WE MUST BE PERFECT!

Algebra Level 2

True or False (or Ambiguous)?

The sum of all the reciprocals of the divisors of a perfect number is ALWAYS 2

False True Ambiguous

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2 solutions

Chris Sapiano
Jun 8, 2019

Call the perfect number x x

Factors of x x are F 1 , F 2 , F 3 F n F_1 , F_2 , F_3 … F_n

i = 1 n F n = 2 x \sum_{i=1}^n F_n = 2x

1 F 1 + 1 F 2 + 1 F n = x F 1 + x F 2 + x F n x \frac{1}{F_1} + \frac{1}{F_2} … + \frac{1}{F_n} = \frac{\frac{x}{F_1} + \frac{x}{F_2} … + \frac{x}{F_n}}{x}

Notice that because x x is a perfect number, x F 1 + x F 2 + x F n \frac{x}{F_1} + \frac{x}{F_2} … + \frac{x}{F_n} is equal to i = 1 n F n = 2 x \sum_{i=1}^n F_n = 2x

2 x x = 2 \frac{2x}{x} = \boxed{2}

Mohammad Farhat
Nov 2, 2018

It's true since,

n + + c + b + a n = 2 n n a + n b + = 2 n 1 a + 1 b + = 2 \frac{n + \dots + c + b + a}{n} = 2n \implies \frac{n}{a} + \frac{n}{b} + \dots = 2n \implies \frac{1}{a} + \frac{1}{b} + \dots = 2

I don't follow your argument... Are a , b , c , a,b,c,\ldots the divisors of n n ? If so, why are you dividing by n n in the first equation? And I don't see how the second equation follows from the first.

Jordan Cahn - 2 years, 7 months ago

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