"friendly numbers"/"amicable numbers"

friendly numbers are pairs of numbers such that each number is the sum of the divisors of the other number.
the "pythagoreans" made the extraordinary discovery that 220 and 284 are friendly numbers.
the divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110, and the sum of all these is 284.
on the other hand the divisors of 284 are 1, 2, 4, 71, 142, and the sum of these is 220.

the pair is said to be symbolic of friendship.
Martin Gardners book "mathematical magic show" tells of talismans sold in the middle ages which were inscribed with these numbers on the grounds that wearing the charms would promote love.
an arab numerologist documents the practice of carving 220 on one fruit and 284 on another, and then eating the first one and offering the second one to a lover as a form of mathematical aphrodisiac.

taken from "fermats last theorem" by Simon Singh

so now you know what to do if quoting pi to 100 decimals fails to allure the object of your affections.

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