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I thought of this problem when playing Sudoku. Let A = {1,2,3,4}. I have to make a 4x4 box (i.e. the size of A in both dimensions) and fill it with data such that no dimension has repetition. How many solutions I can make such that repetition in all the 4 vertical, 4 horizontal, and 2 diagonal directions is 0?

Example solution without repeated data:

1    3    4    2
2    4    3    1
3    1    2    4
4    2    1    3

Example solution with repeated data:

1    2    3    4
2    1    4    3
3    4    1    2
4    3    2    1

Although the data are distinct in the horizontal and vertical directions, they repeat along the diagonals.

For n=1 there is a single possible solution. For n=2 and n=3 there are no possible solutions for 0 repetition. How many solutions are possible for n=4?

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    $\begingroup$ OEIS A274806 $\endgroup$
    – Daniel Mathias
    Sep 21 at 8:02
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    $\begingroup$ That sequence says 48 for n=4. Note that you can permute/relabel the numbers in n! ways, so there are really only 2, and this is given in A274171. $\endgroup$
    – Jaap Scherphuis
    Sep 21 at 8:28

1 Answer 1

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For any solution, we can relabel the numbers so that the top row reads 1,2,3,4. Given that, there are only

two ways to fill the bottom left square (must be a 2 or 3). Each of these options then forces the remaining squares on the antidiagonal, and then the first column follows, and then the rest of the board as well:

1 2 3 4   1 2 3 4   1 2 3 4
. . . .   . . 1 .   3 4 1 2
. . . .   . 3 . .   4 3 2 1
2 . . .   2 . . .   2 1 4 3

1 2 3 4   1 2 3 4   1 2 3 4
. . . .   . . 2 .   4 3 2 1
. . . .   . 1 . .   2 1 4 3
3 . . .   3 . . .   3 4 1 2

This means there are

$2\cdot4!=48$ solutions.

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  • $\begingroup$ the for n=5 will it be 3*5! sir ? $\endgroup$
    – HelptimeCode
    Sep 21 at 9:51
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    $\begingroup$ @HelptimeCode Apparently it will be 8*5!. After filling the top row and bottom left corner (3 ways) there are now several possibilities for filling the antidiagonal and the rest of the board. I don't know of any way to figure out the 8 solutions without searching through every possibility. For n=4 that was quite easy, but for n=5 that is beginning to get complicated. For larger n it would be best done by computer. $\endgroup$
    – Jaap Scherphuis
    Sep 21 at 9:55
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    $\begingroup$ @JaapScherphuis For n=5 with top row fixed there are 2 options for center square, then 2 ways to fill the bottom corners. Half of the remaining squares are forced, leaving an obviously binary choice for completion. $\endgroup$
    – Daniel Mathias
    Sep 21 at 11:22
  • $\begingroup$ is there anything to compute the diagonals ? $\endgroup$
    – HelptimeCode
    Sep 21 at 11:22
  • $\begingroup$ @HelptimeCode Choose center $x$ first, then there are only 2 choices for corner $y$ and 2 ways to complete the grid from there. There is no simple formula, just refer to the values shown in OEIS. $\endgroup$
    – Daniel Mathias
    Sep 21 at 11:41

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