.
Here are the results of quite long calculations. (I had previous results related to the much smaller 972 database, but that was too small to make any conjecture about it.)
All the known 9x9 sudoku puzzles in T&E(3) are indeed in T&E(W2, 2) or less.
All the terms are essential:
1) "sudoku puzzles" excludes sukakus
2) "9x9" excludes larger puzzles
3) "known" means both published (i.e. it refers to the last database published by mith *) and it implies that the proof relies on testing this database (not on a formal proof). Notice that (due to the consistency of the classification wrt adding candidates) testing the 375,759 minimal database has been done by testing only its 63,137 min-expands.
Considering my universal T&E(n) classification of any finite binary CSP-instance and the sub-classifications of each level:
- T&E(1) sub-classified by braids(k)
- T&E(2) sub-classified by Bk-braids or equivalently by T&E(Bk, 1)
- T&E(3) sub-classified by T&E(Bk, 2)
and applying it to 9x9 Sudoku puzzles, we have:
1) all the known puzzles in T&E(1) are in B29 or less (Mauricio's example in B29)
2) all the known puzzles in T&E(2) are in B7B = T&E(B7, 1) or less (3 examples currently known to be exactly in B7B)
3) all the known puzzles in T&E(3) are in B2BB = T&E(W2, 2)
Each of these 3 experimental results relies on a sufficiently large number of test cases to be turned into a conjecture by deleting the word "known". Note that the resulting second conjecture is a modified version of my very old B7B conjecture, before I found that mith's puzzle Loki is not in T&E(2).
Notice however that my T&E(2) and B7B conjectures relied on a database assembled from many independent seeds. The T&E(3) mith database seems to be assembled from a much smaller number of seeds (maybe ultimately only 1: Loki). This leaves some open questions about other seeds in T&E(3).
* 375,759 minimal puzzles not in T&E(2): http://forum.enjoysudoku.com/the-hardest-sudokus-new-thread-t6539-1299.html
and the corresponding databases of 63,137 min-expands and 15,606 max-expands:
http://forum.enjoysudoku.com/the-hardest-sudokus-new-thread-t6539-1304.html