Much material previously posted to the site has been permanently lost, so we urge all readers to help us re-build the site by contributing new material of their own.

Please share your answers to exercises in the text, your problem sets, or pose (and answer) questions for other readers.

To deter future hacking, several changes in site policy have been implemented.

1. Open enrollment in the site has been discontinued. To gain access, new and returning users must enroll again.
2. To enroll, users must provide:
1. a verifiable email address with an academic or government domain name: that is, the address must contain .edu, .ac or .gov in it.
2. the address of a personal or institutional webpage where the email address and name provided can be verified.
3. To request an exception to these requirements, contact the Site Administrator.
4. Enrolled users will have access to all areas of the site, and may submit material for posting to the discussion forums in the Exercises area. This material will be reviewed by the site admin before being "published" for others to view.