MikeS>Hensel's applet is blazingly fast, In case anyone missed it, Rockicki&Trevorrow's Golly (now 1.2?) application blazes in the ultraviolet. One of its predefined patterns stops growing iff there's another Fermat prime. --rwg PRESUMING PRIMES GUN PS, Can't resist plugging the Pattern Info for Hashing Examples> totalperiodic.rle: #C Without the block, this is Gosper's "total aperiodic" #C pattern (Nov '97), in which every cell in the universe #C eventually becomes aperiodic. #C #C With the block, every cell eventually becomes periodic, #C but incredibly slowly. The block remains untouched for #C very roughly 3^63 generations. It deletes the nth glider #C (and is shifted) at about generation 3^(57.5+5.5n). #C #C Bill Gosper, 24 Jun 2004