For every perfect positive integer
n
, the sum of the reciprocals of the positive divisors of
n
is 2.
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C o r r e c t !
Perfect Solution ! Upvoted 👍🏻.
Nice! Upvoted :)
For a perfect number 1 + d 1 + d 2 + . . . + d k + n = 2 n divide by n n 1 + n d 1 + n d 2 + . . . + n d k + 1 = 2 n 1 + d k 1 + d k − 1 1 + . . . + d 1 1 + 1 = 2
The most concise solution ! Thanks bro :)
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Suppose a perfect number n has factors a 1 n , a 2 n , a 3 n , . . . . . , a k n .
So, the divisors would be 1 , a 1 n , a 2 n , a 3 n , . . . . . , a k n , n
Hence the sum of their reciprocals would be 1 + a 1 n 1 + a 2 n 1 + . . . . . + a k n 1 + n 1 → 1 + n a 1 + n a 2 + . . . . + n a k + n 1 → 1 + n a 1 + a 2 + a 3 + . . . . + a k + 1
Since a 1 , a 2 , a 3 . . . . a k divide n it means they themselves are factors.
We know that perfect numbers are the sum of their divisors excluding the number it self. This implies that 1 + a 1 + a 2 + a 3 + . . . . + a k = n
Hence 1 + n a 1 + a 2 + a 3 + . . . . + a k + 1 = 1 + n n = 1 + 1 = 2