(Apologies in advance if this is excessively commercial; in my defense, I'm not out to make any money, and it all derives directly from threads on this list) A few years ago I plotted out the iterations to reach 1 for the Collatz conjecture for n \in{1,1e9} (and posted the results here). The graph was so interesting that I made a T-shirt out of it, and today I finally got around to adding it to one of their "marketplace" accounts: http://www.cafepress.com/lunkwill.254254793 I included a $1 markup on the items just for the heck of it, but let me know if you'd like it at cost. I also did a version of the shirt for my friend doing a postdoc at MIT, changing the text from "COLLATZ" to "Increment, Halve, Triple. Forever, Please.": http://www.cafepress.com/lunkwill.254237008 The preview isn't intelligible, so here's the image: http://lunkwill.org/ihtfp-black.png (If you want this one, you may want to wait until my friend gets it and lets me know if it looks okay) I also became enamored of Bill's space-filling function, and made some sweatshirts of it: http://www.cafepress.com/lunkwill.254257188