General formula for an integral involving powers of logarithms

$$\int\limits_0^1 \dfrac{\log^{m} (1+x)\log^n x}{x}\; dx =
(-1)^{n+1}(n!) (m)! \sum_{\{m-1\}} \sum_{k=1}^\infty \frac{(-1)^k}{k^{n+2}}
\prod^{l’}_{j=1}\frac{(-1)^{i_j}}{(i_j)!} \left(
\frac{H_{k-1}^{(r_j)}}{r_j}\right)^{i_j}$$

$$\textit{solution}$$

Stirling numbers of the first kind might be useful here, Consider

$$m! \sum_{k=m}^\infty (-1)^{k-m} \left[k\atop m\right] \frac{x^k}{k!} = \log^m(1+x)$$

$$\int\limits_0^1 \dfrac{\log^m (1+x)\log^n x}{x}\; dx = m! \sum_{k=m}^\infty (-1)^{k-m} \left[k\atop m\right] \frac{1}{k!} \int^1_0 x^{k-1} \log^n(x)\,dx$$

Now it is easy to see that

$$\int^1_0 x^{k-1} \,dx = \frac{1}{k}$$

By differentiation $n$ times with respect to $k$

$$\int^1_0 x^{k-1} \log^n(x)\,dx = (-1)^n\frac{n!}{k^{n+1}}$$

Substituting back we have

$$\int\limits_0^1 \dfrac{\log^m (1+x)\log^n x}{x}\; dx =(m!)(n!)
\sum_{k=m}^\infty (-1)^{k-m+n} \left[k\atop m\right] \frac{1}{k!\,
k^{n+1}}$$

Now the Striling numbers could related to Euler sums through equations like

$$\frac{\left[k\atop 3\right]}{k!} =\frac{ (H_{k-1})^2-H^{(2)}_{k-1}}{2k}$$

and

$$\frac{\left[k\atop 4\right]}{k!} =\frac{ (H_{k-1})^3-3H^{(2)}_{k-1}H_{k-1}+2H^{(3)}_{k-1}}{6k}$$

 

Case \( m=2 , n=2 \)


$$\int\limits_0^1 \dfrac{\log^2 (1+x)\log^2 x}{x}\; dx =4
\sum_{k=2}^\infty (-1)^{k} \left[k\atop 2\right] \frac{1}{k!\,
k^{3}}$$

Note that

$$\frac{\left[k\atop 2\right]}{k!} = \frac{H_{k-1}}{k}$$

Hence we deduce that

$$\int\limits_0^1 \dfrac{\log^2 (1+x)\log^2 x}{x}\; dx =4
\sum_{k=2}^\infty (-1)^{k} \frac{H_{k-1}}{\,
k^{4}}$$

Note that

$$\begin{align}
\sum_{k=2}^\infty (-1)^{k} \frac{H_{k-1}}{\,
k^{4}} &=\sum_{k=2}^\infty (-1)^{k} \frac{H_{k}}{
k^{4}} -\sum_{k=2}^\infty (-1)^{k} \frac{1}{
k^{5}} \\
&=\sum_{k=1}^\infty (-1)^{k} \frac{H_{k}}{
k^{4}} -\sum_{k=1}^\infty (-1)^{k} \frac{1}{
k^{5}}\\
&= \frac{\zeta(2) \zeta(3)}{2} – \frac{ 29\zeta(5)}{32}
\end{align}$$

We deduce that

$$\boxed{\int\limits_0^1 \dfrac{\log^2 (1+x)\log^2 x}{x}\; dx = 2\zeta(2) \zeta(3)- \frac{ 29}{8}\zeta(5)}$$

This implies we can represent the special case $m=2$

$$\int\limits_0^1 \dfrac{\log^2 (1+x)\log^n x}{x}\; dx =2 (-1)^n(n!)
\left[ \sum_{k=1}^\infty (-1)^{k} \frac{H_k}{ k^{n+2}} +
\left(1-2^{-n-2} \right) \zeta(n+3) \right]$$


General formula in terms of nonlinear Euler sums

Define \( \{ m\} \) as the \(l\) partitions of \(m \) where \( m = i_1r_1+\cdots i_l r_l\)

$$ \frac{1}{(m+1)!} \log^{m+1}(1+x) =\sum_{\{m\}} \sum_{k=1}^\infty \prod^l_{j=1}\frac{(-1)^{i_j+1}}{(i_j)!} \left( \frac{H_{k-1}^{(r_j)}}{r_j}\right)^{i_j} \frac{(-x)^k}{k} $$

Substitute back in the integral

$$\int\limits_0^1 \dfrac{\log^{m} (1+x)\log^n x}{x}\; dx =
(-1)^{n+1}(n!) (m)! \sum_{\{m-1\}} \sum_{k=1}^\infty \frac{(-1)^k}{k^{n+2}}
\prod^{l’}_{j=1}\frac{(-1)^{i_j}}{(i_j)!} \left(
\frac{H_{k-1}^{(r_j)}}{r_j}\right)^{i_j}$$

Reference

https://arxiv.org/pdf/math/0607514.pdf

This entry was posted in Euler sum, Harmonic numbers, stirling, Striling numbers of first kind and tagged , , , , , . Bookmark the permalink.

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