Tweaking the Fibonacci Sequence!

Calculus Level 5

Let k ( 0 , 1 ) k \in (0,1) , and let the sequence ( B n ) n = 0 (B_n)_{n=0}^{\infty} be defined by:

B 0 = k , B 1 = k 2 B_0 = k, B_1 = k^2 and B n + 2 = k B n + 1 + k 2 B n B_{n+2} = kB_{n+1} + k^2B_n for integers n 0 n \geq 0 .

If the value of the expression n = 0 B n n + 1 \large{\displaystyle \sum_{n=0}^{\infty} \dfrac{B_n}{n+1}} can be expressed as:

1 γ ln ( 1 β k 1 α k ) \large{\dfrac1{\sqrt{\gamma}} \ln \left(\dfrac{1-\beta k}{1-\alpha k}\right)}

where γ Z ; α , β R \gamma \in \mathbb Z; \alpha, \beta \in \mathbb R and α > 0 , β < 0 \alpha >0, \beta<0 . Find the value of α + β + γ \large{\lfloor \alpha + \beta + \gamma \rfloor} .


The answer is 6.

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1 solution

Abhishek Sinha
Jul 31, 2015

It is clear that B n = k n + 1 F n B_n=k^{n+1}F_n (check by direct substitution!), where F n F_n is the n n th Fibonacci number. Now it is well-known that the Generating function of the Fibonacci series is given by : n = 0 F n z n = 1 1 z z 2 \sum_{n=0}^\infty F_nz^n=\frac{1}{1-z-z^2} within the radius of convergence z < 1 ϕ |z|<\frac{1}{\phi} . Hence for all k z < 1 ϕ |kz|< \frac{1}{\phi} , we have : n = 0 B n z n = k n = 0 F n ( k z ) n = k 1 k z k 2 z 2 \sum_{n=0}^\infty B_nz^n=k\sum_{n=0}^\infty F_n(kz)^n=\frac{k}{1-kz-k^2z^2} Take k < 1 ϕ k < \frac{1}{\phi} and hence the above series is uniformly convergent in the interval ( 0 , 1 ) (0,1) . Thus we can integrate both sides of the power series to yield n = 0 B n n + 1 = 0 k 1 1 x x 2 d x = 1 5 log ( 1 + k ( 5 1 ) / 2 1 k ( 5 + 1 ) / 2 ) \sum_{n=0}^\infty \frac{B_n}{n+1}= \int_{0}^{k} \frac{1}{1-x-x^2} dx=\frac{1}{\sqrt{5}}\log \bigg(\frac{1+k(\sqrt{5}-1)/2}{1-k(\sqrt{5}+1)/2}\bigg) which yields the result.

Note : I doubt whether the series is convergent for k > 1 ϕ k>\frac{1}{\phi} . Hence the statement is the problem needs to be modified by restricting 0 < k < 1 ϕ 0<k<\frac{1}{\phi} , where ϕ \phi is the Golden ratio.

We can also show the solution of recurrence relation by "formally finding linear recurrence solutions" by using characteristic equation method.

Kartik Sharma - 5 years, 10 months ago

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Yup! Post your solution!

Satyajit Mohanty - 5 years, 10 months ago

This is actually a much easier method.

Jake Lai - 5 years, 10 months ago

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Checking by direct substitution and using generalized formulas is not an easy method, at least for me. You never know what problem you're getting tomorrow(not a fibonacci maybe).

Kartik Sharma - 5 years, 10 months ago

Well, actually the series converges if: ϕ k < ϕ \large{-\phi \leq k < \phi} .

We can show B n = k n + 1 F n B_{n} = k^{n+1}F_n by induction as well.

Satyajit Mohanty - 5 years, 10 months ago

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Yes, then you should change the statement to ϕ < k < ϕ -\phi<k<\phi .

Abhishek Sinha - 5 years, 10 months ago

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Well, I can. But it doesn't matter as (0,1) lies within the valid range.

Satyajit Mohanty - 5 years, 10 months ago

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@Satyajit Mohanty I think you have a confusion here. From the ratio test, the seies converges if k < 1 / ϕ |k|<1/\phi , which definitely does not contain ( 0 , 1 ) (0,1) . Check again.

Abhishek Sinha - 5 years, 10 months ago

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