Who cares about perfection?

For every perfect positive integer n n , the sum of the reciprocals of the positive divisors of n n is 2.

NOTE :

  • A positive integer k k is said to be perfect if the sum of its proper positive divisors is k k itself.
Always True Always False Sometimes True

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2 solutions

Vaibhav Prasad
Jul 19, 2015

Suppose a perfect number n n has factors n a 1 , n a 2 , n a 3 , . . . . . , n a k \frac{n}{a_1}, \frac{n}{a_2}, \frac{n}{a_3},....., \frac{n}{a_k} .

So, the divisors would be 1 , n a 1 , n a 2 , n a 3 , . . . . . , n a k , n 1, \frac{n}{a_1}, \frac{n}{a_2}, \frac{n}{a_3},....., \frac{n}{a_k}, n

Hence the sum of their reciprocals would be 1 + 1 n a 1 + 1 n a 2 + . . . . . + 1 n a k + 1 n 1 + a 1 n + a 2 n + . . . . + a k n + 1 n 1 + a 1 + a 2 + a 3 + . . . . + a k + 1 n 1+\frac{1}{\frac{n}{a_1}}+\frac{1}{\frac{n}{a_2}}+.....+\frac{1}{\frac{n}{a_k}}+\frac{1}{n}\\\rightarrow 1+\frac{a_1}{n}+\frac{a_2}{n}+....+\frac{a_k}{n}+\frac{1}{n}\\\rightarrow 1+\frac{a_1+a_2+a_3+....+a_k+1}{n}

Since a 1 , a 2 , a 3 . . . . a k a_1, a_2, a_3 ....a_k divide n 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 1+a_1+a_2+a_3+....+a_k=n

Hence 1 + a 1 + a 2 + a 3 + . . . . + a k + 1 n = 1 + n n = 1 + 1 = 2 1+\frac{a_1+a_2+a_3+....+a_k+1}{n}\\=1+\frac{n}{n}\\=1+1=\huge{\boxed{2}}

C o r r e c t ! \huge Correct!

Nihar Mahajan - 5 years, 11 months ago

Perfect Solution ! Upvoted 👍🏻.

Venkata Karthik Bandaru - 5 years, 11 months ago

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Thank you !!

Vaibhav Prasad - 5 years, 11 months ago

Nice! Upvoted :)

Kazem Sepehrinia - 5 years, 10 months ago
Kazem Sepehrinia
Jul 19, 2015

For a perfect number 1 + d 1 + d 2 + . . . + d k + n = 2 n 1+d_1+d_2+...+d_{k}+n=2n divide by n n 1 n + d 1 n + d 2 n + . . . + d k n + 1 = 2 1 n + 1 d k + 1 d k 1 + . . . + 1 d 1 + 1 = 2 \frac{1}{n}+\frac{d_1}{n}+\frac{d_2}{n}+...+\frac{d_k}{n}+1=2 \\ \frac{1}{n}+\frac{1}{d_k}+\frac{1}{d_{k-1}}+...+\frac{1}{d_1}+1=2

The most concise solution ! Thanks bro :)

Venkata Karthik Bandaru - 5 years, 10 months ago

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