Strange Radical

Calculus Level 5

T ( a ) = a F 1 a F 2 a F 3 , a > 0 \Large\mathfrak T(\color{#3D99F6}{a})=\sqrt{\dfrac{\color{#3D99F6}{a}^{F_1}}{\sqrt{\dfrac{\color{#3D99F6}{a}^{F_2}}{\sqrt{\dfrac{\color{#3D99F6}{a}^{F_3}}{\sqrt{\cdots}}}}}}}~~,\color{#3D99F6}{a}>0

where F n F_n denotes the n th n^\text{th} Fibonacci number .

Then T ( 4 ) = α β γ \large\mathfrak T(\color{#3D99F6}{4})=\alpha^{\frac{\beta}{\gamma}} ,where α \alpha is a prime and β \beta and γ \gamma are co prime positive integers. Then:

α + β + γ = ? \Large \alpha+\beta+\gamma=\ ?


The answer is 11.

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2 solutions

Rishabh Jain
Apr 15, 2016

T ( a ) = a F 1 2 F 2 2 2 + F 3 2 3 + P \Large\mathfrak T(\color{#3D99F6}{a})=\color{#3D99F6}{a}^{\overbrace{\frac{F_1}{2}-\frac{F_2}{2^2}+\frac{F_3}{2^3}+\cdots}^{\color{#D61F06}{\mathfrak{\Large P}}}}

P = 1 2 1 2 2 + 2 2 3 3 2 4 + P 2 = 1 2 2 + 1 2 3 2 2 4 + \begin{aligned}\color{#D61F06}{\mathfrak{\large P}}=&\dfrac{1}{2}-\dfrac{1}{2^2}+\dfrac{2}{2^3}-\dfrac{3}{2^4}+\cdots\\-\dfrac{\color{#D61F06}{\mathfrak{\large P}}}{2}=&~~~-\dfrac{1}{2^2}+\dfrac{1}{2^3}-\dfrac{2}{2^4}+\cdots\end{aligned}

Subtracting and multiplying by 2 2 we get:

3 P = 1 + 1 2 2 1 2 3 + 2 2 4 + P 2 3\color{#D61F06}{\mathfrak{\large P}}=1+\underbrace{\color{#D61F06}{\dfrac{1}{2^2}-\dfrac{1}{2^3}+\dfrac{2}{2^4}+\cdots}}_{\dfrac{\color{#D61F06}{\mathfrak{\large P}}}{2}} P = 2 5 \Large\implies\color{#D61F06}{\mathfrak{\large P}}=\dfrac 25

T ( a ) = a 2 5 \Large\implies \mathfrak T(\color{#3D99F6}{a})=\color{#3D99F6}{a}^{\frac 25} T ( 4 ) = 4 2 5 = 2 4 5 \Large \mathfrak T(\color{#3D99F6}{4})=4^{\frac 25}=2^{\frac 45}

2 + 4 + 5 = 11 \huge \therefore 2+4+5=\boxed{11}

To be completely rigorous, one needs to explain why P \mathfrak{P} converges ;) (Inspired by @Abhishek Sinha )

Otto Bretscher - 5 years, 1 month ago

. Did the same solution again. I think you'll get bored seeing this comment

Vignesh S - 5 years, 2 months ago

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No no .... Great... :-)

Rishabh Jain - 5 years, 2 months ago

Exactly the same way

Aakash Khandelwal - 5 years, 2 months ago

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Cool... BTW Someone is down voting all our comments secretly.. :-)

Rishabh Jain - 5 years, 1 month ago
Otto Bretscher
Apr 16, 2016

The generating function of F n F_n is G ( x ) = n = 1 F n x n = x 1 x x 2 G(x)=\sum_{n=1}^{\infty}F_nx^n=\frac{x}{1-x-x^2} , and n = 1 ( 1 ) n + 1 F n 2 n \sum_{n=1}^{\infty}(-1)^{n+1}\frac{F_n}{2^n} = n = 1 F n ( 1 2 ) n = G ( 1 2 ) = 2 5 =-\sum_{n=1}^{\infty}F_n(-\frac{1}{2})^n=-G\left(-\frac{1}{2}\right)=\frac{2}{5} . Thus T ( 4 ) = 4 2 / 5 = 2 4 / 5 \mathfrak{T}(4)=4^{2/5}=2^{4/5} . The answer is 2 + 4 + 5 = 11 2+4+5=\boxed{11} .

To be completely rigorous, one needs to state under what condition the generating function is valid. In other words, what is the region of convergence of G ( x ) G(x) ? Does 1 / 2 -1/2 fall into it ?

Abhishek Sinha - 5 years, 1 month ago

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Sure! The roots of x 2 + x 1 = 0 x^2+x-1=0 are ϕ , ϕ 1 -\phi,\phi^{-1} , so that the radius of convergence is ϕ 1 > 1 2 . \phi^{-1}>\frac{1}{2}.

Otto Bretscher - 5 years, 1 month ago

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