Perfect Numbers and their reciprocals.

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In summary, a Perfect Number is a number n for which the sum of its divisors is equal to 2n, and there is a neat fact about them stating that the reciprocals of the divisors will always add up to 2. This raises the question of whether there is any possible relation between this fact and the summation of (1/2)^n, which converges to two as n approaches infinity. A possible connection is explored through a proof that if N is perfect, then 1 = (1/N) + (1/p2m) + ... + (1/p1).
  • #1
Slats18
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For those who don't know, here's the definition:

A Perfect Number is some number n such that the sum of its divisors is equal to 2n (or just n if you don't count n|n =P )
Ex: Let n=6, and the divisors of 6 are 1,2,3 and 6

1+2+3+6 = 12 = 2*6.

Moving on, there's also the neat fact about them that states that the reciprocals of the divisors will always add up to 2.

Ex: n=6, 1/6 + 1/3 + 1/2 + 1/1 = 2

Now, my question/wonderment: is there any possible relation between this fact, and the summation of (1/2)^n, which converges to two as n approaches infinity?
 
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  • #2
Let

[tex]1 < p_{1} < p_{2} < \ldots < p_{2m} < N[/tex]

be all the divisors of [itex]N[/itex] excluding 1 and itself.

Notice that the number of such divisors must be even and that:

[tex]
p_{1} p_{2 m} = p_{2} p_{2 m - 1} = \ldots p_{m - 1} p_{m + 1} = N
[/tex]

If [itex]N[/itex] is perfect then we also have:

[tex]
N = 1 + p_{1} + \ldots + p_{2m}
[/tex]

Divide this equality by [itex]N[/itex] and you get:

[tex]
1 = \frac{1}{N} + \frac{p_{1}}{N} + \ldots + \frac{p_{2m}}{N}
[/tex]

But, according to the upper equalities:

[tex]
\frac{p_{1}}{N} = \frac{1}{p_{2m}}, \frac{p_{2m}}{N} = \frac{1}{p_{1}}, \ldots
[/tex]

and we get:

[tex]
1 = \frac{1}{N} + \frac{1}{p_{2m}} + \ldots + \frac{1}{p_{1}}
[/tex]

Q.E.D.
 
Last edited:
  • #3
Thank you for the speedy reply, Dickfore, and thank you on the proof of why the fact holds. But, I'm more interested in the discussion of whether they are related in any way (excluding the trivial), not just proof-wise =)
 

What is a perfect number?

A perfect number is a positive integer that is equal to the sum of its proper divisors (positive divisors excluding the number itself). For example, 6 is a perfect number because its proper divisors are 1, 2, and 3, and 1+2+3=6.

How many perfect numbers are there?

As of 2021, there are 51 known perfect numbers. It is not known if there are an infinite number of perfect numbers.

What is the significance of perfect numbers?

Perfect numbers have been studied for thousands of years and have intrigued mathematicians for their unique properties. They are also used in some areas of mathematics, such as number theory and cryptography.

What is the relationship between perfect numbers and their reciprocals?

The reciprocals of perfect numbers are also perfect numbers. For example, the reciprocal of 6 is 1/6, which is also a perfect number (1+2+3=6).

Are all perfect numbers even?

As of now, all known perfect numbers are even. It is not known if there are any odd perfect numbers, but it is highly unlikely based on current mathematical theories.

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