 10-n version of the MC grid suffice??

Tarek

[eleven]

For me the internet was much better 15 years ago, when the fools did not have the tools to spoil it with their rubbish.

Now it's probably impossible to find, what you want, though we can be sure, that it is still published somewhere.
All the links to "worst case" puzzles for backtracking solvers go to this 17 clue, which has been in the wikipedia.

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

[m_b_metcalf]

eleven,
Thanks. That's perfect. I'm in discussion with the author of a brute-force code. It takes 61s to solve this puzzle. By replacing n by 10-n the time falls to 0.1s! (Of course, in real life, no solver is actually necessary as it is completely solvable with singles only.)

Regards,

Mike