"I am Back" says Integration Part 5

Calculus Level 5

0 x ln x e x 1 d x \large \int_{0}^{\infty}{\dfrac{x \ln x}{e^x -1} \, dx}

The integral above can be expressed as π a b ( ln ( c π ) d ln ( A ) + f ) , \dfrac{\pi^a}{b} \left( \ln(c\pi) - d\ln(A) + f \right),

where a , b , c , d , f a,b,c,d,f are positive integers.

Find a + b + c + d + f a+b+c+d+f .

Notations :

  • γ \gamma denotes the Euler-Mascheroni constant , γ = lim n ( ln n + k = 1 n 1 k ) 0.5772 \displaystyle \gamma = \lim_{n\to\infty} \left( - \ln n + \sum_{k=1}^n \dfrac1k \right) \approx 0.5772 .
  • A denotes the Glaisher-Kinkelin constant , A = lim n 1 1 2 2 n n e n 2 / 4 n n 2 / 2 + n / 2 + 1 / 12 1.2824 \displaystyle A = \lim_{n\to\infty} \dfrac{1^1 \cdot 2^2 \cdots n^n}{e^{-n^2/4} \cdot n^{n^2/2 + n/2 + 1/12}} \approx 1.2824 .

This is an original problem.


The answer is 23.

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1 solution

Mark Hennings
Feb 12, 2016

If we define F ( α ) = 0 x α e x 1 d x = 0 x α e x ( 1 e x ) 1 d x = n = 0 0 x α e ( n + 1 ) x d x = n = 0 1 ( n + 1 ) α + 1 0 y α e y d y = ζ ( α + 1 ) Γ ( α + 1 ) \begin{array}{rcl} F(\alpha) & = & \displaystyle \int_0^\infty \frac{x^\alpha}{e^x-1}\,dx \; = \; \int_0^\infty x^\alpha e^{-x}(1 - e^{-x})^{-1}\,dx \\ & = & \displaystyle \sum_{n=0}^\infty \int_0^\infty x^{\alpha} e^{-(n+1)x}\,dx \; = \; \sum_{n=0}^\infty \frac{1}{(n+1)^{\alpha+1}} \int_0^\infty y^\alpha e^{-y}\,dy \\ & = & \zeta(\alpha+1) \Gamma(\alpha+1) \end{array} for α > 1 \alpha > -1 , noting that the term-by-term integration is justified by the Monotone Convergence Theorem, then it is clear that I = 0 x ln x e x 1 d x = F ( 1 ) = ζ ( 2 ) Γ ( 2 ) + ζ ( 2 ) Γ ( 2 ) = ζ ( 2 ) + 1 6 π 2 Γ ( 2 ) = 1 6 π 2 [ γ + ln ( 2 π ) 12 ln A ] + 1 6 π 2 ( 1 γ ) = 1 6 π 2 [ γ + ln ( 2 π ) 12 ln A ( γ 1 ) ] \begin{array}{rcl} \displaystyle I \; = \; \int_0^\infty \frac{x \ln x}{e^x-1}\,dx & = & \displaystyle F'(1) \; = \; \zeta'(2)\Gamma(2) + \zeta(2)\Gamma'(2) \\ & = & \zeta'(2) + \tfrac16\pi^2\Gamma'(2) \; = \; \tfrac16\pi^2\big[\gamma + \ln(2\pi) - 12\ln A\big] + \tfrac16\pi^2(1 - \gamma) \\ & = & \tfrac16\pi^2\big[ \gamma + \ln(2\pi) - 12\ln A - (\gamma-1)\big] \end{array} so that the answer is 2 + 6 + 2 + 12 + 1 = 23 2 + 6 + 2 + 12 + 1 = \boxed{23} . It is a bit strange that the answer does not cancel the γ \gamma terms.

Yes, the format in which the answer has to be submitted is quite persplexing at first glance.

But anyways, awesome problem @Rajdeep Dhingra

Keep posting more!

Harsh Shrivastava - 5 years, 4 months ago

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I had not followed rajdeep. This question reminded to follow him :P. Nice one @Rajdeep Dhingra

Aditya Kumar - 5 years, 4 months ago

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