# LogSine Integral $\int_0^{\pi/3}\ln^n\big(2\sin\frac{\theta}{2}\big)\mathrm d\theta$

I am trying to integrate a special case of the log sine integral $$\rm{Ls}_n(\sigma)$$ at $$\sigma=\pi/3$$ : $$\rm{Ls}_{n}\big(\tfrac{\pi}{3}\big)=-\int_0^{\pi/3}\bigg[\ln\big(2\sin\tfrac{\theta}{2}\big)\bigg]^{n-1}\mathrm d\theta$$ where $$n$$ is a non-negative integer. This problem is strongly related to the hypergeometric form of the Log Sine integral.

The closed form is rather simple, although I am having trouble computing it. We can use standard log rules on the inside expression, although I am not sure how this will help us...Thanks

## 2 Answers

$$\text{Consider }\int_0^\frac{\pi}{3}\left(2\sin\frac{\theta}{2}\right)^a~d\theta~,$$

$$\int_0^\frac{\pi}{3}\left(2\sin\frac{\theta}{2}\right)^a~d\theta$$

$$=2^a\int_0^\frac{\pi}{3}\sin^a\frac{\theta}{2}d\theta$$

$$=2^{a+1}\int_0^\frac{\pi}{6}\sin^a\theta~d\theta$$

$$=2^{a+1}\int_0^\frac{1}{2}x^a~d(\sin^{-1}x)$$

$$=2^{a+1}\int_0^\frac{1}{2}\dfrac{x^a}{\sqrt{1-x^2}}dx$$

$$=2^{a+1}\int_0^\frac{1}{4}\dfrac{x^\frac{a}{2}}{\sqrt{1-x}}d\left(x^\frac{1}{2}\right)$$

$$=2^a\int_0^\frac{1}{4}\dfrac{x^\frac{a-1}{2}}{\sqrt{1-x}}dx$$

$$=2^aB\left(\dfrac{1}{4};\dfrac{a+1}{2},\dfrac{1}{2}\right)$$

$$\therefore\int_0^\frac{\pi}{3}\ln^n\left(2\sin\frac{\theta}{2}\right)d\theta=\dfrac{d^n}{da^n}\left(2^aB\left(\dfrac{1}{4};\dfrac{a+1}{2},\dfrac{1}{2}\right)\right)(a=0)$$

I. Special case

The log sine integral, $$\rm{Ls}_{n}\big(\tfrac{\pi}{3}\big)=-\int_0^{\pi/3}\bigg[\ln\big(2\sin\tfrac{\theta}{2}\big)\bigg]^{n-1}\mathrm d\theta$$

can be given a nice closed-form in terms of $$\pi$$, the zeta function $$\zeta(s)$$, and Clausen function $$\rm{Cl}_n(x)$$ for the first few even $$n$$. Let,

$$\rm{Cl}_n(x) =\sum_{m=1}^\infty \frac{\sin(mx)}{m^n},\quad \text{even}\;m$$ $$\rm{Cl}_n(x) =\sum_{m=1}^\infty \frac{\cos(mx)}{m^n},\quad \text{odd}\;m$$

Then,

\begin{aligned} \rm{Ls}_2\big(\tfrac\pi3\big) &= \rm{Cl}_2(\tfrac\pi3\big)\\ \rm{Ls}_4\big(\tfrac\pi3\big) &= \tfrac92\rm{Cl}_4(\tfrac\pi3\big)+\tfrac12\pi\,\zeta(3)\\ \rm{Ls}_6\big(\tfrac\pi3\big) &= \tfrac{135}2\rm{Cl}_6(\tfrac\pi3\big)+\tfrac{35}{36}\pi^3\,\zeta(3)+\tfrac{15}{2}\pi\,\zeta(5) \end{aligned}

See Borwein's "Special Values of Generalized Log-sine Integrals". Note that the special case $$n=2$$ is Gieseking's constant.

Unfortunately, the simple pattern seems to stop at $$n=6$$. I tried to find $$n=8$$ using $$\rm{Cl}_8(\tfrac\pi3\big)$$ and analogous products of $$\pi$$ and $$\zeta(s)$$, but an integer relations algorithm couldn't find it. (Presumable a new function comes to play at $$n=8$$.)

II. General case

More generally, we have, $$\rm{Ls}_n^{(k)}(\sigma) = \int_0^{\sigma}\theta^k \Big(\ln\big(2\sin\tfrac{\theta}{2}\big)\Big)^{n-1-k}\,d\theta$$

If we focus on the case $$n-1-k=1$$, or $$n=k+2$$,

$$\rm{Ls}_{k+2}^{(k)}(\sigma) = \int_0^{\sigma}\theta^k \ln\big(2\sin\tfrac{\theta}{2}\big)\,d\theta$$

then for all $$k$$ we have the concise formula in terms of the Clausen function and zeta function, $$\frac{1}{k!}\int_0^{\sigma}\theta^k \ln\big(2\sin\tfrac{\theta}{2}\big)\,d\theta = P(k)+\sum_{j=2}^{k+2} \frac{(-1)^{j+\lfloor(j+1)/2\rfloor }}{(k+2-j)!} \rm{Cl}_j(\sigma)\,\sigma^{k+2-j}$$

with floor function $$\lfloor x\rfloor$$ and where, $$P(k)=\frac{1-(-1)^k}{2}\,\zeta(k+2)\,i^{k-1}$$

and $$|\sigma| < 2\pi$$.

P.S. Note that for even $$k$$, then $$P(k) = 0$$, while for odd $$k$$, it is a real number.

• @Lucian: Ok. No luck though. The algorithm couldn't find anything. – Tito Piezas III May 3 at 2:15
• Related post – Tito Piezas III May 5 at 8:03