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Any idea how ot approach

$$I=\int_0^1\frac{1-2x}{2x^2-2x+1}\ln(x)\text{Li}_2(x)dx\ ?$$

I came across this integral while I was trying to find a different solution for $\Re\ \text{Li}_4(1+i)$ posted here.

here is how I came across it;

using the identity

$$\int_0^1\frac{\ln(x)\text{Li}_2(x)}{1-ax}dx=\frac{\text{Li}_2^2(a)}{2a}+3\frac{\text{Li}_4(a)}{a}-2\zeta(2)\frac{\text{Li}_2(a)}{a}$$

multiply both sides by $\frac{a}{3}$ then replace $a$ by $1+i$ and consider the the real parts of both sides we have

$$\Re\ \text{Li}_4(1+i)=-\frac16\Re\ \text{Li}_2^2(1+i)+\frac23\zeta(2)\Re\ \text{Li}_2(1+i)+\frac13\Re \int_0^1\frac{(1+i)}{1-(1+i)x}\ln(x)\text{Li}_2(x)dx$$

For the integral, use $\Re\frac{1+i}{1-(1+i)x}=\frac{1-2x}{2x^2-2x+1}$ which gives $I$.

What I tried is subbing $1-2x=y$ which gives

$$I=\int_{-1}^1\frac{-y}{1+y^2}\ln\left(\frac{1+y}{2}\right)\text{Li}_2\left(\frac{1+y}{2}\right)dy=\int_{-1}^1 f(y)dy=\underbrace{\int_{-1}^0 f(y)dy}_{y\to\ -y}+\int_{0}^1 f(y)dy$$

$$=\int_0^1\frac{y}{1+y^2}\ln\left(\frac{1-y}{2}\right)\text{Li}_2\left(\frac{1-y}{2}\right)dy-\int_0^1\frac{y}{1+y^2}\ln\left(\frac{1+y}{2}\right)\text{Li}_2\left(\frac{1+y}{2}\right)dy$$

I think I made it more complicated. Any help would be appriciated.

Ali Shadhar
  • 25,498

4 Answers4

1

$$I=\int_0^1\frac{y}{1+y^2}\ln\left(\frac{1-y^2}{4}\right)\text{Li}_2\left(\frac{1-y}{2}\right)dy+\frac{7}{32} \zeta (3) \log (2)+\frac{ \text{Li}_4\left(\frac{1}{2}\right)}{8}-\frac{157 \pi ^4}{46080}-\frac{ 11\log ^4(2)}{48}+\frac{19}{384} \pi ^2 \log ^2(2)$$ Put $$y=\frac{1-x}{1+x}$$ $$\int_0^1\frac{y}{1+y^2}\ln\left(\frac{1-y^2}{4}\right)\text{Li}_2\left(\frac{1-y}{2}\right)dy=\int_0^1\frac{\ln(x)}{1+x}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx-2\int_0^1\frac{\ln(1+x)}{1+x}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx-\int_0^1\frac{x\ln(x)}{1+x^2}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx+2\int_0^1\frac{x\ln(1+x)}{1+x^2}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx$$ be continued $$\int_0^1\frac{\ln(1+x)}{1+x}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx=3\operatorname{Li}_4\left(\frac12\right)-\frac{\pi^4}{30}+\frac{21}8\ln2\zeta(3)-\frac{\pi^2}{12}\ln^22$$ $$\int_0^1\frac{x\ln(x)}{1+x^2}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx=\frac{C^2}{2}+\frac{15 \text{Li}_4\left(\frac{1}{2}\right)}{16}-\frac{701 \pi ^4}{46080}+\frac{7}8\ln2\zeta(3)+\frac{5 \log ^4(2)}{128}-\frac{3}{64} \pi ^2 \log ^2(2)$$

user178256
  • 5,467
1

$$\int_0^1\frac{\ln(1+x)}{1+x}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx=3\operatorname{Li}_4\left(\frac12\right)-\frac{\pi^4}{30}+\frac{21}8\ln2\zeta(3)-\frac{\pi^2}{12}\ln^22$$ $$\int_0^1\frac{x\ln(x)}{1+x^2}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx=\frac{C^2}{2}+\frac{15 \text{Li}_4\left(\frac{1}{2}\right)}{16}-\frac{701 \pi ^4}{46080}+\frac{7}8\ln2\zeta(3)+\frac{5 \log ^4(2)}{128}-\frac{3}{64} \pi ^2 \log ^2(2)$$

user178256
  • 5,467
1

$$\int_0^1\frac{\ln(x)}{1+x}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx=3\text{Li}_4(2)+\text{Li}_2(2)\log^22-3\text{Li}_3(2)\log2+6\operatorname{Li}_4\left(\frac12\right)+\frac{21}4\ln2\zeta(3)-\frac{\pi^2}{8}\log^22+\frac{1}{4}\log^42-\frac{29\pi^4}{288}$$ I being known,I deduce $$\int_0^1\frac{x\ln(1+x)}{1+x^2}\text{Li}_2\left(\frac{x}{1+x}\right)\ dx=-\frac{1}{16}\operatorname{Li}_4\left(\frac12\right)+\frac{21}{64}\ln2\zeta(3)-\frac{41}{768} \pi ^2 \log ^2(2)-\frac{1}{96}\log^42+\frac{1609\pi^4}{92160}-\frac{3}{2}\text{Li}_4(2)-\frac{1}{2}\text{Li}_2(2)\log^22+\frac{3}{2}\text{Li}_3(2)\log2$$ Sorry,i couldn't deduct this integral. $$3\text{Li}_4(2)+\text{Li}_2(2)\log^22-3\text{Li}_3(2)\log2=-3\operatorname{Li}_4\left(\frac12\right)-\frac{21}8\ln2\zeta(3)-\frac{1}{8}\log^42+\frac{\pi^4}{15}$$

user178256
  • 5,467
1

$$=\int_0^1\frac{y}{1+y^2}\ln\left(\frac{1-y}{2}\right)\text{Li}_2\left(\frac{1-y}{2}\right)dy=2\operatorname{Li}_4\left(\frac12\right)+\frac{133}{64}\ln2\zeta(3)-\frac{37}{768} \pi ^2 \log ^2(2)+\frac{77}{384}\log^42-\frac{3197\pi^4}{92160}-\frac{C^2}{2}-\frac{1}{8} \pi C \log (2)+\frac{3}{2}\text{Li}_4(2)+\frac{1}{2}\text{Li}_2(2)\log^22-\frac{3}{2}\text{Li}_3(2)\log2$$ $$\int_0^1\frac{y}{1+y^2}\ln\left(\frac{1+y}{2}\right)\text{Li}_2\left(\frac{1+y}{2}\right)dy=\frac{47}{16}\operatorname{Li}_4\left(\frac12\right)+\frac{133}{64}\ln2\zeta(3)-\frac{61}{768} \pi ^2 \log ^2(2)+\frac{23}{96}\log^42-\frac{4367\pi^4}{92160}+\frac{1}{8} \pi C \log (2)+\frac{3}{2}\text{Li}_4(2)+\frac{1}{2}\text{Li}_2(2)\log^22-\frac{3}{2}\text{Li}_3(2)\log2$$

user178256
  • 5,467
  • Very impressive! I'm also working on similar integrals. – Infiniticism May 14 '20 at 07:52
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    I'm not sure how does this answer the question. OP clearly wants an idea how to approach the integral, not some results that should be on the comments section. Given the length I can understand why you posted an answer, but why 4 answers and not just 1 for all 4 that could be comments? – Zacky May 23 '20 at 09:58
  • @ I did the best I could, but in fact I made a mistake that I could not correct – user178256 May 23 '20 at 10:12