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28 (number)
28 (twenty-eight) is the even composite number that is the second perfect number, after 6, its proper divisors 1, 2, 4, 7, and 14 summing to 28.
Abundant number
An abundant number, or excessive number, is a positive integer whose proper divisors add up to more than the number itself, the smallest being 12.
Amicable numbers
Amicable numbers, or amicable pairs, are two numbers each equal to the sum of the other's proper divisors; the smallest pair, 220 and 284, was known to Thābit ibn Qurra in the ninth century.
Perfect numbers
A perfect number is a positive integer equal to the sum of its proper divisors, such as 6 and 28; every even one arises from a Mersenne prime.