A limit regarding Fibonacci and Phi

Calculus Level 3

Given that

L = lim n f n ϕ n \large L=\lim_{n \to \infty} \dfrac{f_n}{\phi^n}

where f n f_n is the n n th Fibonacci number and ϕ = lim n f n + 1 f n \displaystyle\phi = \lim_{n \to \infty} \dfrac{f_{n+1}}{f_n} , find the value of L L to 4 decimal places.


The answer is 0.4472.

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1 solution

Chan Lye Lee
Dec 29, 2017

It is known that f n = 1 5 ( ( 1 + 5 2 ) n ( 1 5 2 ) n ) f_n = \frac{1}{\sqrt{5}} \left( \left(\frac{1+\sqrt{5}}{2} \right)^n -\left(\frac{1-\sqrt{5}}{2} \right)^n\right) and that ϕ = 1 + 5 2 \phi = \frac{1+\sqrt{5}}{2} .

Now L = lim n f n ϕ n = lim n 1 5 ( ( 1 + 5 2 ) n ( 1 5 2 ) n ) ( 1 + 5 2 ) n = lim n 1 5 ( 1 ( 1 5 1 + 5 ) n ) = 1 5 0.4472 \displaystyle L=\lim_{n \to \infty} \dfrac{f_n}{\phi^n} = \lim_{n \to \infty} \dfrac{\frac{1}{\sqrt{5}} \left( \left(\frac{1+\sqrt{5}}{2} \right)^n -\left(\frac{1-\sqrt{5}}{2} \right)^n\right)}{\left(\frac{1+\sqrt{5}}{2}\right)^n} = \lim_{n \to \infty} \frac{1}{\sqrt{5}} \left(1- \left( \frac{1-\sqrt{5}}{1+\sqrt{5}}\right)^n \right) = \frac{1}{\sqrt{5}} \approx 0.4472 .

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