Sorry, I seem to be posting about sudoku again. I'm not nearly as into these as my recent posting history would suggest, it's just that I thought about these a few weeks ago and still have bits and pieces in my head. Rich wrote:
Sudoku: Until the 17-clue example from yesterday, I was wondering what all the fuss was about. The 17er actually requires some more interesting reasoning, but no trial. I used things like "these two cells must be {3,4} in some order", and subtracted them from larger sets. Also "since these two are {3,4} (or {4,3}), and they are in the same row, I can exclude 3&4 from other cells in the row".
Those *should* be required lines of argument in any non-easy puzzle. (But all generation programs were not created equal.)
The main work is scanning the array looking for an application of one of the simple rules like "this 3x3 box must have a 7 somewhere, and all but one of the empty cells is excluded by having a 7 elsewhere in an intersecting row or column".
There are sudoku helpers that do some of the drudge work and let you spend your time on the interesting bits. Um, www.sudohelper.com has one, but it looks like it just maintains the sets of legal numbers for each square, and you'd still need to do the above step yourself. But I think there are some which will apply a larger set of rules and leave you with only more challenging bits. Check the ones listed in the Wikipedia's external links. --Michael Kleber -- It is very dark and after 2000. If you continue you are likely to be eaten by a bleen.