Some Tricky Integral

Calculus Level 5

x 2 e x ( 1 + e x ) 2 d x \displaystyle \int_{-\infty}^{\infty} \dfrac{x^{2}e^{x}}{\left(1+e^{x}\right)^{2}} \, dx

Find the value of the closed form of the above integral to 3 decimal places.


The answer is 3.289868.

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1 solution

Mark Hennings
Feb 8, 2017

If we define A ( a ) = e a x ( 1 + e x ) 2 d x = 0 u a 1 ( 1 + u ) 2 d u = B ( a , 2 a ) = Γ ( a ) Γ ( 2 a ) A(a) \; =\; \int_{-\infty}^\infty \frac{e^{ax}}{(1 + e^x)^2}\,dx \; = \; \int_0^\infty \frac{u^{a-1}}{(1+u)^2}\,du \; = \; B(a,2-a) \; = \; \Gamma(a)\Gamma(2-a) for a > 0 a > 0 , then A ( a ) = Γ ( a ) Γ ( 2 a ) { ψ ( a ) ψ ( 2 a ) } A ( a ) = Γ ( a ) Γ ( 2 a ) { ( ψ ( a ) ψ ( 2 a ) ) 2 + ψ ( a ) + ψ ( 2 a ) } \begin{aligned} A'(a) & = \Gamma(a)\Gamma(2-a)\big\{\psi(a) - \psi(2-a)\big\} \\ A''(a) & = \Gamma(a)\Gamma(2-a)\big\{(\psi(a) - \psi(2-a))^2 + \psi'(a) + \psi'(2-a)\big\} \end{aligned} so that x 2 e x ( 1 + e x ) 2 d x = A ( 1 ) = 2 ψ ( 1 ) = 1 3 π 2 = 3.2898681 \int_{-\infty}^\infty \frac{x^2 e^x}{(1 + e^x)^2}\,dx \; = \; A''(1) \; = \; 2\psi'(1) \; = \; \tfrac13\pi^2 \; = \; \boxed{3.2898681}

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