1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120.
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A perfect number equals the sum of its proper divisors. A deficient number is greater than the sum of its proper divisors. An abundant number is less than the sum of its proper divisors. Proper divisors of a number do not include the number itself.
Because 26 is greater than the sum of its proper divisors.
Amicable numbers are pairs of numbers for which the sum of the proper divisors (the divisors except for the number itself) equals the other number in the pair. The smallest amicable pair is 220 and 284. The proper divisors of 220 are and the factors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, and 110, the sum of which is 284. The proper divisors of 284 are 1, 2, 4, 71, and 142, the sum of which is 220.
Because 51 is greater than the sum of its proper divisors.
a perfect number is defined as a positive integer which is the sum of its proper positive divisors, that is, the sum of the positive divisors excluding the number itself 137,438,691,328 is the 7th perfect number.