You certainly need the "box rule" too: that's the obvious rule about numbers appearing at the intersection of a box with a row/column. I think a good statement of it is: if a set of digits appears within a box only where it intersects a certain row/column, then you can eliminate those digits from the rest of that row/column, and conversely (with row/column swapped with box). I hope that's at least enough of the idea for you to figure out what I mean even if I didn't say it right. --Joshua Zucker On 7/6/06, Henry Baker <hbaker1@pipeline.com> wrote:
I've come to the conclusion that the LATimes does _not_ use a program to rate the Sudoku's. In the past several days, there have been two Moderate's: one was easily solved by my Level 1 (k=1&8), while the other wasn't solved by Level 3 (considering k=1,2,3, as well as implicitly k=8,7,6). This last Moderate should be relabeled at least "Tough", or worse.
It also appears that the utility of the higher levels of k is very limited. I've only found 1 or 2 Sudokus where k=3/6 helped.
Also, the "k rule" (pigeonhole rule) doesn't capture the power of some very useful rules, which relatively unskilled Sudoku people (like myself) use.
I'm still trying to formalize a simple characterization of non-lookahead, non-backtracking strategies for Sudoku.
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