MHB A few tricky integrals.... Or are they?

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The discussion centers on evaluating specific integrals related to Vardi's results, particularly focusing on the integral of log functions involving the tangent function. The integrals in question include expressions for log squared and log cubed of log(tan x), which are connected to the Dirichlet Beta function and involve complex analysis techniques. A method is outlined for evaluating these integrals by differentiating under the integral sign and employing contour integration. The final results link back to classical constants and gamma functions, demonstrating a deep connection between these integrals and special functions. The conversation highlights advanced techniques in integral calculus and their applications in mathematical analysis.
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Here are a few Vardi-type integrals I recently posted on another forum (some of you might have seen them)...Assuming the following classic result - due to Vardi - holds...\int_{\pi/4}^{\pi/2}\log\log(\tan x)\,dx=\frac{\pi}{2}\log\left[\sqrt{2\pi}\frac{\Gamma(3/4)}{\Gamma(1/4)}\right]Prove that:
\int_{\pi/4}^{\pi/2}\log^2[\log(\tan x)]\,dx=

\beta''(1)+\frac{\pi^3}{24}-\frac{\pi\gamma^2}{4}-\pi\gamma\log\left[\sqrt{2\pi}\frac{\Gamma(3/4)}{\Gamma(1/4)}\right]and...\int_{\pi/4}^{\pi/2}\log^3[\log(\tan x)]\,dx=

\beta'''(1)-3\gamma\,\beta''(1)+\frac{\pi\gamma^3}{2}-\frac{\pi}{2}\zeta(3) + \left(\frac{\pi^3}{4}+\frac{3\pi\gamma^2}{2}\right)\log\left[\sqrt{2\pi}\frac{\Gamma(3/4)}{\Gamma(1/4)}\right]
Where $$\beta(x)\,$$ is the Dirichlet Beta function, defined by:$$\beta(x)=\sum_{k=0}^{\infty}\frac{(-1)^k}{(2k+1)^x}$$
 
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I saw your other post, so I'm not going to say much.

But here's a brief outline of a way to evaluate Vardi's integral.Let $ \displaystyle I(a) = \int_{0}^{\infty}\frac{\ln(a^{2}+x^{2})}{\cosh x} \ dx $ and differentiate inside of the integral to get $ \displaystyle I'(a) = \int_{0}^{\infty} \frac{2a}{(a^{2}+x^{2}) \cosh x} \ dx $.

$I'(a)$ can be evaluated be letting $ \displaystyle f(z) = \frac{2a}{(a^{2}+z^{2}) \cosh z}$ and integrating around a rectangle or circle in the upper-half complex plane.

After somewhat tedious calculations, you''ll find that $ \displaystyle I'(a) = \Big[ \psi \Big(\frac{3}{4} + \frac{a}{2 \pi} \Big) - \psi \Big(\frac{1}{4} + \frac{a}{2 \pi} \Big) \Big]$

Then integrate back with respect to $a$.

Finding the constant of integration is tricky. It involves rewriting the integral and letting $a$ go to $\infty$.

But it will turn out that $ \displaystyle I(a) = 2 \pi \ln \Bigg[ \sqrt{2 \pi} \frac{\Gamma(\frac{3}{4} + \frac{a}{2 \pi})}{\Gamma(\frac{1}{4} + \frac{a}{2 \pi})} \Bigg] $

Then $ \displaystyle \lim_{a \to 0^{+}} I(a) = 2 \int_{0}^{\infty} \frac{\ln x}{\cosh x} \ dx = 2 \pi \ln \Bigg[ \sqrt{2 \pi} \frac{\Gamma(\frac{3}{4})}{\Gamma(\frac{1}{4})} \Bigg] $

Finally make the change of variables $x = \ln (\tan u) $.
 
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Very nice, RV! (Rock)
 
No worries if not, but just in case anyone's interested, here's a little hint to get you started in the right direction...

(Bandit)
Consider the integral:$$\int_0^{\infty}\frac{x^{q-1}}{\cosh x}\,dx$$for the real parameter $$q\in\mathbb{R}^+$$
 
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