Evaluate $\int_{-\pi/4}^{\pi/4}\frac{x}{\sin x}\mathrm{d}x$

I am working on the integral $$I=\int_{-\pi/4}^{\pi/4}\frac{x}{\sin x}\mathrm{d}x=2\int_0^{\pi/4}\frac{x}{\sin x}\mathrm{d}x$$ Which I am fairly confident has a closed form, as $$\int_{0}^{\pi/2}\frac{x}{\sin x}\mathrm{d}x=2G$$ Where $G$ is Catalan's constant.

Preforming a tangent half angle substitution, we have that $$I=4\int_0^{\sqrt{2}-1}\frac{\arctan x}{x}\mathrm{d}x$$ Then using $$\arctan x=\sum_{n\geq0}(-1)^n\frac{x^{2n+1}}{2n+1}$$ We have $$I=4\sum_{n\geq0}\frac{(-1)^n}{(2n+1)^2}(\sqrt{2}-1)^{2n+1}$$ Which is painfully similar to $G$. I do not know how to deal with that extra $(\sqrt{2}-1)^{2n+1}$ bit though...

In another post of mine I showed that $$I=\pi\sum_{n\geq1} n\log\bigg(\frac{4n+1}{4n-1}\bigg)\prod_{k\geq1\\k\neq n}\frac{k^2}{k^2-n^2}$$ And similarly I showed that $$\sum_{n\geq1}n\log\bigg(\frac{2n+1}{2n-1}\bigg)\prod_{k\geq1\\k\neq n}\frac{k^2}{k^2-n^2}=\frac{4G}\pi$$ So I have to questions. How do I find an exact value for $I$? And are the last two series representations correct? Thanks.

Major Edit:

Okay so I found a closed form for the integral. Wolfy gave me $$\int\frac{x}{\sin x}\mathrm{d}x=i\bigg(\text{Li}_2(-e^{ix})-\text{Li}_2(e^{ix})\bigg)+x\log\frac{1-e^{ix}}{1+e^{ix}}$$ I guess that Wolfy didn't want to do the algebra, so I did it by hand. It took me like $10$ minutes, but I am pretty sure that $$I=-\frac34G+\frac{\pi^2}4\bigg(\frac{13}{24}-i\bigg)-\frac{i\pi}4\log(1+\sqrt{2})+\frac{i-1}{32\sqrt{2}}\bigg[\psi^{(1)}\bigg(\frac{5}{8}\bigg)-\psi^{(1)}\bigg(\frac{1}{8}\bigg)\bigg]+\frac{i+1}{32\sqrt{2}}\bigg[\psi^{(1)}\bigg(\frac{3}{8}\bigg)-\psi^{(1)}\bigg(\frac{7}{8}\bigg)\bigg]$$ Where $\psi^{(1)}$ is the first derivative of the di-gamma function.

Solution 1:

Let $$I = \int_{-\pi/4}^{\pi/4} x \, \text{cosec} \, x \, dx = 2 \int_0^{\pi/4} x \, \text{cosec} \, x \, dx.$$ After integrating by parts we have \begin{align} I &= -\frac{\pi}{2} \ln (1 + \sqrt{2}) + 2 \int_0^{\pi/4} \ln (\text{cosec} \, x + \cot x) \, dx\\ &= -\frac{\pi}{2} \ln (1 + \sqrt{2}) + 2 \int_0^{\pi/4} \ln (1 + \cos x) \, dx - 2 \int_0^{\pi/4} \ln (\sin x) \, dx. \end{align}

The second of these integrals is perhaps (?) reasonably well known (for an evaluation, see here). The result is: $$\int_0^{\pi/4} \ln (\sin x) \, dx = -\frac{1}{2} \mathbf{G} - \frac{\pi}{4} \ln 2,$$ where $\mathbf{G}$ is Catalan's constant. Thus

$$I = -\frac{\pi}{2} \ln (1 + \sqrt{2}) + \mathbf{G} + \frac{\pi}{2} \ln 2 + 2I_1.$$

For the first of the integrals we will make use of the following formula, a proof of which can be found here $$\ln (1 + \cos x) = 2 \sum_{n = 1}^\infty (-1)^{n + 1} \frac{\cos (nx)}{n} - \ln 2.$$

So \begin{align} I_1 &= \int_0^{\pi/4} \ln (1 + \cos x) \, dx\\ &= 2 \sum_{n = 1}^\infty \frac{(-1)^{n + 1}}{n} \int_0^{\pi/4} \cos (nx) \, dx - \ln 2 \int_0^{\pi/4} dx\\ &= -\frac{\pi}{4} \ln 2 + 2 \sum_{n = 1}^\infty \frac{(-1)^{n + 1}}{n^2} \sin \left (\frac{n \pi}{4} \right )\\ &= -\frac{\pi}{4} \ln 2 + 2 \sum_{\stackrel{{\Large{n = 1}}}{n \in \text{odd}}}^\infty \frac{(-1)^{n + 1}}{n^2} \sin \left (\frac{n \pi}{4} \right ) + + 2 \sum_{\stackrel{{\Large{n = 1}}}{n \in \text{even}}}^\infty \frac{(-1)^{n + 1}}{n^2} \sin \left (\frac{n \pi}{4} \right )\\ &= -\frac{\pi}{4} \ln 2 + 2 \sum_{n = 1}^\infty \frac{1}{(2n - 1)^2} \sin \left [\frac{\pi}{4} (2n - 1) \right ] - \frac{1}{2} \sum_{n = 1}^\infty \frac{1}{n^2} \sin \left (\frac{n \pi}{2} \right )\\ &= -\frac{\pi}{4} \ln 2 + 2 S_1 - \frac{1}{2} S_2. \end{align}

For the second of these sums, \begin{align} S_2 &= \sum_{\stackrel{{\Large{n = 1}}}{n \in \text{odd}}}^\infty \frac{1}{n^2} \sin \left (\frac{\pi n}{2} \right ) + \sum_{\stackrel{{\Large{n = 1}}}{n \in \text{even}}} \frac{1}{n^2} \sin \left (\frac{\pi n}{2} \right )\\ &= \sum_{n = 0}^\infty \frac{(-1)^n}{(2n + 1)^2}\\ &= \mathbf{G}, \end{align} after a shift of the index $n \mapsto 2n + 1$ in the odd sum has been made while the even sum is identically equal to zero.

For the first of the sums, as it converges absolutely we can split it up as folows: $$S_1 = \sum_{\stackrel{{\Large{n = 1}}}{n \in 1,5,9,\ldots}}^\infty \frac{1}{(2n - 1)^2} \sin \left [\frac{\pi}{4} (2k - 1) \right ] + \sum_{\stackrel{{\Large{n = 1}}}{n \in 2,6,10,\ldots}}^\infty \frac{1}{(2n - 1)^2} \sin \left [\frac{\pi}{4} (2k - 1) \right ] + \sum_{\stackrel{{\Large{n = 1}}}{n \in 3,7,11,\ldots}}^\infty \frac{1}{(2n - 1)^2} \sin \left [\frac{\pi}{4} (2k - 1) \right ] + \sum_{\stackrel{{\Large{n = 1}}}{n \in 4,8,12,\ldots}}^\infty \frac{1}{(2n - 1)^2} \sin \left [\frac{\pi}{4} (2k - 1) \right ].$$

Shifting the indices as follows: $n \mapsto 4n - 3, n \mapsto 4n - 2, n \mapsto 4n - 1, n \mapsto 4n$ leads to \begin{align} S_1 &= \frac{1}{\sqrt{2}} \left [\sum_{n = 1}^\infty \frac{1}{(8n - 7)^2} + \sum_{n = 1}^\infty \frac{1}{(8n - 5)^2} - \sum_{n = 1}^\infty \frac{1}{(8n - 3)^2} - \sum_{n = 1}^\infty \frac{1}{(8n - 1)^2} \right ]\\ &= \frac{1}{\sqrt{2}} \left [\sum_{n = 0}^\infty \frac{1}{(8n + 1)^2} + \sum_{n = 0}^\infty \frac{1}{(8n + 3)^2} - \sum_{n = 0}^\infty \frac{1}{(8n + 5)^2} - \sum_{n = 0}^\infty \frac{1}{(8n + 7)^2} \right ]\\ &= \frac{1}{64 \sqrt{2}} \left [\sum_{n = 0}^\infty \frac{1}{(n + 1/8)^2} + \sum_{n = 0}^\infty \frac{1}{(n + 3/8)^2} - \sum_{n = 0}^\infty \frac{1}{(n + 5/8)^2} - \sum_{n = 0}^\infty \frac{1}{(n + 7/8)^2} \right ]\\ &= \frac{1}{64 \sqrt{2}} \left [\psi^{(1)} \left (\frac{1}{8} \right ) + \psi^{(1)} \left (\frac{3}{8} \right ) - \psi^{(1)} \left (\frac{5}{8} \right ) - \psi^{(1)} \left (\frac{7}{8} \right ) \right ], \end{align} where we have made use of the series representation for the polygamma function of order one (as known as the trigamma function). So the value for $I_1$ is: $$I_1 = -\frac{\pi}{4} \ln 2 - \frac{1}{2} \mathbf{G} + \frac{1}{32 \sqrt{2}} \left [\psi^{(1)} \left (\frac{1}{8} \right ) + \psi^{(1)} \left (\frac{3}{8} \right ) - \psi^{(1)} \left (\frac{5}{8} \right ) - \psi^{(1)} \left (\frac{7}{8} \right ) \right ],$$ leading to a final result of $$\int_{-\pi/4}^{\pi/4} x \, \text{cosec} \, x \, dx = -\frac{\pi}{2} \ln (1 + \sqrt{2}) + \frac{1}{16 \sqrt{2}} \left [\psi^{(1)} \left (\frac{1}{8} \right ) + \psi^{(1)} \left (\frac{3}{8} \right ) - \psi^{(1)} \left (\frac{5}{8} \right ) - \psi^{(1)} \left (\frac{7}{8} \right ) \right ].$$

Converting the trigamma functions to Clausen functions of order two

Inspired by the answer given by Zacky in terms of the Clausen function of order two, here I will show how to convert my answer in terms of the 4 trigamma functions into 2 Clausen functions of order 2.

The relation between the Clausen function of order two and the trigamma function is given by (a proof of this can be found here) $$\text{Cl}_2 \left (\frac{q \pi}{p} \right ) = \frac{1}{(2p)^{2m} (2m - 1)!} \sum_{n = 1}^p \sin \left (\frac{qn\pi}{p} \right ) \left [\psi^{(1)} \left (\frac{n}{2p} \right ) + (-1)^q \psi^{(1)} \left (\frac{n + p}{2p} \right ) \right ].$$ Setting $m = 1, q = 1, p = 4$ gives \begin{align} \text{Cl}_2 \left (\frac{\pi}{4} \right ) &= \frac{1}{64} \left [\frac{1}{\sqrt{2}} \left \{\psi^{(1)} \left (\frac{1}{8} \right ) - \psi^{(1)} \left (\frac{5}{8} \right ) \right \} + \psi^{(1)} \left (\frac{1}{4} \right ) - \psi^{(1)} \left (\frac{3}{4} \right ) \right.\\ & \qquad \left. + \frac{1}{\sqrt{2}} \left \{\psi^{(1)} \left (\frac{3}{8} \right ) - \psi^{(1)} \left (\frac{7}{8} \right ) \right \} \right ], \qquad (*) \end{align} and setting $m = 1, q = 3, p = 4$ gives \begin{align} \text{Cl}_2 \left (\frac{3\pi}{4} \right ) &= \frac{1}{64} \left [\frac{1}{\sqrt{2}} \left \{\psi^{(1)} \left (\frac{1}{8} \right ) - \psi^{(1)} \left (\frac{5}{8} \right ) \right \} - \psi^{(1)} \left (\frac{1}{4} \right ) + \psi^{(1)} \left (\frac{3}{4} \right ) \right.\\ & \qquad \left. + \frac{1}{\sqrt{2}} \left \{\psi^{(1)} \left (\frac{3}{8} \right ) - \psi^{(1)} \left (\frac{7}{8} \right ) \right \} \right ]. \qquad (**) \end{align} On adding ($*$) to ($**$) we see that $$\psi^{(1)} \left (\frac{1}{8} \right ) + \psi^{(1)} \left (\frac{3}{8} \right ) - \psi^{(1)} \left (\frac{5}{8} \right ) - \psi^{(1)} \left (\frac{7}{8} \right ) = 32 \sqrt{2} \left [\text{Cl}_2 \left (\frac{\pi}{4} \right ) + \text{Cl}_2 \left (\frac{3\pi}{4} \right ) \right ],$$ giving $$\int_{-\pi/4}^{\pi/4} \frac{x}{\sin x} \, dx = -\frac{\pi}{2} \ln (1 + \sqrt{2}) + 2 \, \text{Cl}_2 \left (\frac{\pi}{4} \right ) + 2 \, \text{Cl}_2 \left (\frac{3\pi}{4} \right ).$$

Solution 2:

For the purpose of an alternative method, mostly relying on Clausen function. We have: $$I=\int_{-\pi/4}^{\pi/4}\frac{x}{\sin x} dx=2\int_0^{\pi/4}x\left(\ln\left(\tan\frac{x}{2}\right)\right)' dx=2x\ln\left(\tan\frac{x}{2}\right)\bigg|_0^\frac{\pi}{4}-2\int_0^\frac{\pi}{4}\ln\left(\tan\frac{x}{2}\right)dx=$$ $$=\frac{\pi}{2}\ln(\sqrt 2-1)-2\int_0^\frac{\pi}{4}\left(\ln\left(2\sin\frac{x}{2}\right)-\ln\left(2\cos\frac{x}{2}\right)\right)dx=$$$$=\frac{\pi}{2}\ln(\sqrt 2-1)+2\text{Cl}_2\left(\frac{\pi}{4}\right)+2\text{Cl}_2\left(\frac{3\pi}{4}\right)$$ The last two integrals can be found on the first link. Now is up to the reader if using Clausen function gives any satisfaction, because in desguise is still a series, but same goes with the trigamma function.

An interesting question might be: For what values of $\phi$ does the following integral have an elementary answer (Catalan's constant included)? $\ \displaystyle{I(\phi)=\int_0^\phi \frac{x}{\sin x}dx}$. So far I only know about $I\left(\frac{\pi}{6}\right)$.

Solution 3:

Too long for comments.

$$I=4\sum_{n\geq0}\frac{(-1)^n}{(2n+1)^2}(\sqrt{2}-1)^{2n+1}=4 \left(\sqrt{2}-1\right) \, _3F_2\left(\frac{1}{2},\frac{1}{2},1;\frac{3}{2},\frac{3}{2};2 \sqrt{2}-3\right)$$

On the other hand, working from Wolfram Alpha expression for the antiderivative and trying to simplify as much as I could the integral, I got $$I=-\frac{\pi}{4} \, \log \left(3+2 \sqrt{2}\right)+\frac 1 {16 \sqrt 2} \left(\psi ^{(1)}\left(\frac{1}{8}\right)+\psi ^{(1)}\left(\frac{3}{8}\right)-\psi ^{(1)}\left(\frac{5}{8}\right)-\psi ^{(1)}\left(\frac{7}{8}\right) \right)$$

What is interesting to mention is that, doing calculations with another CAS, intermediate steps show $G$ appearing a few times.

Solution 4:

$\newcommand{\bbx}[1]{\,\bbox[15px,border:1px groove navy]{\displaystyle{#1}}\,} \newcommand{\braces}[1]{\left\lbrace\,{#1}\,\right\rbrace} \newcommand{\bracks}[1]{\left\lbrack\,{#1}\,\right\rbrack} \newcommand{\dd}{\mathrm{d}} \newcommand{\ds}[1]{\displaystyle{#1}} \newcommand{\expo}[1]{\,\mathrm{e}^{#1}\,} \newcommand{\ic}{\mathrm{i}} \newcommand{\mc}[1]{\mathcal{#1}} \newcommand{\mrm}[1]{\mathrm{#1}} \newcommand{\pars}[1]{\left(\,{#1}\,\right)} \newcommand{\partiald}[3][]{\frac{\partial^{#1} #2}{\partial #3^{#1}}} \newcommand{\root}[2][]{\,\sqrt[#1]{\,{#2}\,}\,} \newcommand{\totald}[3][]{\frac{\mathrm{d}^{#1} #2}{\mathrm{d} #3^{#1}}} \newcommand{\verts}[1]{\left\vert\,{#1}\,\right\vert}$ \begin{align} I & \equiv \int_{-\pi/4}^{\pi/4}{x \over \sin\pars{x}}\,\dd x,\ \pars{~\mbox{OP already shows that}\ I = \bbox[10px,#ffd]{\!\!\!\!\! 4\!\int_{0}^{\root{2} - 1}\!\!{\arctan\pars{x} \over x}\,\dd x}~} \end{align}