An Infinite Summation Involving Sinc Function

Calculus Level 5

n = 1 1 n 2 π n sin z z d z = π ( A log B π ) \sum_{n=1}^\infty \dfrac{1}{n}\int_{2\pi n} ^\infty \dfrac{\sin{z}}{z}dz = \pi(A-\log{\sqrt{B\pi}})

If the equation above holds true for positive integers A A and B B , then find A + B A+B .


The answer is 3.

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1 solution

The given integral can be written as I = n = 1 1 n 0 sin z z + 2 π n d z = n = 1 1 n 0 sin n z z + 2 π d z = n = 1 1 n m = 1 2 ( m 1 ) π 2 m π sin n z z + 2 π d z = n = 1 1 n m = 1 0 2 π sin n z z + 2 m π d z = m = 1 0 2 π 1 z + 2 m π n = 1 sin n z n d z = m = 1 0 2 π π z 2 ( z + 2 m π ) d z = π m = 1 [ ( m + 1 ) ln ( 1 + 1 m ) 1 ] = π S I=\sum_{n=1}^{\infty}\frac{1}{n}\int_{0}^{\infty}\frac{\sin z}{z+2\pi n}dz\\=\sum_{n=1}^{\infty}\frac{1}{n}\int_0^{\infty}\frac{\sin nz}{z+2\pi}dz\\=\sum_{n=1}^{\infty}\frac{1}{n}\sum_{m=1}^{\infty}\int_{2(m-1)\pi}^{2m\pi}\frac{\sin nz}{z+2\pi}dz\\=\sum_{n=1}^{\infty}\frac{1}{n}\sum_{m=1}^{\infty}\int_{0}^{2\pi}\frac{\sin nz}{z+2m\pi}dz\\=\sum_{m=1}^{\infty}\int_{0}^{2\pi}\frac{1}{z+2m\pi}\sum_{n=1}^{\infty}\frac{\sin nz}{n}dz\\=\sum_{m=1}^{\infty}\int_{0}^{2\pi}\frac{\pi-z}{2(z+2m\pi)}dz\\=\pi\sum_{m=1}^{\infty}\left[\left(m+1\right)\ln\left(1+\frac{1}{m}\right)-1\right]=\pi\cdot S Now, e S = lim M e M m = 1 M ( 1 + 1 / m ) m + 1 / 2 = lim M e M ( M + 1 ) M M + 1 M ! = lim M e M ( M + 1 ) M M + 1 M M e M 2 π M ( Using Stirling ) = 1 2 π lim M ( 1 + 1 M ) M = e 2 π e^S=\lim_{M\to \infty}e^{-M}\prod_{m=1}^M \left(1+1/m\right)^{m+1/2}\\=\lim_{M\to \infty}e^{-M}\frac{(M+1)^M\sqrt{M+1}}{M!}\\=\lim_{M\to \infty}\frac{e^{-M}(M+1)^M\sqrt{M+1}}{M^Me^{-M}\sqrt{2\pi M}}\quad (\mbox{Using Stirling})\\=\frac{1}{\sqrt{2\pi}}\lim_{M\to \infty}\left(1+\frac{1}{M}\right)^M=\frac{e}{\sqrt{2\pi}} Thus, I = π ln ( e 2 π ) = π ( 1 ln 2 π ) I=\pi\ln\left(\frac{e}{\sqrt{2\pi}}\right)=\pi(1-\ln\sqrt{2\pi}) Which gives the answer 3 \boxed{3} .

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