Amicable Number
The two numbers 220 and 284 are called amicable number. The sum of the divisors of 220 (1,2,4,5…110) is 284, and vice versa.
Flipping through the book Fermat’s Last Theorem the other day, I found this interesting story:
An Arab numerologist documents the practice of carving 220 on one fruit and 284 on another, then eating the first one and offering the second one to a lover as a form of mathematical aphrodisiac.
Wonder if any of our modern-day geeks would like to pick up this fantastic ritual again — perhaps as laser-engravings onto mobile phones?
Also try a geekier pair: 9363584 and 9437056, discovered by Descartes in the Age of Reason.