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Math Help - Definite integral in closed form?

  1. #1
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    Definite integral in closed form?

    Hi,
    I'd like to compute the following integral in closed form:

    \int_0^\infty \frac{\log(1+x) e^{-x}}{1+x} dx

    I assume this is possible via complex analysis, but I can't figure out how to extend this to a complex integration problem. Any ideas?
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  2. #2
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    Re: Definite integral in closed form?

    Quote Originally Posted by jens View Post
    Hi,
    I'd like to compute the following integral in closed form:

    \int_0^\infty \frac{\log(1+x) e^{-x}}{1+x} dx

    I assume this is possible via complex analysis, but I can't figure out how to extend this to a complex integration problem. Any ideas?
    Maybe try integration by parts with \displaystyle \begin{align*} u = e^{-x} \end{align*} and \displaystyle \begin{align*} dv = \frac{\log{(1 + x)}}{1 + x} \end{align*}...
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  3. #3
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    Re: Definite integral in closed form?

    Quote Originally Posted by Prove It View Post
    Maybe try integration by parts with \displaystyle \begin{align*} u = e^{-x} \end{align*} and \displaystyle \begin{align*} dv = \frac{\log{(1 + x)}}{1 + x} \end{align*}...
    I've tried that already, but it's not conclusive. Whatever I try with integration by parts, I only manage to express the above integral in terms of a similar integral, but with the logarithm squared, or the denominator squared. So it's making things only more difficult instead of simpler.

    By the way, I happened to find the exact value of this integral by playing around with WolframAlpha and doing some extra algebra. It reads as follows:

    e \left[ \frac{1}{12} (\pi^2+6 \gamma^2) - \sideset{_3}{_3}F (1,1,1;2,2,2|-1)\right]

    = e \left[ \frac{1}{12} (\pi^2+6 \gamma^2) - \sum_{k=0}^\infty \frac{(-1)^k}{k!\;k^3(k+1)^3}\right]

    and is in agreement with the numerical computation.

    But that doesn't tell me the steps to get to this resultů
    Last edited by jens; April 26th 2012 at 10:38 AM.
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