Prove that:
$$\int_{0}^{\pi/2}{x\log \left( 1-\cos x \right) \, {\rm d} x}=\frac{35}{16}\zeta \left( 3 \right)-\frac{\pi ^{2}}{8}\log 2-\pi\mathcal{G}$$
Solution
This is a nice exercise and it has very nice extensions, some of which will be added as a comment. As someone may notice the basic idea is our beloved Fourier Series.
\begin{align*} \int_{0}^{\pi/2} x \log \left ( 1-\cos x \right ) &= \int_{0}^{\pi/2} x \log \left ( 2 \sin^2 \frac{x}{2} \right )\, {\rm d}x \\
&= \int_{0}^{\pi/2} x \log 2 \, {\rm d}x + 2 \int_{0}^{\pi/2} x \log \sin \frac{x}{2} \, {\rm d}x\\
&\!\!\!\!\!\!\overset{u=x/2}{=\! =\! =\! =\! =\!} \frac{\pi^2 \log 2}{8} + 8 \int_{0}^{\pi/4} u \log \sin u \, {\rm d}u \\
&= \frac{\pi^2 \log 2}{8} + 8 \int_{0}^{\pi/4} u \left ( - \log 2 - \sum_{m=1}^{\infty} \frac{\cos 2mu}{m} \right ) \, {\rm d}u \\
&= \frac{\pi^2 \log 2}{8} - \frac{2\pi^2 \log 2}{8} - 8 \int_{0}^{\pi/4} u \sum_{m=1}^{\infty} \frac{\cos 2mu}{m} \, {\rm d}u\\
&= - \frac{\pi^2 \log 2}{8} - 8 \sum_{m=1}^{\infty} \frac{1}{m} \int_{0}^{\pi/4} u \cos 2mu \, {\rm d}u \\
&= -\frac{\pi^2 \log 2}{8} - 8 \sum_{m=1}^{\infty} \frac{1}{m}\left [ \frac{\pi \sin \left ( \frac{m \pi}{2} \right )}{8m} + \frac{2 \cos \left ( \frac{m \pi}{2} \right )}{8m^2} - \frac{2}{8m^2} \right ] \\
&= -\frac{\pi^2 \log 2}{8} - \sum_{m=1}^{\infty} \frac{1}{m}\left [ \frac{\pi \sin \left ( \frac{m \pi}{2} \right )}{m} + \frac{2 \cos \left ( \frac{m \pi}{2} \right )}{m^2} - \frac{2}{m^2} \right ] \\
&=-\frac{\pi^2 \log 2}{8} - \pi \mathcal{G} + \frac{35}{16} \zeta(3)
\end{align*}
since it is known that $ \displaystyle {\rm Cl}_2 \left ( \frac{\pi}{2} \right )= \sum_{n=1}^{\infty} \frac{\sin \frac{n \pi}{2}}{n^2}=\mathcal{G}$. (here is a derivation.) Finally the series
$$\displaystyle \sum_{n=1}^{\infty} \frac{\cos \frac{n \pi}{2}}{n^3}$$
can be tackled by either using Fourier or splitting it up to its odd and even components. It eventually evaluates to $\displaystyle \sum_{n=1}^{\infty} \frac{\cos \frac{n \pi}{2}}{n^3} = - \frac{3\zeta(3)}{32}$ completing the exercise.
The exercise can also be found in mathimatikoi.org .
$$\int_{0}^{\pi/2}{x\log \left( 1-\cos x \right) \, {\rm d} x}=\frac{35}{16}\zeta \left( 3 \right)-\frac{\pi ^{2}}{8}\log 2-\pi\mathcal{G}$$
Solution
This is a nice exercise and it has very nice extensions, some of which will be added as a comment. As someone may notice the basic idea is our beloved Fourier Series.
\begin{align*} \int_{0}^{\pi/2} x \log \left ( 1-\cos x \right ) &= \int_{0}^{\pi/2} x \log \left ( 2 \sin^2 \frac{x}{2} \right )\, {\rm d}x \\
&= \int_{0}^{\pi/2} x \log 2 \, {\rm d}x + 2 \int_{0}^{\pi/2} x \log \sin \frac{x}{2} \, {\rm d}x\\
&\!\!\!\!\!\!\overset{u=x/2}{=\! =\! =\! =\! =\!} \frac{\pi^2 \log 2}{8} + 8 \int_{0}^{\pi/4} u \log \sin u \, {\rm d}u \\
&= \frac{\pi^2 \log 2}{8} + 8 \int_{0}^{\pi/4} u \left ( - \log 2 - \sum_{m=1}^{\infty} \frac{\cos 2mu}{m} \right ) \, {\rm d}u \\
&= \frac{\pi^2 \log 2}{8} - \frac{2\pi^2 \log 2}{8} - 8 \int_{0}^{\pi/4} u \sum_{m=1}^{\infty} \frac{\cos 2mu}{m} \, {\rm d}u\\
&= - \frac{\pi^2 \log 2}{8} - 8 \sum_{m=1}^{\infty} \frac{1}{m} \int_{0}^{\pi/4} u \cos 2mu \, {\rm d}u \\
&= -\frac{\pi^2 \log 2}{8} - 8 \sum_{m=1}^{\infty} \frac{1}{m}\left [ \frac{\pi \sin \left ( \frac{m \pi}{2} \right )}{8m} + \frac{2 \cos \left ( \frac{m \pi}{2} \right )}{8m^2} - \frac{2}{8m^2} \right ] \\
&= -\frac{\pi^2 \log 2}{8} - \sum_{m=1}^{\infty} \frac{1}{m}\left [ \frac{\pi \sin \left ( \frac{m \pi}{2} \right )}{m} + \frac{2 \cos \left ( \frac{m \pi}{2} \right )}{m^2} - \frac{2}{m^2} \right ] \\
&=-\frac{\pi^2 \log 2}{8} - \pi \mathcal{G} + \frac{35}{16} \zeta(3)
\end{align*}
since it is known that $ \displaystyle {\rm Cl}_2 \left ( \frac{\pi}{2} \right )= \sum_{n=1}^{\infty} \frac{\sin \frac{n \pi}{2}}{n^2}=\mathcal{G}$. (here is a derivation.) Finally the series
$$\displaystyle \sum_{n=1}^{\infty} \frac{\cos \frac{n \pi}{2}}{n^3}$$
can be tackled by either using Fourier or splitting it up to its odd and even components. It eventually evaluates to $\displaystyle \sum_{n=1}^{\infty} \frac{\cos \frac{n \pi}{2}}{n^3} = - \frac{3\zeta(3)}{32}$ completing the exercise.
The exercise can also be found in mathimatikoi.org .
Closely related are the following integrals:
ReplyDelete1. $\displaystyle \int_0^{\pi/2} x \log \sin x \, {\rm d}x = \frac{7 \zeta(3)}{16} - \frac{\pi^2 \log 2}{8}$
2. $\displaystyle \int_0^{\pi/2} x \log \cos x \, {\rm d}x = - \frac{7\zeta(3)}{16} - \frac{\pi^2 \log 2}{8}$
3. $\displaystyle \int_0^{\pi/2} x \log \tan x \, {\rm d}x = \frac{7 \zeta(3)}{8}$
All of them are dealt using Fourier series of $\log \sin$, $\log \cos$ and $\log \tan $ respectively. Now, giving an extension of the original integral one can prove that:
$$\int_{0}^{\pi/2} x \log \left ( 1+\cos x \right ) \, {\rm d}x = \pi \mathcal{G} - \frac{21 \zeta(3)}{16} - \frac{\pi^2 \log 2}{8}$$
as well as many other interesting facts , mainly:
4. $\displaystyle \int_{0}^{\pi/2} x \log \left ( 1 - \sin x \right ) \, {\rm d}x = - \frac{35 \zeta(3)}{16} - \frac{\pi^2 \log 2}{8}$
5. $\displaystyle \int_{0}^{\pi/2} x \log \left ( 1 + \sin x \right ) \, {\rm d}x = \frac{21 \zeta(3)}{16} - \frac{\pi^2 \log 2}{8}$