the slowest simple sudoku

weia · Posted: Mon Nov 14, 2005 9:37 pm Post subject: the slowest simple sudoku

Many sudoku's in newspapers can be solved by only filling in naked singles and hidden singles, without having to look at more complex patterns. This 'simple sudoku's' (or do they already have another name?) start with a position that often leads to say 3 singles (hidden or not). I call it one round when you fill in those 3. This leads to another say 4 singles for the second round. In the third round you can add 5 singles or so. Now what I am wondering is: what is the slowest simple sudoku that takes the most rounds to find the complete solution?
In theory one could start with 17 hints and so 64 blanks, but the last 4 always go in one round, so theoretically a simple sudoku is possible that takes 60 rounds to complete, with every round existing of finding only one single (naked or hidden).
Have others looked at this? Who knows a very slow sudoku? (Which I like more than the fast ones.)

As an example: on http://www.csse.uwa.edu.au/~gordon/sudokumin.php all sudoku's with 17 hints are given. The first is done after 15 rounds, the second after only 11 rounds, the next three aren't simple. The two simple ones have only one round with only 1 single, their last round.

weia · Posted: Mon Jan 30, 2006 6:02 pm Post subject:

The slowest among the first 2500 of Gordon's 17-sudoku's takes 26 rounds to the solution. This one:
000000063
000100000
200000000
050000100
000400200
009006000
630090000
000200740
008000000
Nine rounds have only 1 (hidden) single.

Ruud · Site Admin

Hi,

I perform these tests on my own generated sudokus, so I was able to run the test on the complete sudoku17 collection (32930 entries)

There are 15198 sudokus in the collection that can be solved with FN only.

The winner, with 27 rounds:

weia · Posted: Tue Jan 31, 2006 8:34 am Post subject:

Ah! You tested them all, thanks!

Weia

daj95376 · Posted: Sat Feb 18, 2006 2:49 am Post subject:

Ruud · Site Admin