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Friday, April 8, 2016

Gazeta Matematica 10/2015, Problem 27137

Problem:
Evaluate \lim_{y \to 0} \int_{y}^1 \left(\arctan x \arctan \dfrac{1}{x}+\dfrac{\log(1+x^2)}{1+x^2}\right) \ dx.

Proposed by Alessandro Ventullo, Milan, Italy
 
Solution:
Observe that \arctan x + \arctan \dfrac{1}{x}=\dfrac{\pi}{2} for all x>0. Setting a=\arctan x, b=\arctan \dfrac{1}{x}, c=-\dfrac{\pi}{2}, we have a+b+c=0, so a^3+b^3+c^3=3abc. This means that
-3\arctan x \cdot \arctan \dfrac{1}{x} \cdot \dfrac{\pi}{2}=\arctan^3 x + \arctan^3 \dfrac{1}{x}-\dfrac{\pi^3}{8}.
Hence
\int_0^1 \arctan x \arctan \dfrac{1}{x} \ dx=-\dfrac{2}{3\pi}\int_0^1 \left( \arctan^3 x + \arctan^3 \dfrac{1}{x}-\dfrac{\pi^3}{8}\right) \ dx.                  (1)
Integrating by part, we obtain
\int \arctan^3 x \ dx=x \arctan^3 x - \dfrac{3}{2}\log(1+x^2)\arctan^2 x+3 \int \dfrac{\log (1+x^2) \arctan x}{1+x^2} \ dx.
and
\int \arctan^3 \dfrac{1}{x} \ dx=x \arctan^3 \dfrac{1}{x} + \dfrac{3}{2}\log(1+x^2)\arctan^2 \dfrac{1}{x}+3 \int \dfrac{\log (1+x^2) \arctan 1/x}{1+x^2} \ dx.
Summing up and integrating from x=0 to x=1 and using the fact that \arctan x+\arctan \dfrac{1}{x}=\dfrac{\pi}{2}, we get
\int_0^1 \left( \arctan^3 x + \arctan^3 \dfrac{1}{x} \right) \ dx=\dfrac{\pi^3}{32}+\dfrac{3\pi}{2} \int_0^1 \dfrac{\log (1+x^2)}{1+x^2} \ dx.                 (2)
Substituting (1) into (2), we get
\int_{0}^1 \left(\arctan x \arctan \dfrac{1}{x}+\dfrac{\log(1+x^2)}{1+x^2}\right)=\dfrac{\pi^2}{16}. 

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