Whose integrals is it?

Calculus Level 5

k = 1 2018 0 sin 2 k ( 1 x ) d x = a π Γ ( b c ) Γ ( d ) \displaystyle \large\sum\limits_{k=1}^{2018} \int_0^{\infty} \sin^{2k} \bigg(\frac{1}{x}\bigg) \mathrm{d}x = \displaystyle a \sqrt{\pi} \cdot \dfrac{ \Gamma \big(\frac{b}{c} \big)}{\Gamma (d) }

The equation above holds true for positive integers a a , b b , c c and d d with gcd ( b , c ) = 1 \gcd(b,c) = 1 . Find a + b + c + d a+b+c+d .


Notation: Γ ( ) \Gamma(\cdot) denotes the Gamma function .


The answer is 6058.

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1 solution

Mark Hennings
May 20, 2017

Start with the integral 0 1 cos a x x 2 = 1 2 π a a > 0 \int_0^\infty \frac{1 - \cos ax}{x^2} \; = \; \tfrac12\pi a \hspace{1cm} a > 0 Using the Binomial Theorem, for any positive integer k k , sin 2 k x = ( e i x e i x 2 i ) k = ( 1 ) k 2 2 k j = 0 2 k ( 1 ) j ( 2 k j ) e 2 ( k j ) i x = 1 2 2 k 1 j = 1 k ( 1 ) j 1 ( 2 k k + j ) ( 1 cos 2 j x ) \begin{aligned} \sin^{2k}x & = \left(\frac{e^{ix} - e^{-ix}}{2i}\right)^k \; = \; \frac{(-1)^k}{2^{2k}}\sum_{j=0}^{2k} (-1)^j {2k \choose j} e^{2(k-j)ix} \\ & = \frac{1}{2^{2k-1}}\sum_{j=1}^k (-1)^{j-1}{2k \choose k+j} (1 - \cos 2jx) \end{aligned} and hence 0 sin 2 k 1 x d x = 0 sin 2 k x x 2 d x = 1 2 2 k 1 j = 1 k ( 1 ) j 1 ( 2 k k + j ) 0 1 cos 2 j x x 2 d x = π 2 2 k 1 j = 1 k ( 1 ) j 1 j ( 2 k k + j ) = π 2 2 k 1 ( 2 k 2 k 1 ) \begin{aligned} \int_0^\infty \sin^{2k}\tfrac{1}{x}\,dx & = \int_0^\infty \frac{\sin^{2k}x}{x^2}\,dx \\ & = \frac{1}{2^{2k-1}}\sum_{j=1}^k (-1)^{j-1} {2k \choose k+j} \int_0^\infty \frac{1 - \cos 2jx}{x^2}\,dx \\ & = \frac{\pi}{2^{2k-1}}\sum_{j=1}^k (-1)^{j-1} j {2k \choose k+j} \; = \; \frac{\pi}{2^{2k-1}}{2k-2 \choose k-1} \end{aligned} and hence k = 1 N + 1 0 sin 2 k 1 x d x = π k = 0 N 1 2 2 k + 1 ( 2 k k ) = π ( 2 N + 1 ) ! ! 2 N + 1 N ! = π Γ ( N + 3 2 ) Γ ( N + 1 ) \begin{aligned} \sum_{k=1}^{N+1} \int_0^\infty \sin^{2k}\tfrac{1}{x}\,dx & = \; \pi\sum_{k=0}^N \frac{1}{2^{2k+1}}{2k \choose k} \; = \; \frac{\pi (2N+1)!!}{2^{N+1}N!} \; = \frac{\sqrt{\pi}\Gamma\big(N + \frac32\big)}{\Gamma(N+1)} \end{aligned} Putting N = 2017 N=2017 gives the answer π Γ ( 4037 2 ) Γ ( 2018 ) \frac{\sqrt{\pi}\Gamma\big(\tfrac{4037}{2}\big)}{\Gamma(2018)} and hence the answer is 1 + 4037 + 2 + 2018 = 6058 1 + 4037 + 2 + 2018 = \boxed{6058}


Alternatively, we can match the original integrals with the more standard Wallis integrals as follows: 0 sin 2 k 1 x d x = 0 sin 2 k x x 2 d x = 1 2 sin 2 k x x 2 d x = 1 2 n Z 0 π sin 2 k x ( x + n π ) 2 d x = 1 2 0 π sin 2 k x c o s e c 2 x d x = 1 2 0 π sin 2 k 2 x d x = 0 1 2 π sin 2 k 2 x d x = π 2 2 k 1 ( 2 k 2 k 1 ) \begin{aligned} \int_0^\infty \sin^{2k} \tfrac{1}{x}\,dx & = \int_0^\infty \frac{\sin^{2k}x}{x^2}\,dx \; = \; \tfrac12\int_{-\infty}^\infty \frac{\sin^{2k}x}{x^2}\,dx \\ & = \tfrac12\sum_{n \in \mathbb{Z}} \int_0^\pi \frac{\sin^{2k}x}{(x + n\pi)^2}\,dx \; = \; \tfrac12 \int_0^\pi \sin^{2k}x \,\mathrm{cosec}^2x\,dx \\ & = \tfrac12\int_0^\pi \sin^{2k-2}x\,dx \; = \; \int_0^{\frac12\pi}\sin^{2k-2}x\,dx \; =\; \frac{\pi}{2^{2k-1}}{2k-2 \choose k-1} \end{aligned}

Wow. I see now why my approach at summing up the even numbered Wallis Integrals miraculously worked. Thank you for showing that!

Efren Medallo - 4 years ago

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How to prove the first integral ??

In ur 2nd line k will be 2k

Kushal Bose - 4 years ago

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One way would be to integrate e i a z 1 z 2 \frac{e^{iaz} - 1}{z^2} around a semicircular contour in the upper half place, indented at the origin to avoid the pole.

Mark Hennings - 4 years ago

Sir could you please explain how you got \begin{aligned} \int_0^\infty \sin^{2k}\tfrac{1}{x}\,dx & = \int_0^\infty \frac{\sin^{2k}x}{x^2}\,dx

senna s - 3 years, 12 months ago

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Sin^(2k)(1/x) as( (sin^(2k)(x))/x^(2))

senna s - 3 years, 12 months ago

Use the substitution x x 1 x \mapsto x^{-1} .

Mark Hennings - 3 years, 12 months ago

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