# Evaluation of the integral $\int_0^1 \frac{\ln(1 - x)}{1 + x}dx$

How can I evaluate the integral $$\int_0^1 \frac{\ln(1 - x)}{1 + x}dx$$ I tried manipulating the known integral $$\int_0^1 \frac{\ln(1 - x)}{x}dx = -\frac{\pi^2}{6}$$ but couldn't do anything with it.

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Wolfram alpha mentions polylogarithm $\operatorname{Li}_2(x)$. May be relevant: $\operatorname{Li}_1(x) = - \ln(1-x)$. –  user2468 Mar 6 '12 at 22:24
I tried and it doesn't seem to work. –  Martin Mar 6 '12 at 22:26

You can use double integration:

$$\int\limits_0^1 {\frac{{\log \left( {1 - x} \right)}}{{1 + x}}dx} = \int\limits_0^1 {\int\limits_0^{ - x} {\frac{{du \cdot dx}}{{\left( {1 + u} \right)\left( {1 + x} \right)}}} }$$

$$\int\limits_0^1 {\int\limits_0^x {\frac{{dm \cdot dx}}{{\left( {m - 1} \right)\left( {1 + x} \right)}}} }$$

Now make

$$m = ux$$

$$\int\limits_0^1 {\int\limits_0^1 {\frac{{x \cdot du \cdot dx}}{{\left( {ux - 1} \right)\left( {1 + x} \right)}}} } = \int\limits_0^1 {\int\limits_0^1 {\frac{{du \cdot dx}}{{\left( {ux - 1} \right)}}} } - \int\limits_0^1 {\int\limits_0^1 {\frac{{du \cdot dx}}{{\left( {ux - 1} \right)\left( {1 + x} \right)}}} }$$

We have that (partial fraction decomposition)

$$\frac{1}{ \left( ux - 1 \right)\left( x + 1 \right) } = \frac{u}{ \left( u + 1 \right)\left( ux - 1 \right) } - \frac{1}{ \left( x + 1 \right)\left( u + 1 \right) }$$

So we get

$$\int\limits_0^1 {\int\limits_0^1 {\frac{{du \cdot dx}}{{\left( {ux - 1} \right)}}} } - \int\limits_0^1 {\int\limits_0^1 {\frac{{u \cdot du \cdot dx}}{{\left( {ux - 1} \right)\left( {u + 1} \right)}}} } + \int\limits_0^1 {\int\limits_0^1 {\frac{{du \cdot dx}}{{\left( {x + 1} \right)\left( {u + 1} \right)}}} }$$

Now:

$$\int\limits_0^1 {\int\limits_0^1 {\frac{{du \cdot dx}}{{\left( {ux - 1} \right)}}} } = \int\limits_0^1 {\frac{{\log \left( {1 - u} \right)}}{u}} du = - \frac{{{\pi ^2}}}{6}$$

$$\int\limits_0^1 {\int\limits_0^1 {\frac{{du\cdot dx}}{{\left( {x + 1} \right)\left( {u + 1} \right)}}} } = {\log ^2}2$$

For our last one,note it is the integral we're looking for

$$\int\limits_0^1 {\int\limits_0^1 {\frac{{u\cdot du\cdot dx}}{{\left( {ux - 1} \right)\left( {u + 1} \right)}}} \mathop = \limits^{ux = m} } \int\limits_0^1 {\int\limits_0^u {\frac{{dm\cdot du}}{{\left( {m - 1} \right)\left( {u + 1} \right)}}} } \mathop = \limits^{m = - x} \int\limits_0^1 {\int\limits_0^{ - u} {\frac{{dx\cdot du}}{{\left( {x + 1} \right)\left( {u + 1} \right)}}} } = \int\limits_0^1 {\frac{{\log \left( {1 - u} \right)}}{{ {u + 1} }}} du$$

We get

$$\int\limits_0^1 {\frac{{\log \left( {1 - u} \right)}}{{ {u + 1} }}} du = {\log ^2}2 - \frac{{{\pi ^2}}}{6} - \int\limits_0^1 {\frac{{\log \left( {1 - u} \right)}}{{ {u + 1} }}} du$$

or

$$\int\limits_0^1 {\frac{{\log \left( {1 - u} \right)}}{{{u + 1} }}} du = \frac{{{{\log }^2}2}}{2} - \frac{{{\pi ^2}}}{{12}}$$

as desired.

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@Martin In a similar fashion, you can try and prove that $$\int\limits_0^1 {\frac{{\log \left( {{x^2} + 1} \right)}}{{x + 1}}dx = \frac{3}{4}{{\log }^2}2 - \frac{{{\pi ^2}}}{{48}}}$$ –  Pedro Tamaroff Mar 7 '12 at 2:40
I looked at it briefly and it seems right to me. Well done! –  Aryabhata Mar 7 '12 at 4:58
+1. The step which replaces the logarithm by an integral to reach a double integral to be further massaged is worth remembering. –  Did Mar 7 '12 at 6:05
This is wonderful derivation Peter. Very clever, clear and accessible to everyone. Thank you very much. –  Martin Mar 7 '12 at 6:50
Everything looks correct (+1). However, when you had $$\color{red}{\int\limits_0^1 {\int\limits_0^1 {\frac{{x \cdot du \cdot dx}}{{\left( {ux - 1} \right)\left( {1 + x} \right)}}} }} =\int\limits_0^1 {\int\limits_0^1 {\frac{{du \cdot dx}}{{\left( {ux - 1} \right)}}} } - \color{red}{\int\limits_0^1 {\int\limits_0^1 {\frac{{u \cdot du \cdot dx}}{{\left( {ux - 1} \right)\left( {u + 1} \right)}}} }} + \int\limits_0^1 {\int\limits_0^1 {\frac{{du \cdot dx}}{{\left( {x + 1} \right)\left( {u + 1} \right)}}} }$$ the red integrals are equal and you could simply add and divide by $2$. –  robjohn Mar 7 '12 at 7:06

You can use the integral you want to use, and the Dilogarithm function as mentioned in the comments.

Below we give a complete proof, including a derivation of the value of the integral you wanted to use.

The Dilogarithm function is defined as

$$\text{Li}_2(z) = -\int_{0}^{z} \frac{\log (1-x)}{x} \text{dx} = \sum_{n=1}^{\infty} \frac{z^n}{n^2}, \quad |z| \le 1$$

The integral which you want to use is $\displaystyle -\text{Li}_2(1)$.

Note that $\displaystyle \text{Li}_2(1) = \sum_{n=1}^{\infty} \frac{1}{n^2} = \zeta(2) = \frac{\pi^2}{6}$. (For multiple proofs of that, see here: Different methods to compute $\sum\limits_{n=1}^\infty \frac{1}{n^2}$)

In your integral(whose value you want), make the substitution $\displaystyle x = 2t -1$ and we get

$$\int_{\frac{1}{2}}^{1} \frac{\log (2(1-t))}{t} \text{dt} = \log^2 2 + \int_{\frac{1}{2}}^{1} \frac{\log (1-t)}{t} \text{dt} = \log^2 2 + \text{Li}_2 \left(\frac{1}{2} \right) - \text{Li}_2(1)$$

Now the Dilogarithm function also satisfies the identity

$$\text{Li}_2(x) + \text{Li}_2(1-x) = \frac{\pi^2}{6}-\log x \log (1-x), 0 \lt x \lt 1$$

This identity can easily be proven by just differentiating and using the value of $\displaystyle \text{Li}_2(1)$:

$$\text{Li}_2'(x) - \text{Li}_2'(1-x) = -\frac{\log (1-x)}{x} + \frac{\log x}{1-x} = (-\log x \log (1-x))'$$

and so $$\text{Li}_2(x) + \text{Li}_2(1-x) = C -\log x \log (1-x), 0 \lt x \lt 1$$

Taking limits as $\displaystyle x \to 1$ gives us $\displaystyle C = \frac{\pi^2}{6}$.

Thus

$$\text{Li}_2(x) + \text{Li}_2(1-x) = \frac{\pi^2}{6}-\log x \log (1-x), 0 \lt x \lt 1$$

Setting $\displaystyle x = \frac{1}{2}$ gives us the value of $\displaystyle \text{Li}_2\left(\frac{1}{2}\right) = \frac{\pi^2}{12} - \frac{\log^2 2}{2}$

$$\log^2 2 + \text{Li}_2 \left(\frac{1}{2} \right) - \text{Li}_2(1) = \frac{\log^2 2}{2} - \frac{\pi^2}{12}$$

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Also see J.M's answer here: math.stackexchange.com/a/60492/1102 –  Aryabhata Mar 6 '12 at 23:05
Thank you Aryabhata. I never saw this Dilogarithm function before, but it looks as very useful one. –  Martin Mar 6 '12 at 23:47
@Martin: You are welcome. Thank you for the nice problem. –  Aryabhata Mar 7 '12 at 1:18
@Aryabhata Nice reflection formula! Check my answer, tell me what you think. –  Pedro Tamaroff Mar 7 '12 at 2:29
@PeterT.off: Thanks! Your answer seems very interesting. I will go through it sometime soon. –  Aryabhata Mar 7 '12 at 3:04
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Maple says it's $${(\log2)^2\over2}-{\pi^2\over12}$$ To get there, I think you will have to understand how the known integral you cite was established, and then use the same ideas to do yours (perhaps after first following Emile's calculations).

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First let $2u=x+1$ and thus $2du=dx$. Then we get: $$\int_0^1\frac{\ln(1-x)}{1+x}dx=\int_{\frac{1}{2}}^1\frac{\ln(2-2u)}{2u}2du$$ $$=\int_{\frac{1}{2}}^1\frac{\ln(2(1-u))}{u}du=\int_{\frac{1}{2}}^1\frac{\ln2+\ln(1-u)}{u}du$$ $$=\int_{\frac{1}{2}}^1\frac{\ln2}{u}du+\int_{\frac{1}{2}}^1\frac{\ln(1-u)}{u}du$$