Bino Harmonic Series 2

Calculus Level 5

S = n = 0 [ ( 2 n n ) 2 H n 2 5 n ] \text{S} = \displaystyle \sum_{n=0}^\infty \left[\binom{2n}{n}^2\frac{ H_n}{2^{5n}} \right]

S \text{S} can be represented as

π A B Γ 2 ( C D ) ( E π F log G ) \dfrac{\sqrt{\pi^{\text{A}}}}{\text{B} \ \Gamma^2\left(\frac{\text{C}}{\text{D}} \right)}(\text{E}\pi - \text{F}\log \text{G})

where A , B , C , D , E , F \text{A}, \text{B}, \text{C}, \text{D}, \text{E}, \text{F} and G \text{G} are positive integers, gcd ( C , D ) = gcd ( B , E ) = 1 \gcd (\text{C},\text{D}) = \gcd(\text{B},\text{E}) = 1 and G \text{G} is a prime number.

Evaluate A + B + C + D + E + F + G \text{A}+\text{B}+\text{C}+\text{D}+\text{E}+\text{F}+\text{G}


See Also : Part 1


The answer is 17.

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1 solution

Mark Hennings
Aug 4, 2016

The complete elliptic function of the first kind (under one convention) is K ( k ) = 0 1 2 π d θ 1 k 2 sin 2 θ = 1 2 π n = 0 ( 2 n n ) 2 k 2 n 2 4 n K(k) \; = \; \int_0^{\frac12\pi} \frac{d\theta}{\sqrt{1-k^2\sin^2\theta}} \; = \; \tfrac12\pi \sum_{n=0}^\infty {2n \choose n}^2 \frac{k^{2n}}{2^{4n}} so that n = 0 ( 2 n n ) 2 k n 2 5 n = 2 π K ( k 2 ) \sum_{n=0}^\infty {2n \choose n}^2 \frac{k^n}{2^{5n}} \; = \; \tfrac{2}{\pi}K\left(\sqrt{\tfrac{k}{2}}\right) Thus, if we write S = n = 1 ( 2 n n ) 2 H n 2 5 n S \; = \; \sum_{n=1}^\infty {2n \choose n}^2 \frac{H_n}{2^{5n}} then S = n = 1 ( 2 n n ) 2 2 5 n 0 1 1 k n 1 k d k = 2 π 0 1 K ( 1 2 ) K ( k 2 ) 1 k d k = 2 π 0 1 d k 1 k 0 1 2 π { 1 1 1 2 sin 2 θ 1 1 1 2 k sin 2 θ } d θ = 1 π 0 1 d k 0 1 2 π sin 2 θ d θ ( 1 1 2 sin 2 θ ) ( 1 1 2 k sin 2 θ ) ( 1 1 2 sin 2 θ + 1 1 2 k sin 2 θ ) = 1 π 0 1 2 π sin 2 θ d θ 1 1 2 sin 2 θ ( 0 1 d k 1 1 2 k sin 2 θ ( 1 1 2 sin 2 θ + 1 1 2 k sin 2 θ ) ) = 2 π 0 1 2 π d θ 1 1 2 sin 2 θ { 2 ln ( 1 + 1 1 2 sin 2 θ ) 2 ln 2 ln ( 1 1 2 sin 2 θ ) } \begin{array}{rcl} \displaystyle S & = & \displaystyle \sum_{n=1}^\infty {2n \choose n}^2 2^{-5n} \int_0^1 \frac{1-k^n}{1-k}\,dk \; = \; \frac{2}{\pi}\int_0^1 \frac{K\left(\sqrt{\frac12}\right) - K\left(\sqrt{\frac{k}{2}}\right)}{1-k}\,dk \\ & = & \displaystyle \frac{2}{\pi} \int_0^1 \frac{dk}{1-k}\int_0^{\frac12\pi} \left\{ \frac{1}{\sqrt{1 - \frac12\sin^2\theta}} - \frac{1}{\sqrt{1 - \frac12k\sin^2\theta}}\right\}\,d\theta \\ & = & \displaystyle \frac{1}{\pi}\int_0^1\,dk \int_0^{\frac12\pi} \frac{\sin^2\theta\,d\theta}{\sqrt{(1-\frac12\sin^2\theta)(1 - \frac12k\sin^2\theta)}\left(\sqrt{1 - \frac12\sin^2\theta} + \sqrt{1 - \frac12k\sin^2\theta}\right)} \\ & = & \displaystyle \frac{1}{\pi} \int_0^{\frac12\pi} \frac{\sin^2\theta\,d\theta}{\sqrt{1-\frac12\sin^2\theta}} \left(\int_0^1 \frac{dk}{\sqrt{1 - \frac12k\sin^2\theta}\left(\sqrt{1-\frac12\sin^2\theta} + \sqrt{1-\frac12k\sin^2\theta}\right)}\right) \\ & = & \displaystyle \frac{2}{\pi} \int_0^{\frac12\pi} \frac{d\theta}{\sqrt{1-\frac12\sin^2\theta}}\left\{ 2\ln\left(1 + \sqrt{1 - \tfrac12\sin^2\theta}\right) - 2\ln2 - \ln\left(1 -\tfrac12\sin^2\theta\right)\right\} \end{array} Since 1 2 \tfrac{1}{\sqrt{2}} is equal to its own complement, we have (Gradshteyn and Rhyzik) 0 1 2 π ln ( 1 + 1 1 2 sin 2 θ ) 1 1 2 sin 2 θ d θ = 1 4 ( π ln 2 ) K ( 1 2 ) \int_0^{\frac12\pi} \frac{\ln\left(1 + \sqrt{1 - \tfrac12\sin^2\theta}\right)}{\sqrt{1 - \frac12\sin^2\theta}}\,d\theta \; = \; \tfrac14(\pi - \ln2)K\big(\tfrac{1}{\sqrt{2}}\big) while this paper tells us that 0 1 2 π ln ( 1 1 2 sin 2 θ ) 1 1 2 sin 2 θ d θ = 1 2 ln 2 K ( 1 2 ) \int_0^{\frac12\pi} \frac{\ln\left(1 - \frac12\sin^2\theta\right)}{\sqrt{1 - \frac12\sin^2\theta}}\,d\theta \; = \; -\tfrac12\ln2 K\big(\tfrac{1}{\sqrt{2}}\big) (on further looking, this integral is in G&R as well) and hence S = 2 π [ 2 × 1 4 ( π ln 2 ) K ( 1 2 ) 2 ln 2 K ( 1 2 ) + 1 2 ln 2 K ( 1 2 ) ] = 1 π K ( 1 2 ) ( π 4 ln 2 ) = π 2 Γ ( 3 4 ) 2 ( π 4 ln 2 ) \begin{array}{rcl} \displaystyle S & = & \displaystyle \tfrac{2}{\pi}\left[ 2\times\tfrac14(\pi - \ln2)K\big(\tfrac{1}{\sqrt{2}}\big) - 2\ln2 K\big(\tfrac{1}{\sqrt{2}}\big) + \tfrac12\ln2 K\big(\tfrac{1}{\sqrt{2}}\big) \right] \\ & = & \displaystyle \tfrac{1}{\pi}K\big(\tfrac{1}{\sqrt{2}}\big)\big(\pi - 4\ln2\big) \; = \; \frac{\sqrt{\pi}}{2\Gamma\left(\frac34\right)^2}\big(\pi - 4\ln2\big) \end{array} making the answer 1 + 2 + 3 + 4 + 1 + 4 + 2 = 17 1+2+3+4+1+4+2 = \boxed{17} .

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