Evaluate an integral without using H(n)

Calculus Level 3

0 1 ln ( 1 x ) Li 2 ( x ) x d x = ? \large \int_0^1 \frac {\ln(1-x)\text{ Li}_2(-x)}x dx = \ ?


The answer is 0.8592.

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1 solution

Mark Hennings
Jan 30, 2019

Ignoring the title of the question, we can use the expansion for the polylogarithm to show that 0 1 ln ( 1 x ) L i 2 ( x ) x d x = k = 1 ( 1 ) k k 2 0 1 x k 1 ln ( 1 x ) d x = k = 1 ( 1 ) k + 1 H k k 3 \int_0^1 \frac{\ln(1-x)\mathrm{Li}_2(-x)}{x}\,dx \; = \; \sum_{k=1}^\infty \frac{(-1)^k}{k^2}\int_0^1 x^{k-1}\ln(1-x)\,dx \; = \; \sum_{k=1}^\infty (-1)^{k+1}\frac{H_k}{k^3} and this paper tells us that this is equal to 1 2 0 1 ( ln x ) 2 ln ( 1 + x ) x ( 1 + x ) d x = 2 L i 2 ( 1 2 ) + 11 4 ζ ( 4 ) + 1 2 ζ ( 2 ) ( ln 2 ) 2 1 12 ( ln 2 ) 4 7 4 ζ ( 3 ) ln 2 = 0.859247 \frac12\int_0^1 \frac{(\ln x)^2\ln(1+x)}{x(1+x)}\,dx \; = \; -2\mathrm{Li}_2(\tfrac12) + \tfrac{11}{4}\zeta(4) + \tfrac12\zeta(2)(\ln2)^2 - \tfrac{1}{12}(\ln 2)^4 - \tfrac74\zeta(3)\ln2 = \boxed{0.859247}

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