denis_berthier wrote: .... I think that the graphical aspect of a puzzle plays a major role in its success. .... in games of this kind, some form of regular grid is necessary ....

I agree. I would never be interested in actually trying to solve one of the theoretical monstrosities I've been proposing. My point was to observe that the black cells don't really exist mathematically -- they're just convenient places to write the sums.

.... but not necessarily a square in a plane, it can be hexagonal or it can lie on a torus or a Klein bottle ....

Yes, Tarek has quite a few Moebius strips, tori, Klein bottles, and projective planes in the Sudoku Variants topic. Those are interesting too.

Bill Smythe