# How to tackle this integral $\int_0^1 \frac{x^p}{1-x} \ln \frac{1}{x} dx\quad (p>-1)$

For improper integral $$\int_0^1 \frac{x^p}{1-x} \ln \left(\frac{1}{x}\right)\mathrm{d} x\qquad(p>-1),$$ we observe that there is a term $$\frac{\ln(1)-\ln(x)}{1-x}$$ in which we can use Taylor expansion of $$\ln(1+x)$$, then we can use $$B$$ function, but I'm stuck, I don't know how to simplify the result, the final result of this integral is $$\sum_{n=1}^{\infty }\frac{1}{\left ( n+p \right )^{2}}.$$ I don't know how to get to this point, could you please help me? Thank you.

• @Lorago it is dx Nov 30, 2023 at 12:46
• Make the substitution $x=\exp(-t)$ and refer to $(25.11.25)$.
– Gary
Nov 30, 2023 at 13:02

\begin{align}\int_{0}^{1}\frac{x^{p}}{1-x}\ln\left(\frac{1}{x}\right)dx=&-\int_{0}^{1}\frac{x^{p}}{1-x}\ln\left(x\right)dx\\ =&-\frac{\partial}{\partial p}\int_{0}^{1}\frac{x^{p}}{1-x}\mathrm{d}x\\=&-\frac{\partial}{\partial p}\int_{0}^{1}\frac{x}{1-x} x^{p-1}\mathrm{d}x\\ \end{align} $$\frac{x}{1-x}=\sum_{n=1}^{\infty}x^n$$ \begin{align}\int_{0}^{1}\frac{x^{p}}{1-x}\ln\left(\frac{1}{x}\right)dx=&-\frac{\partial}{\partial p}\int_{0}^{1}\sum_{n=1}^{\infty}x^n x^{p-1}\mathrm{d}x\\ =&-\frac{\partial}{\partial p}\sum_{n=1}^{\infty}\int_{0}^{1}x^{n+p-1}\mathrm{d}x\\ =&-\frac{\partial}{\partial p}\sum_{n=1}^{\infty}\frac{1}{n+p}\\ =&\sum_{n=1}^{\infty}\frac{1}{(n+p)^2}\\ =&\psi^{(1)}(p+1) \end{align}
Where $$\psi^{(1)}(z)$$ is the polygamma function