Does this really have a closed form?

Algebra Level 4

22 n = 0 F n 4 n \large 22 \sum_{n=0}^\infty \dfrac{F_n}{4^n} Let F n F_n denote the n th n^\text{th} Fibonacci number , where F 0 = 0 , F 1 = 1 F_0 = 0, F_1 = 1 and F n = F n 1 + F n 2 F_n = F_{n-1} + F_{n-2} for n = 2 , 3 , 4 , n=2,3,4,\ldots .

Compute the expression above.


The answer is 8.

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1 solution

Rohit Udaiwal
May 11, 2016

Let S \mathbf{S} denote n = 0 F n 4 n \displaystyle \sum_{n=0}^{\infty} \frac{F_{n}}{4^{n}} .Then we have S = n = 0 F n 4 n = n = 1 F n 4 n [ F 0 = 0 ] = 1 4 + 1 4 2 + 2 4 3 + 3 4 4 + 5 4 5 + 8 4 6 + ( 1 ) S 4 = 1 4 2 + 1 4 3 + 2 4 4 + 3 4 5 + 5 4 6 + 8 4 7 + ( 2 ) 3 S 4 = 1 4 + 1 4 3 + 1 4 4 + 2 4 5 + 3 4 6 + 5 4 7 + [ Subtracting ( 2 ) from ( 1 ) ] 3 S 4 = 1 4 + S 16 S = 4 11 22 S = 8 \begin{aligned} \mathbf{S} & =\displaystyle \sum_{n=0}^{\infty} \frac{F_{n}}{4^{n}}=\sum_{n=1}^{\infty} \frac{F_{n}}{4^{n}} \quad \quad[\because F_{0}=0] \\ & =\frac{1}{4}+\frac{1}{4^2}+\frac{2}{4^3}+\frac{3}{4^4}+\frac{5}{4^5}+\frac{8}{4^6} +\ldots\quad \quad & \ldots(1) \\ \implies \frac{\mathbf{S}}{4} & =\frac{1}{4^2}+\frac{1}{4^3}+\frac{2}{4^4}+\frac{3}{4^5}+\frac{5}{4^6}+\frac{8}{4^7}+\ldots \quad \quad & \ldots(2) \\ \implies \frac{3\mathbf{S}}{4} & =\frac{1}{4}+\frac{1}{4^3}+\frac{1}{4^4}+\frac{2}{4^5} +\frac{3}{4^6}+\frac{5}{4^7}+\ldots \quad[\small{\text{Subtracting}~ (2)~\text{from} ~(1)}] \\ \implies \frac{3\mathbf{S}}{4} & =\frac{1}{4}+\frac{\mathbf{S}}{16} \\ \implies \mathbf{S} & =\frac{4}{11} \\ \therefore 22\mathbf{S} & =\boxed{8} \end{aligned}

You fail to show that the series converges.

Otto Bretscher - 5 years, 1 month ago

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We can simply prove it by ratio test. First we find lim n F n + 1 F n \displaystyle \lim _{ n\rightarrow \infty }{ \frac { { F }_{ n+1 } }{ { F }_{ n } } } .

We can find it by using Binet's formula:

lim n F n + 1 F n = lim n ϕ n + 1 ( 1 ϕ ) n + 1 ϕ n ( 1 ϕ ) n = 1 ϕ < 1 \lim _{ n\rightarrow \infty }{ \frac { { F }_{ n+1 } }{ { F }_{ n } } } =\lim _{ n\rightarrow \infty }{ \frac { { \phi }^{ n+1 }-{ \left( \frac { -1 }{ \phi } \right) }^{ n+1 } }{ { \phi }^{ n }-{ \left( \frac { -1 }{ \phi } \right) }^{ n } } } =\frac { 1 }{ \phi } <1

Now coming back to the series, we'll do the ratio test. lim n F n + 1 F n . 1 4 = 1 ϕ . 1 4 < 1 \displaystyle \lim _{ n\rightarrow \infty }{ \frac { { F }_{ n+1 } }{ { F }_{ n } } .\frac { 1 }{ 4 } } =\frac { 1 }{ \phi } .\frac { 1 }{ 4 } <1

Hence, the series converges.

Aditya Kumar - 5 years, 1 month ago

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Exactly! Or we can use the generating function, as I did here , making it a one-line solution.

Otto Bretscher - 5 years, 1 month ago

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