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Calculus Level 5

0 x ln ( 2 π x ) e 2 π x 1 d x \int_{0}^{\infty}\dfrac{x\ln(2\pi x)}{e^{2\pi x}-1}dx The above integral has the following closed form: ln ( α π ) + ϵ β ϵ α ln ( A ) \dfrac{\ln(\alpha\pi)+\epsilon}{\beta}-\dfrac{\epsilon}{\alpha}\ln(A) Here, α , β , ϵ \alpha, \beta,\epsilon are positive integers and all fractions are irreducible with prime α \alpha then find α + β + ϵ \alpha+\beta+\epsilon .

Notation: A 1.282427 A \approx 1.282427 denotes the Glaisher-Kinkelin constant .


The answer is 27.

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1 solution

Kunal Gupta
Nov 30, 2016

Consider: I ( a ) = 0 x a e 2 π x 1 d x I(a)=\int_{0}^{\infty}\dfrac{x^{a}}{e^{2\pi x}-1}dx Rewrite it as: I ( a ) = 0 x a e 2 π x 1 e 2 π x d x I(a)=\int_{0}^{\infty}\dfrac{x^{a}e^{-2\pi x}}{1-e^{-2\pi x}}dx Using the Geometric Infinite Series and setting t = 2 π x t = 2\pi x : I ( a ) = 1 ( 2 π ) a + 1 r = 1 1 r a + 1 0 t a e t d t I(a) =\dfrac{1}{(2\pi)^{a+1}}\sum_{r=1}^{\infty}\dfrac{1}{r^{a+1}}\int_{0}^{\infty}t^{a}e^{-t}dt which is by definitions of Zeta and Gamma Functions: I ( a ) = Γ ( a + 1 ) ζ ( a + 1 ) ( 2 π ) a + 1 I(a) =\dfrac{\Gamma(a+1)\zeta(a+1)}{(2\pi)^{a+1}} Now, the required integral can be split up as: ln ( 2 π ) I ( 1 ) + I ( 1 ) \ln(2\pi)I(1) + I'(1) Using the fact that ζ ( 1 ) = 1 12 ln ( A ) \zeta'(-1)=\dfrac{1}{12}-\ln(A) ---------------(1)
we can get the integral easily to be ( ln ( 2 π ) + 1 ) 24 1 2 ln ( A ) \dfrac{(\ln(2\pi)+1)}{24}-\dfrac{1}{2}\ln(A) Note that the solution is not complete and the derivation of (1) is needed

@Mark Hennings got any proof of (1) in the solution??

Kunal Gupta - 4 years, 6 months ago

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Try this .

Mark Hennings - 4 years, 6 months ago

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Done! btw quite a lot went over my head

Kunal Gupta - 4 years, 6 months ago

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@Kunal Gupta Also try this

Kunal Gupta - 4 years, 6 months ago

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