Minor comment: It's not really the "Drexel" anything; rather, Drexel has in recent years provided a web-based newsreader for the much older Usenet newsgroup sci.math.research . There are a number of choices for how you can read the newsgroups, but Drexel's operation of """its""" Math Forum has been riddled with incompetence. I recommend using Google Groups for Usenet and sci.math.research in particular. For one thing, you can read *all*, not just some, of the posts there. For another, if someone cancels their erroneous post for any reason, the cancellation is reflected in Google Groups -- but not in """Drexel's""" math forum (which once had about a one-month outage when they screwed up a new software/hardware rollout). << This thing turns up in 3-D analogues of the Penrose tiling, see e.g. the Drexel Math Forum discussion by Joshua Socolar at . . . . . .
--Dan