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| | Perfect, amicable and sociable numbers |
 | | For a number n, we define s(n) to be the sum of the aliquot parts of n, i.e., the sum of the positive divisors of n, excluding n itself: so, for example, s(8)=1+2+4=7, and s(12)=1+2+3+4+6=16. |
 | | A perfect number is a cycle of length 1 of s, i.e., a number whose positive divisors (except for itself) sum to itself. |
 | | However the sum of unitary divisors, bi-unitary divisors, or infinitary divisors of a number is always even, unless the number is a power of 2. |
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