Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Definite integral from 0 to 1 of : ln(x)ln(1-x)dx

  1. Aug 15, 2006 #1
    I would really like to post the work I did, but it is gibberish !
    I don't know how to tackle this integral :
    definite integral from 0 to 1 of : ln(x)ln(1-x)dx

    The "traditionnal" methods don't work but I assure you that I have tried much more !

    Please help
     
  2. jcsd
  3. Aug 16, 2006 #2

    benorin

    User Avatar
    Homework Helper

    That integral is, on cursorary examination, sorta nasty

    a computer told me that

    [tex]\int_{0}^{1}\log x\log (1-x) \, dx=2-\frac{\pi ^2}{6}[/tex]

    so I expect some quick manipulation to produce the Basel problem (proving that [itex]\sum_{k=1}^{\infty}\frac{1}{k^2}=\frac{\pi ^2}{6}[/itex],) a computer also told me that

    [tex]\int\log x\log (1-x) \, dx = (x\log x-x)\log (1-x)-x\log x+2x+\mbox{Li}_2 (x)+\log (-1+x)[/tex]

    where [tex]\mbox{Li}_2 (x)=\sum_{k=1}^{\infty}\frac{x^k}{k^2}[/tex] is the dilogarithm. The dilog gets us to the Basel problem since [tex]\mbox{Li}_2 (1)=\frac{\pi^2}{6}[/tex] and hey, there's even a handy formula for the dilog, namely

    [tex]\mbox{Li}_2 (t)=-\int_{0}^{t}\frac{\log (1-x)}{x}dx[/tex]

    and I'd conjecture that you can pull this off with integration by parts...

    -Ben
     
  4. Aug 16, 2006 #3
    We have
    [tex]I = \int_0^1 \ln x \ln (1-x) dx = \lim_{a\rightarrow 1} \int_0^a \ln x \ln (1-x) dx[/tex].

    Integrating by part

    [tex]I= \lim_{a \rightarrow 1}( - \ln (1-a) + \int_0^a \frac{x \ln x}{1-x} - \int_0^a \frac{x}{1-x})[/tex].

    By performing such a limit we have

    [tex]I= 2 + \int_0^1 \frac{\ln x}{1-x} dx [/tex].

    Now from the substitution [itex]x = e^{-u}[/itex] and by remembering the integral expression for the [itex]\zeta[/itex] function:

    [tex]\zeta(s)= \frac{1}{\Gamma (s)} \int_0^\infty \frac{x^{s-1}}{e^x -1 } d x[/tex]

    we get

    [tex]I= 2 - \zeta (2) = 2 - \frac{\pi^2}{6}[/tex].
     
    Last edited: Aug 16, 2006
  5. Aug 16, 2006 #4

    benorin

    User Avatar
    Homework Helper

    in [tex]\int_{0}^{1}\log x\log (1-x) \, dx[/tex] do integration by parts with [tex]u=\log x[/tex] so that [tex]du=\frac{dx}{x}[/tex] and [tex]dv=\log (1-x) dx[/tex] so that [tex]v=(1-x)\left( 1-\log (1-x)\right) [/tex] to get

    [tex]\int_{0}^{1}\log x\log (1-x) \, dx = \left[ (1-x)\log x\left( 1-\log (1-x)\right) \right]_{x=0}^{1}- \int_{0}^{1}(1-x)\left( 1-\log (1-x)\right)\frac{dx}{x}[/tex]
    [tex]=\left[ (1-x)\log x\left( 1-\log (1-x)\right) \right]_{x=0}^{1}- \int_{0}^{1}\left[ \frac{1}{x}-1-\frac{\log (1-x)}{x}+\log (1-x) \right] \, dx[/tex]
    [tex]=\left[ (1-x)\log x\left( 1-\log (1-x)\right) -\log x +x -(1-x)\left( 1-\log (1-x)\right) \right]_{x=0}^{1}-\mbox{Li}_2 (1)[/tex]

    now it is a matter of taking limits at x=0 and at x=1 and putting in the value [tex]\mbox{Li}_2(1)=\frac{\pi^2}{6}[/tex]
     
  6. Aug 17, 2006 #5

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    [tex] I=\lim_{b\rightarrow 1,a\rightarrow 0}\int_{a}^{b}\ln x\ln \left( 1-x\right) dx=...?[/tex]

    One knows that

    [tex] \ln \left( 1-x\right) =-\sum_{k=1}^{\infty }\frac{x^{k}}{k}.[/tex]

    for x smaller than 1. So

    [tex]I=\lim_{b\rightarrow 1,a\rightarrow 0}\int_{b}^{a}\ln x\left( \sum_{k=1}^{\infty }\frac{x^{k}}{k}\right) dx=\sum_{k=1}^{\infty }\frac{1}{k}\left(\lim_{b\rightarrow 1,a\rightarrow 0}\int_{b}^{a}x^{k}\ln xdx\right) =...=\sum_{k=1}^{\infty }\frac{1}{k\left( k+1\right) ^{2}}. [/tex]

    To evaluate the last sum, use that

    [tex] 1=k+1-k [/tex]

    and the definition of the Riemann zeta function. U'll easily get [itex] 2-\zeta(2) [/itex] you were supposed to get.

    Daniel.

    P.S. Don't worry about pulling sum out of the integral sign and passing sum symbol over the limit sign. It usually works.
     
    Last edited: Aug 17, 2006
  7. Aug 17, 2006 #6

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    To get a better grip of what i did, try to show that

    [tex]\int_{0}^{\infty} \frac{x}{e^{x}+1} \ dx =\eta(2) =\frac{\pi^{2}}{12}[/tex]

    , where \eta is Dirichlet's eta function.

    Use the integral above to compute the integrals on page 154 from Ch.Kittel's "Introduction to Solid State Physics", 7-th edition.

    Daniel.
     
    Last edited: Aug 17, 2006
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Definite integral from 0 to 1 of : ln(x)ln(1-x)dx
  1. Integral of 1/ln(x) (Replies: 25)

Loading...