Problem
Let: f(x)=arctan(1+x2−1x)f(x)=\arctan\left({\sqrt{\frac{\sqrt{1+x^2}-1}{x}}}\right)f(x)=arctan(x1+x2−1) Prove that ∫01f(x2)1−x2dx=π216\int^1_0 \frac{f(x^2)}{\sqrt{1-x^2}}dx =\frac{\pi^2}{16}∫011−x2f(x2)dx=16π2
Solution 1
we have :
arcsin(xi)=arctan(1+x4−1x2)+i arctanh(1+x4−1x2)\arcsin(x\sqrt{i}) =\text{arctan}\left({\sqrt{\frac{\sqrt{1+x^4}-1}{x^2}}}\right) + \text{i } \text{arctanh}\left({\sqrt{\frac{\sqrt{1+x^4}-1}{x^2}}}\right)arcsin(xi)=arctan(x21+x4−1)+i arctanh(x21+x4−1)
therfore:
I=Re(∫01arcsin(xi)1−x2dx)I = \text{Re}\left({\int^1_0 \frac{\arcsin(x\sqrt{i})}{\sqrt{1-x^2}} dx }\right)I=Re(∫011−x2arcsin(xi)dx)
we have :(Easy proof using series expansion ) ∫01arcsin(ax)1−x2dx=Li2(a)−14Li2(a2)\int^1_0 \frac{\arcsin(ax)}{\sqrt{1-x^2}} dx = \text{Li}_2(a)-\frac{1}{4}\text{Li}_2(a^2)∫011−x2arcsin(ax)dx=Li2(a)−41Li2(a2) Therfore: I=Re(Li2(i)−14Li2(i))=π216I = \text{Re}\left({\text{Li}_2(\sqrt{i})-\frac{1}{4}\text{Li}_2(i)}\right) =\frac{\pi^2}{16}I=Re(Li2(i)−41Li2(i))=16π2