| Asimov, Issac. Asimov on Numbers. 1977: Pocket Books, New York, New York. |
| The 1995 Grolier's Multimedia Encyclopedia. Designed Produced, and Published by Grolier Electronic Publishing Inc. Copyright 1993. |
| Britannica Online.* www.britannica.com. (This is a restricted forum. A password is necessary to check any articles here.) |
| Downs, Robert B. Landmarks of Science, Hippocrates to Carson.. 1982: Libraries Unlimited, New York, New York. |
| Hellemans, Alexander. Bunch, Brian. Bunch, Bryan. Timetables of Science. 1991, Touchstone Books. |