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I've found that \begin{align} \mathcal{L} = 2\operatorname{Li}_3\left(-\frac{1}{2}\right)+\operatorname{Li}_3\left(-\frac{1}{3}\right)+2\operatorname{Li}_3\left(\frac{2}{3}\right)+\operatorname{Li}_2\left(-\frac{1}{3}\right) \ln(3)+2\operatorname{Li}_2\left(\frac{2}{3}\right) \ln(3) \end{align} equals to $$ \mathcal{L} = \frac{\pi^2}{3}\ln(2)+\frac{1}{3}\ln^3(2)-\frac{1}{3} \ln^2(3) \ln\left(\frac{27}{8}\right)-\frac{\zeta(3)}{6}. $$

How could we prove this identity?

A numerical approximation: $$ \mathcal{L} \approx 1.701652530545172752791574942340971991312113932043\dots $$

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1 Answers1

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This identity (with $z=\frac13$) implies that $$\operatorname{Li}_3\left(\frac23\right)+\operatorname{Li}_3\left(-\frac12\right)+\operatorname{Li}_3\left(\frac13\right)=\zeta\left(3\right)-\frac{\pi^2}{6}\ln\frac32+\frac12\ln 3\ln^2\frac32-\frac16\ln^3\frac32. \tag{$\spadesuit$}$$ On the other hand this identity gives $$\operatorname{Li}_3\left(-\frac13\right)-2\operatorname{Li}_3\left(\frac13\right)=-\frac{\ln^33}{6}+\frac{\pi^2}{6}\ln3-\frac{13}{6}\zeta(3).\tag{$\clubsuit$}$$ Adding $2(\spadesuit)+(\clubsuit)$ gives the trilogarithmic part of your expression.

The dilogarithmic part can be computed using dilogarithmic identities from the same question. They give $$2\operatorname{Li}_2\left(\frac13\right)-\operatorname{Li}_2\left(-\frac13\right)=\frac{\pi^2}{6}-\frac{\ln^23}{2},$$ or, equivalently, ($\operatorname{Li}_2\left(z\right)+\operatorname{Li}_2\left(1-z\right)=\frac{\pi^2}{6}-\ln z\ln(1-z)$ with $z=\frac13$) $$2\operatorname{Li}_2\left(\frac23\right)+\operatorname{Li}_2\left(-\frac13\right)=\frac{\pi^2}{6}+\frac{\ln^23}{2}-2\ln 3\ln\frac32.\tag{$\diamondsuit$}$$

Therefore $$\mathcal{L}=2(\spadesuit)+(\clubsuit)+\ln3\left(\diamondsuit\right).$$

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