An integral Thing I guess

Calculus Level 3

( sinh 1 ( x ) x ) 2 d x = π a \large \int_{-\infty }^{\infty } \left(\frac{\sinh ^{-1}(x)}{x}\right)^2 \, dx=\pi ^a

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The answer is 2.

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2 solutions

Mark Hennings
Dec 16, 2017

Making the substitutions u = x 2 + 1 + x u = \sqrt{x^2+1} + x and then v = u 1 v = u^{-1} we note that R ( sinh 1 x x ) 2 d x = 2 R ( ln ( x 2 + 1 + x ) x ) 2 d x = 4 1 u 2 + 1 ( u 2 1 ) 2 ( ln u ) 2 d u = 4 0 1 v 2 + 1 ( 1 v 2 ) 2 ( ln v ) 2 d v = 4 0 1 ( n 0 ( 2 n + 1 ) v 2 n ) ( ln v ) 2 d v = 8 n 0 1 ( 2 n + 1 ) 2 = 8 × 3 4 ζ ( 2 ) = π 2 \begin{aligned} \int_\mathbb{R} \left(\frac{\sinh^{-1}x}{x}\right)^2\,dx & = \; 2\int_{\mathbb{R}} \left(\frac{\ln(\sqrt{x^2+1}+x)}{x}\right)^2\,dx \; = \; 4\int_1^\infty \frac{u^2+1}{(u^2-1)^2}(\ln u)^2\,du \\ & = \; 4\int_0^1 \frac{v^2+1}{(1-v^2)^2}(\ln v)^2\,dv \; = \; 4\int_0^1 \left(\sum_{n \ge 0} (2n+1)v^{2n}\right)(\ln v)^2\,dv \\ & = \; 8\sum_{n\ge 0} \frac{1}{(2n+1)^2} \;= \; 8 \times \tfrac34\zeta(2) \; = \; \pi^2 \end{aligned} using the fact that 0 1 v m ( ln v ) 2 d v = 2 ( m + 1 ) 3 m 0 \int_0^1 v^m (\ln v)^2\,dv \; = \; \frac{2}{(m+1)^3} \hspace{2cm} m \ge 0 Thus the answer is 2 \boxed{2} .

integral((sinh^(-1)(x))/x)^2 dx = 2 Li 2(-e^(-sinh^(-1)(x))) - 2 Li 2(e^(-sinh^(-1)(x))) - sinh^(-1)(x) ((sinh^(-1)(x))/x - 2 log(1 - e^(-sinh^(-1)(x))) + 2 log(e^(-sinh^(-1)(x)) + 1)) + constant

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