Harmonic Fibonacci Series

Calculus Level 5

n = 1 H n F n 2 n = ln ( A ) + B C coth 1 ( D E ) \large \sum_{n=1}^{\infty}\frac{H_{n}F_{n}}{2^{n}} = \ln(A) + \frac{B}{\sqrt{C}}\coth^{-1}\left (\frac{D}{\sqrt{E}} \right ) Find ( A + B + C ) 2 D E \large\frac{(A+B+C)^{2}}{DE}


The answer is 15.

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1 solution

We know that a Generating Function of the Harmonic Numbers is given by:-

n = 1 H n z n = ln ( 1 x ) 1 x \sum_{n=1}^{\infty}H_{n}z^{n} = -\frac{\ln(1-x)}{1-x}

Also we know by Binet's Formula that the nth Fibonacci Number is given by F n = φ n ( 1 φ ) n 5 \displaystyle F_{n} = \frac{{\varphi}^{n}-(1-\varphi)^{n}}{\sqrt{5}}

Where φ = 1 + 5 2 \varphi = \frac{1+\sqrt{5}}{2} is the Golden Ratio .

So we have the following summation:-

1 5 n = 1 H n ( ( φ 2 ) n ( 1 φ 2 ) n ) \displaystyle \frac{1}{\sqrt{5}}\sum_{n=1}^{\infty}H_{n}\left(\left(\frac{\varphi}{2}\right)^{n} -\left(\frac{1-\varphi}{2}\right)^{n}\right)

So using the generating function we have our summation equal to:-

1 5 ( ln ( 1 1 φ 2 ) 1 1 φ 2 ln ( 1 φ 2 ) 1 φ 2 ) \displaystyle \frac{1}{\sqrt{5}}\left(\frac{\ln(1-\frac{1-\varphi}{2})}{1-\frac{1-\varphi}{2}} - \frac{\ln(1-\frac{\varphi}{2})}{1-\frac{\varphi}{2}}\right) .

Now it is just a matter of Rearrangements. We know coth 1 ( x ) = 1 2 ln ( 1 + x 1 x ) \displaystyle \coth^{-1}(x) = \frac{1}{2}\ln\left(\frac{1+x}{1-x}\right) .

Using all of these we arrive at the answer:-

ln ( 4 ) + 6 5 coth 1 ( 3 5 ) \displaystyle \ln(4) +\dfrac{6}{\sqrt{5}}\coth^{-1}(\dfrac{3}{\sqrt{5}})

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