# Evaluate $\int_{0}^{\pi/2} \ln\left[ \tan\left ( \frac{\theta}{2}\right) \right ]^2 K\left ( \sin\theta \right )\text{d}\theta$

Let us define $$K(x)$$ as complete elliptic integral of the first kind, where $$x$$ is elliptic modulus. A possible closed-form is ($$G$$ denotes Catalan's constant.) $$\int_{0}^{\pi/2} \ln\left[ \tan\left ( \frac{\theta}{2}\right) \right ]^2 K\left ( \sin\theta \right )\text{d}\theta =\frac{\Gamma\left ( \frac14 \right )^4G }{8\pi}.$$ It looks like a "product" of two solvable integrals (both are elementary): $$\int_{0}^{\pi/2} \ln\left[ \tan\left ( \frac{\theta}{2}\right) \right ]^2\text{d}\theta =\frac{\pi^3 }{8}$$ and $$\int_{0}^{\pi/2} K\left ( \sin\theta \right )\text{d}\theta =\frac{\Gamma\left ( \frac14 \right )^4 }{16\pi}.$$

Question: How can we evaluate the integral? I try to utilize the Fourier series $$K(\sin\theta) =\pi\sum_{n\ge0} \frac{\left ( \frac12 \right )_n^2 }{(n!)^2} \sin\left ( \left ( 4n+1 \right )\theta \right )$$ to prove, but seems not to go well. I appreciate for your help.

An Interesting Observation: We find $$\int_{0}^{\pi/2} \ln\left[ \tan\left ( \frac{\theta}{2}\right) \right ]^4 K\left ( \sin\theta \right )\text{d}\theta =\frac{3\,\Gamma\left ( \frac14 \right )^4}{4\pi}(G^2+\beta(4))$$ where $$\beta(.)$$ is Dirichlet's $$\beta$$ function.

• Using series will probably never give this nice result. Feb 28, 2023 at 14:58
• We have some more questions around regarding values of integrals involving $K$, please share the source of the identities. In a book or paper, such identities have their direct neighborhood, often suggesting a way to attack the integrals, also they come with useful identities. Also, for a structural or systematic way to see these integrals, there is always a benefit of already having all data at one place, please understand, i suppose for your purposes all these posts are not individual integrals with a dead end computation. If numerical tools suggest the identities, please also mention them. Mar 1, 2023 at 11:58

We have $$\int_0^{\pi/2} \log^{2n} (\tan \frac{x}{2}) K(\sin x) dx = \int_0^1 \frac{2 \log^{2n} t}{1+t^2} K(\frac{2t}{1+t^2}) dt$$ Their values can be extracted by differentiating $$\tag{*}\int_0^1 \frac{t^{4a} + t^{-4a}}{1+t^2} K(\frac{2t}{1+t^2}) dt = \frac{\pi}{8}\cot(\pi(\frac{1}{4}-a))\frac{\Gamma \left(a+\frac{1}{4}\right)^2}{\Gamma \left(a+\frac{3}{4}\right)^2} \quad -1/4<\Re(a)<1/4$$

I give two different proofs of $$(*)$$, I come up with the longer proof first.

First proof of $$(*)$$: using quadratic transformation $$(1+t^2)K(t^2) = K(2t/(1+t^2))$$, we have \begin{aligned}\int_0^1 \frac{t^{4a}}{1+t^2} K(\frac{2t}{1+t^2}) dt &= \frac{1}{2} \int_0^1 t^{2a-1/2} K(t) dt \\ &= \frac{1}{2} \frac{\pi}{1+4a}{_3F_2}(\frac{1}{2}, \frac{1}{2}, \frac{1}{4} + a; 1, \frac{5}{4} + a; 1) \end{aligned} here we noted that $$K(t)$$ is a $$_2F_1$$. Using third formulas here gives $${_3F_2}(\frac{1}{2}, \frac{1}{2}, \frac{1}{4} + a; 1, \frac{5}{4} + a; 1) = \frac{4 a+1}{4 a-1} {_3F_2(\frac{1}{2}, \frac{1}{2}, \frac{1}{4} - a; 1, \frac{5}{4} - a; 1)} +\frac{\Gamma \left(\frac{1}{4}-a\right) \Gamma \left(a+\frac{5}{4}\right)}{\Gamma \left(\frac{3}{4}-a\right) \Gamma \left(a+\frac{3}{4}\right)}$$ we see the $$a$$ in $$_3F_2$$ becomes $$-a$$, giving $$(*)$$. Q.E.D.

Second proof of $$(*)$$: It's much longer. See edit history.

• How was $G$ "discovered"? Is there some standard combinatoric algorithm for finding such 'complementary' $G$? Feb 28, 2023 at 18:46
• @FShrike $G$ is found by creative telescoping algorithm. You might want to learn about theory of WZ (Wilf-Zeilberger) pairs on combinatorics first. Feb 28, 2023 at 18:54
• Can $\sum_{k=0}^{n} \frac{(1/2)_k^2}{(1)_k^2} \frac{1}{2n+2k+1}$ be transferred? Jun 3, 2023 at 11:24
• @SetnessRamesory what do you mean by "transferred"? Jun 3, 2023 at 12:08

By expanding $$K(k)$$, we deduce $$\,_5F_4\left ( \frac14,\frac14,\frac14,\frac12,\frac12; 1,\frac54,\frac54,\frac54;1 \right )=\frac{\Gamma\left ( \frac14 \right)^4}{16\pi^2}G,$$ $$\,_7F_6\left ( \frac14,\frac14,\frac14,\frac14,\frac14,\frac12,\frac12; 1,\frac54,\frac54,\frac54,\frac54,\frac54;1 \right )=\frac{\Gamma\left ( \frac14 \right)^4}{32\pi^2}(G^2+\beta(4)).$$

• How did you calculate that? May 27, 2023 at 14:32