Amicable Numbers

Amicable numbers occur in pairs. Each number is equal to the sum of the other's aliquot divisors (including 1 but excluding the number itself).
For example, the smallest such numbers are 220 and 284 which were known to the Pythagoreans.
The proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, and 110. These add up to 284.
The proper divisors of 284 are 1, 2, 4, 71, and 142. They add up to 220.
The Pythagoreans regarded 220 and 284 as symbols of friendship.
Biblical commentators noticed that Jacob gave Esau 220 goats and 220 sheep. (Genesis 32:14).
The commentators believed it was a wise choice because 220, being one of the amicable pair, showed Jacob's great love for Esau.
More than 1,000 pairs of amicable numbers are known today. Here is a list of the amicable numbers with 5 digits or less:
1 220 284
2 1,184 1,210
3 2,620 2,924
4 5,020 5,564
5 6,232 6,368
6 10,744 10,856
7 12,285 14,595
8 17,296 18,416
9 63,020 76,084
10 66,928 66,992
11 67,095 71,145
12 69,615 87,633
13 79,750 88,730

source: Mathematical Magic Show by Martin Gardner

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