Two numbers are said to be amicable if each is equal to the sum of the proper divisors of the other.
- ~850AD Thabit ibn Qurra writes Book on the determination of amicable numbers which contains general methods to construct amicable numbers. He knows the pair of amicable numbers 17296, 18416.
- 1636 Fermat discovers the pair of amicable numbers 17296, 18416 which were known to Thabit ibn Qurra 800 years earlier.
See also: Divisor.