Nice question! Though I wish it were a bit more symmetrical: same question for the largest square *torus* grid. --Dan << Since there are clearly wizards here (both mathematicians and programmers) I thought some here might be interested in the just-started programming contest here: http://infinitesearchspace.dyndns.org/ . It is just for fun. The object is to find the largest square grid that can be colored with C colors so that corners of sub-rectangles are not all the same color, and to do this for various values of C (each value a sub-contest). You just register and then start submitting your best results. Since this is somewhat mathematical and for fun, I hope it is not inappropriate to mention here.

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