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I am trying to evaluate this integral $$I = \int\limits_{\frac{1}{3}}^{\frac{1}{2}} {\frac{{\ln \left( {1 - x} \right)\ln \left( {1 - 2x} \right)}}{x}dx} $$ Here is my try: $${\text{Let}}:x \to 1 - 2x \Rightarrow I = \int\limits_0^{\frac{1}{3}} {\frac{{\ln \left( x \right)\ln \left( {1 + x} \right)}}{{1 - x}}dx} - \ln \left( 2 \right)\int\limits_0^{\frac{1}{3}} {\frac{{\ln \left( x \right)}}{{1 - x}}dx} $$ Since the antiderivative of $${\frac{{\ln \left( x \right)}}{{1 - x}}}$$ is $${{\text{L}}{{\text{i}}_2}\left( { 1-x} \right)}$$ then $$I = \int\limits_0^{\frac{1}{3}} {\frac{{\ln \left( x \right)\ln \left( {1 + x} \right)}}{{1 - x}}dx} - \ln \left( 2 \right)\left( {{\text{L}}{{\text{i}}_2}\left( {\frac{2}{3}} \right) - \frac{{{\pi ^2}}}{6}} \right)$$ I am stucking at the remaining integral, I tried to use series expansion of:$\displaystyle \frac{1}{{1 - x}} = \sum\limits_{n = 0}^\infty {{x^n}}$ but after changing summation and integration, the integral became more complicated.

May I ask for help? Thank you so much. By the way, the closed form is $$\frac{{3{\text{L}}{{\text{i}}_3}\left( {\frac{1}{4}} \right)}}{4} - {\text{L}}{{\text{i}}_2}\left( {\frac{2}{3}} \right)\ln (2) + {\text{L}}{{\text{i}}_2}\left( {\frac{1}{4}} \right)\ln (2) + \frac{{\zeta (3)}}{8} + {\ln ^3}(2) + \frac{{{{\ln }^3}(3)}}{3} - {\ln ^2}(3)\ln (2) + \frac{1}{{12}}{\pi ^2}\ln (2)$$

OnTheWay
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    One can simplify the result a bit as follows: $$\frac{3 \text{Li}_3\left(\frac{1}{4}\right)}{4}+\frac{1}{2} \text{Li}_2\left(\frac{1}{4}\right) \log (2)+\frac{\zeta (3)}{8}+\frac{\log ^3(3)}{3}-\frac{1}{2} \log (2) \log ^2(3)$$ this one can possibly be done by calculating the indefinite integral. – pisco Aug 07 '23 at 08:52
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    There are some extensions, which can no longer be done via indefinite integral, but nonetheless have closed-forms using polylogs. E.g. $$\int_{\frac{1}{3}}^{\frac{1}{2}} \frac{\log ^2(1-x) \log (1-2 x)}{x} , dx$$ $$\int_{\frac{1}{3}}^{\frac{1}{2}} \frac{\log^2(1-x) \log (1-2 x)}{x} , dx$$ $$\int_{\frac{1}{3}}^{\frac{1}{2}} \frac{\log(1-x) \log (x) \log (1-2 x)}{x} , dx$$ – pisco Aug 07 '23 at 08:53
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    A more general point of view : see the end of the answer here generalizing your issue. – Jean Marie Aug 07 '23 at 09:07

1 Answers1

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Since $a^2 +b^2 - (a-b)^2 = 2ab$ we get \begin{align} \int_{0}^{\xi}\frac{\ln(t)\ln(1+t)}{1-t}\, \mathrm{d}t & = \frac12\int_{0}^{\xi} \frac{\ln^2(t)}{1-t}\, \mathrm{d}t + \frac12\int_{0}^{\xi} \frac{\ln^2(1+t)}{1-t}\, \mathrm{d}\color{blue}{t} - \frac12\int_{0}^{\xi} \frac{\ln^2\left( \frac{t}{t+1}\right)}{1-t}\, \mathrm{d}\color{purple}{t}\\ & \overset{\substack{\color{blue}{t+1\to t} \\ \color{purple}{t/(t+1) \to t}}}{=} \frac12\int_{0}^{\xi} \frac{\ln^2(t)}{1-t}\, \mathrm{d}t + \frac12\int_{1}^{\xi +1} \frac{\ln^2(t)}{2-t}\, \mathrm{d}\color{blue}{t}- \frac12\int_{0}^{\frac{\xi}{\xi +1}} \ln^2\left(t\right)\left(\frac{1}{\frac12-t} - \frac{1}{1-t}\right)\, \mathrm{d}\color{purple}{t} \\ & = \frac12\int_{0}^{\xi} \frac{\ln^2(t)}{1-t}\, \mathrm{d}t - \frac12\int_{0}^{1} \frac{\ln^2(t)}{2-t}\, \mathrm{d}t + \frac12\int_{0}^{\xi +1} \frac{\ln^2(t)}{2-t}\, \mathrm{d}t- \frac12\int_{0}^{\frac{\xi}{\xi +1}} \frac{\ln^2\left(t\right)}{\frac12-t} \, \mathrm{d}t + \frac12\int_{0}^{\frac{\xi}{\xi +1}} \frac{\ln^2\left(t\right)}{1-t}\, \mathrm{d}t\tag{1}\\ \end{align} This reduces the problem to evaluating \begin{equation} \int_{0}^{\alpha} \frac{\ln^2(t)}{\beta -t}\, \mathrm{d}t \overset{\color{blue}{t/\beta \to t}}{=} \int_{0}^{\frac{\alpha}{\beta}} \frac{\ln^2(\beta) + 2 \ln(\beta)\ln (t)+\ln^2(t)}{1-t}\, \mathrm{d}t \tag{2} \end{equation} Which in turn reduces the problem to evaluating $\int_{0}^{x} \frac{\ln^n(t)}{1-t}\, \mathrm{d}t$ for $n=0,1,2$.

The $n=0$ case is simply $\int_{0}^{x} \frac{\mathrm{d}t}{1-t} = - \ln(1-x)$. For $n=1$ we recall that the polylogatithm follows the recursion $\mathrm{Li}_{s}(z) = \int_{0}^{z} \frac{\mathrm{Li}_{s-1}(z)}{t}\, \mathrm{d}t$ and that $\mathrm{Li}_{1}(z) =- \ln(1-z)$. Thus

$$ \int_{0}^{x}\frac{\ln(t)}{1-t}\, \mathrm{d}t \overset{\color{blue}{1-t \to t}}{=} -\int_{1}^{1-x}\frac{\ln(1-t)}{t}\, \mathrm{d}t = \mathrm{Li}_{2} (1-x) - \mathrm{Li}_{2} (1) = \mathrm{Li}_{2} (1-x) - \zeta(2) $$ since $ \mathrm{Li}_{s} (1) = \zeta(s)$ for $s>1$ is also a property of the polylogarithm function.

Lastly, for $n=2$ we get \begin{align} \int_{0}^{x} \frac{\ln^2(t)}{1-t}\, \mathrm{d}t &\overset{\text{IBP}}{=} -\ln^2(x)\ln(1-x) + 2 \int_{0}^{x} \ln(t)\frac{\ln(1-t)}{t}\, \mathrm{d}t\\ &\overset{\text{IBP}}{=} -\ln^2(x)\ln(1-x) - 2 \ln(x)\mathrm{Li}_2(x)+ 2 \int_{0}^{x} \frac{\mathrm{Li}_2(t)}{t}\, \mathrm{d}t\\ & = -\ln^2(x)\ln(1-x) - 2 \ln(x)\mathrm{Li}_2(x)+ 2 \mathrm{Li}_3(x) \end{align} Plugging the $n=0,1,2$ cases into $(2)$ gives \begin{align} \int_{0}^{\alpha} \frac{\ln^2(t)}{\beta -t}\, \mathrm{d}t& = -\ln^2(\beta)\ln\left(1- \frac{\alpha}{\beta} \right) +2 \ln(\beta)\mathrm{Li}_2\left(1- \frac{\alpha}{\beta} \right) -2\ln(\beta)\zeta(2) -\ln^2\left(\frac{\alpha}{\beta} \right)\ln\left(1- \frac{\alpha}{\beta} \right) - 2 \ln\left(\frac{\alpha}{\beta} \right)\mathrm{Li}_2\left(\frac{\alpha}{\beta} \right)+ 2 \mathrm{Li}_3\left(\frac{\alpha}{\beta} \right) \end{align} There's now a bit of simplification that can be done. Since \begin{align} \mathrm{Li}_2(x) +\mathrm{Li}_2(1-x) & = \zeta(2)-\int_{0}^{x} \frac{\ln(1-t)}{t}- \frac{\ln(t)}{1-t}\mathrm{d}t \\ & = \zeta(2)-\int_{0}^{x} \left(\ln(t)\ln(1-t)\right)'\mathrm{d}t\\ & = \zeta(2) - \ln(x)\ln(1-x) \end{align} We get that $$ \int_{0}^{\alpha} \frac{\ln^2(t)}{\beta -t}\, \mathrm{d}t = 2\mathrm{Li}_3\left(\frac{\alpha}{\beta} \right) - 2 \ln(\alpha) \mathrm{Li}_2\left(\frac{\alpha}{\beta} \right) -\ln^2(\alpha)\ln\left(1- \frac{\alpha}{\beta} \right) $$ And using the previous equation in $(1)$ we get that \begin{align} \int_{0}^{\xi}\frac{\ln(t)\ln(1+t)}{1-t}\, \mathrm{d}t& = \text{Li}_3(\xi)+\text{Li}_3\left(\frac{\xi}{1+\xi}\right)-\text{Li}_3\left(\frac{2 \xi}{1+\xi}\right)+\text{Li}_3\left(\frac{1+\xi}{2}\right)-\text{Li}_2(\xi) \log (\xi)-\text{Li}_2\left(\frac{\xi}{1+\xi}\right) \log \left(\frac{\xi}{\xi+1}\right)+\text{Li}_2\left(\frac{2 \xi}{1+\xi}\right) \log \left(\frac{\xi}{\xi+1}\right)-\text{Li}_2\left(\frac{1+\xi}{2}\right) \log (\xi+1)-\frac{1}{2} \log (1-\xi) \log ^2(\xi)+\frac{1}{2} \log \left(\frac{1-\xi}{\xi+1}\right) \log ^2\left(\frac{\xi}{\xi+1}\right)+\frac{1}{2} \log ^2\left(\frac{\xi}{\xi+1}\right) \log (\xi+1)-\frac{1}{2} \log \left(\frac{1-\xi}{2}\right) \log ^2(\xi+1)+\frac{1}{24} \left(-21 \zeta (3)-4 \log ^3(2)+2 \pi ^2 \log (2)\right) \end{align} and plugging in $\xi = \frac13$ gives the evaluation of the desired integral as $$ \int_{0}^{\frac13}\frac{\ln(t)\ln(1+t)}{1-t}\, \mathrm{d}t = \text{Li}_3\left(\frac{1}{4}\right)+\text{Li}_3\left(\frac{1}{3}\right)+\text{Li}_3\left( \frac{2}{3}\right)+\frac{1}{2} \log \left(\frac{4}{3}\right) \left(\log ^2(4)-2 \text{Li}_2\left(\frac{2}{3}\right)\right)+\text{Li}_2\left(\frac{1}{4}\right) \log (4)+\text{Li}_2\left(\frac{1}{3}\right) \log (3)-\frac{7 \zeta (3)}{4}+\frac{2 \log ^3(2)}{3}+\frac{1}{2} \log ^2\left(\frac{4}{3}\right) \log (3)+\frac{1}{2} \log \left(\frac{3}{2}\right) \log ^2(3)-\frac{1}{2} \log (2) \log ^2(4) $$

Robert Lee
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