A little thought

Calculus Level 5

( n = 1 φ ( n ) ( 1 ϕ ) n F n ) 2 = ? \large \left( \sum_{n=1}^{\infty} \frac{\varphi(n)(1-\phi)^{n}}{F_{n}} \right)^{-2} = \ ?

Note :

  • φ \varphi is the Euler totient function.
  • ϕ = 1 + 5 2 \phi = \frac{1+\sqrt{5}}{2} .
  • F n F_{n} is the n n -th Fibonacci number.


The answer is 5.

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1 solution

Isaac Buckley
Aug 7, 2015

For this we need to know two things:

Lambert series

Binets formula

Using binets formula we know F n = ϕ n ( 1 ϕ ) n 5 F_n=\frac{\phi^n-(1-\phi)^n}{\sqrt{5}}

S = n = 1 φ ( n ) ( 1 ϕ ) n F n = 5 n = 1 φ ( n ) ( 1 ϕ ) n ϕ n ( 1 ϕ ) n = 5 n = 1 φ ( n ) ( 1 ϕ ϕ ) n 1 ( 1 ϕ ϕ ) n S= \sum_{n=1}^{\infty} \frac{\varphi(n)(1-\phi)^{n}}{F_{n}}= \sqrt{5} \sum_{n=1}^{\infty} \frac{\varphi(n)(1-\phi)^{n}}{\phi^n-(1-\phi)^n}= \sqrt{5}\sum_{n=1}^{\infty} \frac{\varphi(n)\left(\frac{1-\phi}{\phi}\right)^{n}}{1-\left(\frac{1-\phi}{\phi}\right)^{n}}

Now we know that the lambert series for φ ( x ) φ(x) where q < 1 |q|<1 is n = 1 φ ( n ) q n 1 q n = q ( 1 q ) 2 \sum_{n=1}^{\infty}\frac{φ(n)q^n}{1-q^n}=\frac{q}{(1-q)^2}

Note 1 ϕ ϕ < 1 |\frac{1-\phi}{\phi}|<1

S = 5 1 ϕ ϕ ( 1 1 ϕ ϕ ) 2 = 5 ϕ ( 1 ϕ ) 4 ϕ 2 4 ϕ + 1 = 5 5 \therefore S=\sqrt{5}\frac{\frac{1-\phi}{\phi}}{\left(1-\frac{1-\phi}{\phi}\right)^2}=\sqrt{5}\frac{\phi(1-\phi)}{4\phi^2-4\phi+1}=-\frac{\sqrt{5}}{5}

We can then evaluate the answer to be ( 5 5 ) 2 = 5 \left(-\frac{\sqrt{5}}{5}\right)^{-2}=\boxed{5}

This is quite nice, pleasantly elegant.

Michael Mendrin - 5 years, 10 months ago

@Jake Lai Your questions have became a research project.

Julian Poon - 5 years, 10 months ago

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Go read Apostol's Analytic Number Theory. PDFs can be found online. It'll give you a lot of inspiration.

Jake Lai - 5 years, 10 months ago

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Thanks!

Julian Poon - 5 years, 10 months ago

Did the exact same! @Jake Lai Love your problems!

Kartik Sharma - 5 years, 10 months ago

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