Fibonacci series

Calculus Level 5

n = 0 1 F 2 n + 1 + 1 = A B \large \sum_{n=0}^\infty \frac{1}{F_{2n+1}+1}=\frac{\sqrt{A}}{B}

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 .

If the equation above holds true for positive integers A A and B B , with A A square-free, find A + B A+B .


The answer is 7.

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1 solution

Mark Hennings
Jun 16, 2016

Relevant wiki: Fibonacci Sequence

If we write ϕ = 1 2 ( 5 + 1 ) \phi = \tfrac12(\sqrt{5}+1) , then (by BInet's Formula) F 2 n + 1 + 1 = 1 5 [ ϕ 2 n + 1 + 5 + ϕ 2 n 1 ] = 1 5 ( ϕ n + ϕ n ) ( ϕ n + 1 + ϕ n 1 ) F_{2n+1} + 1 \; = \; \frac{1}{\sqrt{5}}\big[\phi^{2n+1} + \sqrt{5} + \phi^{-2n-1}\big] \; = \; \frac{1}{\sqrt{5}}(\phi^n + \phi^{-n})(\phi^{n+1} + \phi^{-n-1}) and hence 1 F 2 n + 1 + 1 = 5 ( ϕ n + ϕ n ) ( ϕ n + 1 + ϕ n 1 ) = 5 ϕ 2 n + 1 ( ϕ 2 n + 1 ) ( ϕ 2 n + 2 + 1 ) = 5 ( 1 ϕ 2 n + 1 1 ϕ 2 n + 2 + 1 ) \begin{array}{rcl} \displaystyle \frac{1}{F_{2n+1}+1} & = & \displaystyle \frac{\sqrt{5}}{(\phi^n + \phi^{-n})(\phi^{n+1} + \phi^{-n-1})} \; = \; \frac{\sqrt{5} \phi^{2n+1}}{(\phi^{2n}+ 1)(\phi^{2n+2} + 1)} \\ & = & \displaystyle \sqrt{5}\left(\frac{1}{\phi^{2n}+1} - \frac{1}{\phi^{2n+2}+1}\right) \end{array} from which it is clear that n = 0 N 1 F 2 n + 1 + 1 = 5 ( 1 2 1 ϕ 2 N + 2 + 1 ) \sum_{n=0}^N \frac{1}{F_{2n+1}+1} \; = \; \sqrt{5}\left(\frac12 - \frac{1}{\phi^{2N+2}+1}\right) and hence the infinite sum is n = 0 1 F 2 n + 1 + 1 = 5 2 \sum_{n=0}^\infty \frac{1}{F_{2n+1}+1} \; = \; \frac{\sqrt{5}}{2} making the answer 5 + 2 = 7 5+2=\boxed{7} .

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