Beautiful Constant

Calculus Level 5

P = 0 e π x sin x x d x \displaystyle \large P = \int_0 ^{\infty} e^{-\pi x}\dfrac{ \sin x}{x} dx Find the value of 100 P \lfloor 100P \rfloor .


The answer is 30.

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2 solutions

Tanishq Varshney
Jan 7, 2016

Using Laplace transformation

L ( sin x x ) = s 1 s 2 + 1 d s \large{L \left(\frac{\sin x}{x}\right)=\displaystyle \int^{\infty}_{s}\frac{1}{s^2+1}ds}

L ( sin x x ) = arctan s \large{L \left(\frac{\sin x}{x}\right)=\arctan |^{\infty}_{s}}

s = π s=\pi

L ( sin x x ) = arccot ( π ) \large{\Rightarrow L \left(\frac{\sin x}{x}\right)=\text{arccot} (\pi)}

100 a r c c o t ( π ) = 30 \large{\lfloor 100 arccot (\pi) \rfloor =30}

No need to use such tranformations.

Just use expansion of sin(x) and gamma function and few skills of integration

Aakash Khandelwal - 4 years, 10 months ago
First Last
May 29, 2017

0 e π x sin x x d x = 1 2 i 0 e ( π i ) x x e ( π + i ) x x d x \displaystyle\int_0^\infty e^{-\pi x}\frac{\sin{x}}{x}dx = \frac1{2i}\int_0^\infty \frac{e^{-(\pi-i)x}}{x}-\frac{e^{-(\pi+i)x}}{x}dx

Individually each integral cannot be evaluated like a Gaussian because Γ ( 0 ) = \Gamma(0)=\infty (below)

0 x b e a x n d x = 1 a b + 1 n n Γ ( b + 1 n ) \displaystyle\int_0^\infty x^be^{-ax^n}dx = \frac1{a^{\frac{b+1}{n}}n}\Gamma(\frac{b+1}{n})

Using Feynman's trick: I ( a ) = 0 e a x 1 x = ln a + C \displaystyle I(a) = \int_0^\infty\frac{e^{-ax}-1}{x} = -\ln{a}+C , the original integral becomes:

( ln ( π + i π i ) 2 i ) = ( ln ( π + i ) i ) = arctan 1 π \displaystyle\Re(\frac{\ln(\frac{\pi+i}{\pi-i})}{2i}) = \Re(\frac{\ln(\pi+i)}{i}) = \boxed{\arctan{\frac1{\pi}}}

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