Dear readers, here's something I'm pleased with, which I like to advertise every so often. It's a program running on my Web server which demonstrates category theory by calculating constructs in the category of sets: http://www.j-paine.org/cgi-bin/webcats/webcats.php . The page contains instructions, followed by a form which lets you ask the server to calculate products, equalisers, etc. For example, you can ask it to calculate the product of sets A and B. You either type in values for the sets, or let it choose them, and it will calculate the product object and projection arrows. It will then send back the page, but headed with a link to your results. These contain a diagram, values for all its objects and arrows, and an explanation. And, just to make the diagram more vivid, it rendered in VRML so that you can pick it up and move it about. To show how general category theory is, I used the same code to calculate the limit and colimit of simple diagrams representing systems of interacting objects. You can see a demo of that at http://www.j-paine.org/slp_demo.html . Cheers, Jocelyn Ireson-Paine 07768 534 091 http://www.jocelyns-cartoons.uk http://www.j-paine.org [For admin and other information see: http://www.mta.ca/~cat-dist/ ]