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I am working on this integral: How to evaluate $\int_0^{\pi /2}\frac{u^2\ln{(2\cos u)}}{(u^2+\ln^2{(2\cos u)})^2}du$?

Question

How to evaluate $$\int_{0}^{\frac{\pi}{2}} \frac{x^2 \ln(2 \cos x)}{(x^2 + \ln^2(2 \cos x))^2} \, dx $$

My attempt

\begin{align*} \int_{0}^{\frac{\pi}{2}} \frac{x^2 \ln(2 \cos x)}{(x^2 + \ln^2(2 \cos x))^2} \, dx &= -\frac{1}{4} \mathfrak{J} \cdot \int_{-\frac{\pi}{2}}^{\frac{\pi}{2}} \frac{x}{\ln^2(1 + e^{2ix})} \, dx \\ &= -\frac{1}{16} \mathfrak{J} \cdot \int_{-\pi}^{\pi} \frac{x}{\ln^2(1 + e^{ix})} \, dx \\ &= -\frac{\pi}{8} \mathfrak{R} \int_{-\pi}^{\pi} \int_{0}^{1} \int_{0}^{y} (1+e^{ix})^{y-2} \, dy \, dy \left(\frac{\partial}{\partial \omega} \bigg|_{\omega=1} e^{i \omega x}\right) \, dx \\ &= -\frac{\pi}{8} \mathfrak{R} \int_{0}^{1} \int_{0}^{y} \, dy \, dy \frac{\partial}{\partial \omega} \bigg|_{\omega=1} \int_{-\pi}^{\pi} (1 + e^{ix})^{y-2} e^{-i \omega x} \, dx \\ &= -\frac{\pi}{8} \int_{0}^{1} \int_{0}^{y} \, dy \, dy \frac{\partial}{\partial \omega} \bigg|_{\omega=1} \binom{y-2}{\omega} \\ &= -\frac{\pi}{8} \int_{0}^{1} \int_{0}^{y} (y - 2) \left(\psi(y - 2) - \psi(2)\right) \, dy \, dy \end{align*}

Can someone help me to evaluate

$$\int_{0}^{1} \int_{0}^{y} (y - 2) \left(\psi(y - 2) - \psi(2)\right) \, dy \, dy$$

Don’t mark this as a duplicate. I want to know how to evaluate the last integral where I am stuck. My question is not about evaluating the main integral.

  • There seems to be a problem. WolframAlpha says that this last integral does not converge. Using $\frac{d}{dy}\psi_{\alpha}(y)=\psi_{\alpha+1}(y)$ for integration by parts, and guessing at having $y$ as the variable and limit, I got $2\psi_{-1}(-2)$ plus some purely finite stuff, which is infinite. – Darmani V Sep 01 '24 at 05:44
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    I think that the first $\frac 18$ should be $\frac 1{16}$. and the first $1/2$ be $1/4$ – Claude Leibovici Sep 03 '24 at 12:04

0 Answers0