<s>[INST] Diagonal-free Sudoku grid I have a Sudoku grid with the property that diagonally adjacent elements are distinct (it is also a torus under the same property).
My question is up to isomorphism, is the grid unique?
Here's the grid:
$$\begin{array}{|c|c|c|c|c|c|c|c|c|}
\hline
6& 5& 7& 3& 4& 2& 1& 9& 8\\ \hline 
9& 8& 1& 5& 6& 7& 4& 3& 2\\ \hline
3& 2& 4& 8& 9& 1& 6& 5& 7\\ \hline
5& 7& 6& 2& 3& 4& 9& 8& 1\\ \hline
8& 1& 9& 7& 5& 6& 3& 2& 4\\ \hline
2& 4& 3& 1& 8& 9& 5& 7& 6\\ \hline
7& 6& 5& 4& 2& 3& 8& 1& 9\\ \hline
1& 9& 8& 6& 7& 5& 2& 4& 3\\ \hline
4& 3& 2& 9& 1& 8& 7& 6& 5\\ \hline
\end{array}$$
To explain the diagonal-free property, if we have:
\begin{array}{c|c}
a&b\\ \hline
c&d\\
\end{array}
then $a\ne d$ and $b\ne c$. This does NOT imply the the whole diagonal is distinct (as in X-factor).
If we wrap the grid into a cylinder, and then bend the tube into a torus, the diagonal property still holds, so, for example, the $9$ on the base row is considered to be diagonally adjacent to the $4$ and $7$ on the top row.
There are two links on OEIS:


*

*De-diagonalized grid

*Diagonalized grid
Isomorphism in this instance implies we can change the orientation, change the permutations of the numbers, perform row/column swaps as long as the conditions still hold. This can create grids with no internal diagonals but that are not torii, for example (this is my original grid by the way!):
$$\begin{array}{|c|c|c|c|c|c|c|c|c|}
\hline
1&2&3&7&8&9&4&5&6\\ \hline
7&8&9&4&5&6&1&2&3\\ \hline
4&5&6&1&2&3&7&8&9\\ \hline
6&1&2&3&7&8&9&4&5\\ \hline
9&4&5&6&1&2&3&7&8\\ \hline
3&7&8&9&4&5&6&1&2\\ \hline
5&6&1&2&3&7&8&9&4\\ \hline
8&9&4&5&6&1&2&3&7\\ \hline
2&3&7&8&9&4&5&6&1\\ \hline
\end{array}$$
We can see that the $2$ on the base line is diagonally related to the $2$ on the top line, and so this grid is not a torus. The torus example is derived from this one using only the operations defined above.
So I am asking if there is a diagonal-free grid with a quintessentially different structure to either of the two grids given here.
ADDENDUM
For reference here is a Sudoku grid with diagonals, from Conceptis Puzzles:

The diagonally adjacent givens are highlighted - there may be more!
 [/INST] In the general case, a grid will be in an isomorphism equivalence class of $9!\cdot 3!^4 \cdot 2$ corresponding to permutations of the symbols, permutations of the rows and columns, and the symmetry of the square which is not already covered by permutation of the rows and columns. I'm only counting $3!^2$ permutations of the rows rather than $9!$ because most of those $9!$ will break one or more of the $3\times 3$ squares with a Sudoku constraint.
I've set a computer program to generate solutions which match the constraints (and the additional constraint that the first row is $123456789$); so far it has found more than $20000$. The first two are
$$\begin{array}{|c|c|c|c|c|c|c|c|c|}
\hline
1& 2& 3& 4& 5& 6& 7& 8& 9\\ \hline
4& 7& 8& 1& 3& 9& 2& 5& 6\\ \hline
\textbf{5}& \textbf{9}& 6& 7& 2& 8& 4& 1& 3\\ \hline
6& 3& 4& 8& 1& 5& 9& 7& 2\\ \hline
\textbf{9}& \textbf{5}& 2& 3& 4& 7& 1& 6& 8\\ \hline
7& 8& 1& 9& 6& 2& 5& 3& 4\\ \hline
2& 4& 7& 5& 8& 3& 6& 9& 1\\ \hline
3& 1& 9& 6& 7& 4& 8& 2& 5\\ \hline
8& 6& 5& 2& 9& 1& 3& 4& 7\\ \hline
\end{array}$$
$$\begin{array}{|c|c|c|c|c|c|c|c|c|}
\hline
1& 2& 3& 4& 5& 6& 7& 8& 9\\ \hline
4& 7& 8& 1& 3& 9& 2& 5& 6\\ \hline
\textbf{9}& \textbf{5}& 6& 7& 2& 8& 4& 1& 3\\ \hline
6& 3& 4& 8& 1& 5& 9& 7& 2\\ \hline
\textbf{5}& \textbf{9}& 2& 3& 4& 7& 1& 6& 8\\ \hline
7& 8& 1& 9& 6& 2& 5& 3& 4\\ \hline
2& 4& 7& 5& 8& 3& 6& 9& 1\\ \hline
3& 1& 9& 6& 7& 4& 8& 2& 5\\ \hline
8& 6& 5& 2& 9& 1& 3& 4& 7\\ \hline
\end{array}
$$
which, while clearly closely related, I don't believe to be actually isomorphic. And they're certainly not isomorphic to the grid in your question, which has no rectangle whose four corners match diagonally in this way.
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